[PATCH] Fix ALTER SYSTEM empty string bug for GUC_LIST_QUOTE parameters

From 4642e4e0b0a63d9da6ac4479963dec6c7b71ac68 Mon Sep 17 00:00:00 2001
From: Andrew Klychkov <andrew.a.klychkov@gmail.com>
Date: Wed, 3 Sep 2025 10:48:49 +0200
Subject: [PATCH] Fix ALTER SYSTEM empty string bug for GUC_LIST_QUOTE
 parameters

When ALTER SYSTEM SET is used with an empty string for parameters with
GUC_LIST_QUOTE flag (like shared_preload_libraries), the empty string
was being double-quoted, resulting in '""' being written to
postgresql.auto.conf. This caused server crashes on restart due to
malformed configuration syntax.

The fix detects when a GUC_LIST_QUOTE parameter has the value '""' and
writes it as '' instead, preventing the double-quoting issue while
maintaining proper configuration file syntax.

Fixes bug where 'ALTER SYSTEM SET "shared_preload_libraries" TO '''
would write 'shared_preload_libraries = '""'' to postgresql.auto.conf.
---
 src/backend/utils/misc/guc.c | 39 ++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 46fdefebe3..7494cbd56b 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -4497,6 +4497,45 @@ write_auto_conf_file(int fd, const char *filename, ConfigVariable *head)
 	for (item = head; item != NULL; item = item->next)
 	{
 		char	   *escaped;
+		bool		is_guc_list_quote = false;
+
+		/*
+		 * Check if this is a GUC_LIST_QUOTE parameter to handle empty strings
+		 * specially and prevent double-quoting issues.
+		 */
+		if (item->value[0] == '\0' || strcmp(item->value, "\"\"") == 0)
+		{
+			struct config_generic *record = find_option(item->name, false, true, WARNING);
+			if (record && (record->flags & GUC_LIST_QUOTE))
+				is_guc_list_quote = true;
+		}
+
+		/*
+		 * Special handling for GUC_LIST_QUOTE parameters with empty strings.
+		 * When ALTER SYSTEM SET is used with an empty string for such parameters,
+		 * the value comes through as '""' (quoted empty string). We want to write
+		 * this as an empty string rather than the quoted version to avoid the
+		 * double-quoting issue.
+		 */
+		if (is_guc_list_quote && (item->value[0] == '\0' || strcmp(item->value, "\"\"") == 0))
+		{
+			/* For GUC_LIST_QUOTE parameters, write empty string as '' */
+			resetStringInfo(&buf);
+			appendStringInfoString(&buf, item->name);
+			appendStringInfoString(&buf, " = ''\n");
+
+			errno = 0;
+			if (write(fd, buf.data, buf.len) != buf.len)
+			{
+				/* if write didn't set errno, assume problem is no disk space */
+				if (errno == 0)
+					errno = ENOSPC;
+				ereport(ERROR,
+						(errcode_for_file_access(),
+						 errmsg("could not write to file \"%s\": %m", filename)));
+			}
+			continue;
+		}
 
 		resetStringInfo(&buf);
 
-- 
2.47.0

diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 3d6e6bdbfd2..27ba1920e68 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -3086,9 +3086,10 @@ pg_get_functiondef(PG_FUNCTION_ARGS)
 				 * rules used there aren't exactly like SQL's, we have to
 				 * break the list value apart and then quote the elements as
 				 * string literals.  (The elements may be double-quoted as-is,
-				 * but we can't just feed them to the SQL parser; it would do
-				 * the wrong thing with elements that are zero-length or
-				 * longer than NAMEDATALEN.)
+				 * but we can't just feed them to the SQL parser that way; it
+				 * would truncate elements that are longer than NAMEDATALEN,
+				 * which would be wrong if they're paths.)  Also, we need a
+				 * special case for empty lists.
 				 *
 				 * Variables that are not so marked should just be emitted as
 				 * simple string literals.  If the variable is not known to
@@ -3106,6 +3107,9 @@ pg_get_functiondef(PG_FUNCTION_ARGS)
 						/* this shouldn't fail really */
 						elog(ERROR, "invalid list syntax in proconfig item");
 					}
+					/* Special case: represent an empty list as '' */
+					if (namelist == NIL)
+						appendStringInfoString(&buf, "''");
 					foreach(lc, namelist)
 					{
 						char	   *curname = (char *) lfirst(lc);
diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index 2c398cd9e5c..d5ef492ee41 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -2753,7 +2753,7 @@ SplitIdentifierString(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
@@ -2777,6 +2777,8 @@ SplitIdentifierString(char *rawstring, char separator,
 				nextp = endp;
 			}
 			/* endp now points at the terminating quote */
+			if (curname == endp)
+				return false;	/* empty quoted name not allowed */
 			nextp = endp + 1;
 		}
 		else
@@ -2880,7 +2882,7 @@ SplitDirectoriesString(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new directory. */
 	do
@@ -2904,6 +2906,8 @@ SplitDirectoriesString(char *rawstring, char separator,
 				nextp = endp;
 			}
 			/* endp now points at the terminating quote */
+			if (curname == endp)
+				return false;	/* empty quoted name not allowed */
 			nextp = endp + 1;
 		}
 		else
@@ -3001,7 +3005,7 @@ SplitGUCList(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
@@ -3025,6 +3029,8 @@ SplitGUCList(char *rawstring, char separator,
 				nextp = endp;
 			}
 			/* endp now points at the terminating quote */
+			if (curname == endp)
+				return false;	/* empty quoted name not allowed */
 			nextp = endp + 1;
 		}
 		else
diff --git a/src/backend/utils/misc/guc_funcs.c b/src/backend/utils/misc/guc_funcs.c
index b9e26982abd..a0557d164d8 100644
--- a/src/backend/utils/misc/guc_funcs.c
+++ b/src/backend/utils/misc/guc_funcs.c
@@ -210,12 +210,30 @@ flatten_set_variable_args(const char *name, List *args)
 	else
 		flags = 0;
 
-	/* Complain if list input and non-list variable */
-	if ((flags & GUC_LIST_INPUT) == 0 &&
-		list_length(args) != 1)
-		ereport(ERROR,
-				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("SET %s takes only one argument", name)));
+	/*
+	 * Handle special cases for list input.
+	 */
+	if (flags & GUC_LIST_INPUT)
+	{
+		/* A single empty-string item is treated as an empty list. */
+		if (list_length(args) == 1)
+		{
+			Node	   *arg = (Node *) linitial(args);
+
+			if (IsA(arg, A_Const) &&
+				nodeTag(&((A_Const *) arg)->val) == T_String &&
+				*strVal(&((A_Const *) arg)->val) == '\0')
+				return pstrdup("");
+		}
+	}
+	else
+	{
+		/* Complain if list input and non-list variable. */
+		if (list_length(args) != 1)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("SET %s takes only one argument", name)));
+	}
 
 	initStringInfo(&buf);
 
@@ -269,6 +287,9 @@ flatten_set_variable_args(const char *name, List *args)
 					Datum		interval;
 					char	   *intervalout;
 
+					/* gram.y ensures this is only reachable for TIME ZONE */
+					Assert(!(flags & GUC_LIST_QUOTE));
+
 					typenameTypeIdAndMod(NULL, typeName, &typoid, &typmod);
 					Assert(typoid == INTERVALOID);
 
@@ -286,9 +307,17 @@ flatten_set_variable_args(const char *name, List *args)
 				else
 				{
 					/*
-					 * Plain string literal or identifier.  For quote mode,
+					 * Plain string literal or identifier.  In a list GUC,
+					 * disallow empty-string elements (so that the preceding
+					 * hack for empty lists is unambiguous).  For quote mode,
 					 * quote it if it's not a vanilla identifier.
 					 */
+					if ((flags & GUC_LIST_INPUT) && *val == '\0')
+						ereport(ERROR,
+								(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+								 errmsg("SET %s does not accept empty-string elements",
+										name)));
+
 					if (flags & GUC_LIST_QUOTE)
 						appendStringInfoString(&buf, quote_identifier(val));
 					else
diff --git a/src/bin/pg_dump/dumputils.c b/src/bin/pg_dump/dumputils.c
index 05b84c0d6e7..00a369c8861 100644
--- a/src/bin/pg_dump/dumputils.c
+++ b/src/bin/pg_dump/dumputils.c
@@ -781,7 +781,7 @@ SplitGUCList(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
@@ -805,6 +805,8 @@ SplitGUCList(char *rawstring, char separator,
 				nextp = endp;
 			}
 			/* endp now points at the terminating quote */
+			if (curname == endp)
+				return false;	/* empty quoted name not allowed */
 			nextp = endp + 1;
 		}
 		else
@@ -891,8 +893,9 @@ makeAlterConfigCommand(PGconn *conn, const char *configitem,
 	 * array.  However, because the quoting rules used there aren't exactly
 	 * like SQL's, we have to break the list value apart and then quote the
 	 * elements as string literals.  (The elements may be double-quoted as-is,
-	 * but we can't just feed them to the SQL parser; it would do the wrong
-	 * thing with elements that are zero-length or longer than NAMEDATALEN.)
+	 * but we can't just feed them to the SQL parser that way; it would
+	 * truncate elements that are longer than NAMEDATALEN, which would be
+	 * wrong if they're paths.)  Also, we need a special case for empty lists.
 	 *
 	 * Variables that are not so marked should just be emitted as simple
 	 * string literals.  If the variable is not known to
@@ -908,6 +911,9 @@ makeAlterConfigCommand(PGconn *conn, const char *configitem,
 		/* this shouldn't fail really */
 		if (SplitGUCList(pos, ',', &namelist))
 		{
+			/* Special case: represent an empty list as '' */
+			if (*namelist == NULL)
+				appendPQExpBufferStr(buf, "''");
 			for (nameptr = namelist; *nameptr; nameptr++)
 			{
 				if (nameptr != namelist)
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index bea793456f9..9d36a6a5aaf 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -13699,8 +13699,9 @@ dumpFunc(Archive *fout, const FuncInfo *finfo)
 		 * aren't exactly like SQL's, we have to break the list value apart
 		 * and then quote the elements as string literals.  (The elements may
 		 * be double-quoted as-is, but we can't just feed them to the SQL
-		 * parser; it would do the wrong thing with elements that are
-		 * zero-length or longer than NAMEDATALEN.)
+		 * parser that way; it would truncate elements that are longer than
+		 * NAMEDATALEN, which would be wrong if they're paths.)  Also, we need
+		 * a special case for empty lists.
 		 *
 		 * Variables that are not so marked should just be emitted as simple
 		 * string literals.  If the variable is not known to
@@ -13716,6 +13717,9 @@ dumpFunc(Archive *fout, const FuncInfo *finfo)
 			/* this shouldn't fail really */
 			if (SplitGUCList(pos, ',', &namelist))
 			{
+				/* Special case: represent an empty list as '' */
+				if (*namelist == NULL)
+					appendPQExpBufferStr(q, "''");
 				for (nameptr = namelist; *nameptr; nameptr++)
 				{
 					if (nameptr != namelist)
diff --git a/src/test/regress/expected/guc.out b/src/test/regress/expected/guc.out
index 7f9e29c765c..e49e609415b 100644
--- a/src/test/regress/expected/guc.out
+++ b/src/test/regress/expected/guc.out
@@ -31,6 +31,24 @@ SELECT '2006-08-13 12:34:56'::timestamptz;
  2006-08-13 12:34:56-07
 (1 row)
 
+-- Check handling of list GUCs
+SET search_path = 'pg_catalog', Foo, 'Bar';
+SHOW search_path;
+      search_path       
+------------------------
+ pg_catalog, foo, "Bar"
+(1 row)
+
+SET search_path = '';  -- means empty list
+SHOW search_path;
+ search_path 
+-------------
+ 
+(1 row)
+
+SET search_path = '', 'foo';  -- error, empty list elements not OK
+ERROR:  SET search_path does not accept empty-string elements
+RESET search_path;
 -- SET LOCAL has no effect outside of a transaction
 SET LOCAL vacuum_cost_delay TO 50;
 WARNING:  SET LOCAL can only be used in transaction blocks
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 35e8aad7701..43d5cf10266 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -3549,21 +3549,23 @@ CREATE FUNCTION func_with_set_params() RETURNS integer
     SET extra_float_digits TO 2
     SET work_mem TO '4MB'
     SET datestyle to iso, mdy
-    SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
+    SET temp_tablespaces to ''
+    SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
     IMMUTABLE STRICT;
 SELECT pg_get_functiondef('func_with_set_params()'::regprocedure);
-                                                                            pg_get_functiondef                                                                            
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
- CREATE OR REPLACE FUNCTION public.func_with_set_params()                                                                                                                +
-  RETURNS integer                                                                                                                                                        +
-  LANGUAGE sql                                                                                                                                                           +
-  IMMUTABLE STRICT                                                                                                                                                       +
-  SET search_path TO 'pg_catalog'                                                                                                                                        +
-  SET extra_float_digits TO '2'                                                                                                                                          +
-  SET work_mem TO '4MB'                                                                                                                                                  +
-  SET "DateStyle" TO 'iso, mdy'                                                                                                                                          +
-  SET local_preload_libraries TO 'Mixed/Case', 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'+
- AS $function$select 1;$function$                                                                                                                                        +
+                                                                          pg_get_functiondef                                                                          
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ CREATE OR REPLACE FUNCTION public.func_with_set_params()                                                                                                            +
+  RETURNS integer                                                                                                                                                    +
+  LANGUAGE sql                                                                                                                                                       +
+  IMMUTABLE STRICT                                                                                                                                                   +
+  SET search_path TO 'pg_catalog'                                                                                                                                    +
+  SET extra_float_digits TO '2'                                                                                                                                      +
+  SET work_mem TO '4MB'                                                                                                                                              +
+  SET "DateStyle" TO 'iso, mdy'                                                                                                                                      +
+  SET temp_tablespaces TO ''                                                                                                                                         +
+  SET local_preload_libraries TO 'Mixed/Case', 'c:/''a"/path', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'+
+ AS $function$select 1;$function$                                                                                                                                    +
  
 (1 row)
 
diff --git a/src/test/regress/sql/guc.sql b/src/test/regress/sql/guc.sql
index f65f84a2632..65630135f18 100644
--- a/src/test/regress/sql/guc.sql
+++ b/src/test/regress/sql/guc.sql
@@ -12,6 +12,14 @@ SHOW vacuum_cost_delay;
 SHOW datestyle;
 SELECT '2006-08-13 12:34:56'::timestamptz;
 
+-- Check handling of list GUCs
+SET search_path = 'pg_catalog', Foo, 'Bar';
+SHOW search_path;
+SET search_path = '';  -- means empty list
+SHOW search_path;
+SET search_path = '', 'foo';  -- error, empty list elements not OK
+RESET search_path;
+
 -- SET LOCAL has no effect outside of a transaction
 SET LOCAL vacuum_cost_delay TO 50;
 SHOW vacuum_cost_delay;
diff --git a/src/test/regress/sql/rules.sql b/src/test/regress/sql/rules.sql
index fdd3ff1d161..5c5ff5e9cca 100644
--- a/src/test/regress/sql/rules.sql
+++ b/src/test/regress/sql/rules.sql
@@ -1217,7 +1217,8 @@ CREATE FUNCTION func_with_set_params() RETURNS integer
     SET extra_float_digits TO 2
     SET work_mem TO '4MB'
     SET datestyle to iso, mdy
-    SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
+    SET temp_tablespaces to ''
+    SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
     IMMUTABLE STRICT;
 SELECT pg_get_functiondef('func_with_set_params()'::regprocedure);
 

From dd33d106fc67086dde59056d5ad39c61621b4dde Mon Sep 17 00:00:00 2001
From: Andrew Klychkov <andrew.a.klychkov@gmail.com>
Date: Fri, 5 Sep 2025 10:59:08 +0200
Subject: [PATCH] Allow SET to an empty list

---
 doc/src/sgml/config.sgml                      |  2 +
 doc/src/sgml/ref/set.sgml                     |  6 +++
 src/backend/utils/adt/ruleutils.c             | 10 +++--
 src/backend/utils/adt/varlena.c               | 12 ++++--
 src/backend/utils/misc/guc_funcs.c            | 43 ++++++++++++++++---
 src/bin/pg_dump/dumputils.c                   | 12 ++++--
 src/bin/pg_dump/pg_dump.c                     |  8 +++-
 src/test/modules/test_misc/meson.build        |  1 +
 .../modules/test_misc/t/009_alter_system.pl   | 31 +++++++++++++
 src/test/regress/expected/guc.out             | 18 ++++++++
 src/test/regress/expected/rules.out           | 28 ++++++------
 src/test/regress/sql/guc.sql                  |  8 ++++
 src/test/regress/sql/rules.sql                |  3 +-
 13 files changed, 150 insertions(+), 32 deletions(-)
 create mode 100644 src/test/modules/test_misc/t/009_alter_system.pl

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 0a4b3e55ba..8d2fba2a5e 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -52,6 +52,8 @@
        quotes within the value.  Quotes can usually be omitted if the value
        is a simple number or identifier, however.
        (Values that match an SQL keyword require quoting in some contexts.)
+       For parameters that take a list of strings, do not use an empty
+       string as an element, as that is not allowed.
       </para>
      </listitem>
 
diff --git a/doc/src/sgml/ref/set.sgml b/doc/src/sgml/ref/set.sgml
index 2218f54682..ce81993911 100644
--- a/doc/src/sgml/ref/set.sgml
+++ b/doc/src/sgml/ref/set.sgml
@@ -141,6 +141,12 @@ SET [ SESSION | LOCAL ] TIME ZONE { <replaceable class="parameter">value</replac
       value it would have had if no <command>SET</command> had been executed
       in the current session).
      </para>
+     <para>
+      For a parameter that takes a list of strings, a single empty string
+      (<literal>''</literal>) may be specified to set the parameter to an
+      empty list.  This is a special case; empty strings are not otherwise
+      allowed as elements of such lists.
+     </para>
     </listitem>
    </varlistentry>
   </variablelist>
diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 3d6e6bdbfd..27ba1920e6 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -3086,9 +3086,10 @@ pg_get_functiondef(PG_FUNCTION_ARGS)
 				 * rules used there aren't exactly like SQL's, we have to
 				 * break the list value apart and then quote the elements as
 				 * string literals.  (The elements may be double-quoted as-is,
-				 * but we can't just feed them to the SQL parser; it would do
-				 * the wrong thing with elements that are zero-length or
-				 * longer than NAMEDATALEN.)
+				 * but we can't just feed them to the SQL parser that way; it
+				 * would truncate elements that are longer than NAMEDATALEN,
+				 * which would be wrong if they're paths.)  Also, we need a
+				 * special case for empty lists.
 				 *
 				 * Variables that are not so marked should just be emitted as
 				 * simple string literals.  If the variable is not known to
@@ -3106,6 +3107,9 @@ pg_get_functiondef(PG_FUNCTION_ARGS)
 						/* this shouldn't fail really */
 						elog(ERROR, "invalid list syntax in proconfig item");
 					}
+					/* Special case: represent an empty list as '' */
+					if (namelist == NIL)
+						appendStringInfoString(&buf, "''");
 					foreach(lc, namelist)
 					{
 						char	   *curname = (char *) lfirst(lc);
diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index 2c398cd9e5..d5ef492ee4 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -2753,7 +2753,7 @@ SplitIdentifierString(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
@@ -2777,6 +2777,8 @@ SplitIdentifierString(char *rawstring, char separator,
 				nextp = endp;
 			}
 			/* endp now points at the terminating quote */
+			if (curname == endp)
+				return false;	/* empty quoted name not allowed */
 			nextp = endp + 1;
 		}
 		else
@@ -2880,7 +2882,7 @@ SplitDirectoriesString(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new directory. */
 	do
@@ -2904,6 +2906,8 @@ SplitDirectoriesString(char *rawstring, char separator,
 				nextp = endp;
 			}
 			/* endp now points at the terminating quote */
+			if (curname == endp)
+				return false;	/* empty quoted name not allowed */
 			nextp = endp + 1;
 		}
 		else
@@ -3001,7 +3005,7 @@ SplitGUCList(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
@@ -3025,6 +3029,8 @@ SplitGUCList(char *rawstring, char separator,
 				nextp = endp;
 			}
 			/* endp now points at the terminating quote */
+			if (curname == endp)
+				return false;	/* empty quoted name not allowed */
 			nextp = endp + 1;
 		}
 		else
diff --git a/src/backend/utils/misc/guc_funcs.c b/src/backend/utils/misc/guc_funcs.c
index b9e26982ab..a0557d164d 100644
--- a/src/backend/utils/misc/guc_funcs.c
+++ b/src/backend/utils/misc/guc_funcs.c
@@ -210,12 +210,30 @@ flatten_set_variable_args(const char *name, List *args)
 	else
 		flags = 0;
 
-	/* Complain if list input and non-list variable */
-	if ((flags & GUC_LIST_INPUT) == 0 &&
-		list_length(args) != 1)
-		ereport(ERROR,
-				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("SET %s takes only one argument", name)));
+	/*
+	 * Handle special cases for list input.
+	 */
+	if (flags & GUC_LIST_INPUT)
+	{
+		/* A single empty-string item is treated as an empty list. */
+		if (list_length(args) == 1)
+		{
+			Node	   *arg = (Node *) linitial(args);
+
+			if (IsA(arg, A_Const) &&
+				nodeTag(&((A_Const *) arg)->val) == T_String &&
+				*strVal(&((A_Const *) arg)->val) == '\0')
+				return pstrdup("");
+		}
+	}
+	else
+	{
+		/* Complain if list input and non-list variable. */
+		if (list_length(args) != 1)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("SET %s takes only one argument", name)));
+	}
 
 	initStringInfo(&buf);
 
@@ -269,6 +287,9 @@ flatten_set_variable_args(const char *name, List *args)
 					Datum		interval;
 					char	   *intervalout;
 
+					/* gram.y ensures this is only reachable for TIME ZONE */
+					Assert(!(flags & GUC_LIST_QUOTE));
+
 					typenameTypeIdAndMod(NULL, typeName, &typoid, &typmod);
 					Assert(typoid == INTERVALOID);
 
@@ -286,9 +307,17 @@ flatten_set_variable_args(const char *name, List *args)
 				else
 				{
 					/*
-					 * Plain string literal or identifier.  For quote mode,
+					 * Plain string literal or identifier.  In a list GUC,
+					 * disallow empty-string elements (so that the preceding
+					 * hack for empty lists is unambiguous).  For quote mode,
 					 * quote it if it's not a vanilla identifier.
 					 */
+					if ((flags & GUC_LIST_INPUT) && *val == '\0')
+						ereport(ERROR,
+								(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+								 errmsg("SET %s does not accept empty-string elements",
+										name)));
+
 					if (flags & GUC_LIST_QUOTE)
 						appendStringInfoString(&buf, quote_identifier(val));
 					else
diff --git a/src/bin/pg_dump/dumputils.c b/src/bin/pg_dump/dumputils.c
index 05b84c0d6e..00a369c886 100644
--- a/src/bin/pg_dump/dumputils.c
+++ b/src/bin/pg_dump/dumputils.c
@@ -781,7 +781,7 @@ SplitGUCList(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
@@ -805,6 +805,8 @@ SplitGUCList(char *rawstring, char separator,
 				nextp = endp;
 			}
 			/* endp now points at the terminating quote */
+			if (curname == endp)
+				return false;	/* empty quoted name not allowed */
 			nextp = endp + 1;
 		}
 		else
@@ -891,8 +893,9 @@ makeAlterConfigCommand(PGconn *conn, const char *configitem,
 	 * array.  However, because the quoting rules used there aren't exactly
 	 * like SQL's, we have to break the list value apart and then quote the
 	 * elements as string literals.  (The elements may be double-quoted as-is,
-	 * but we can't just feed them to the SQL parser; it would do the wrong
-	 * thing with elements that are zero-length or longer than NAMEDATALEN.)
+	 * but we can't just feed them to the SQL parser that way; it would
+	 * truncate elements that are longer than NAMEDATALEN, which would be
+	 * wrong if they're paths.)  Also, we need a special case for empty lists.
 	 *
 	 * Variables that are not so marked should just be emitted as simple
 	 * string literals.  If the variable is not known to
@@ -908,6 +911,9 @@ makeAlterConfigCommand(PGconn *conn, const char *configitem,
 		/* this shouldn't fail really */
 		if (SplitGUCList(pos, ',', &namelist))
 		{
+			/* Special case: represent an empty list as '' */
+			if (*namelist == NULL)
+				appendPQExpBufferStr(buf, "''");
 			for (nameptr = namelist; *nameptr; nameptr++)
 			{
 				if (nameptr != namelist)
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index bea793456f..9d36a6a5aa 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -13699,8 +13699,9 @@ dumpFunc(Archive *fout, const FuncInfo *finfo)
 		 * aren't exactly like SQL's, we have to break the list value apart
 		 * and then quote the elements as string literals.  (The elements may
 		 * be double-quoted as-is, but we can't just feed them to the SQL
-		 * parser; it would do the wrong thing with elements that are
-		 * zero-length or longer than NAMEDATALEN.)
+		 * parser that way; it would truncate elements that are longer than
+		 * NAMEDATALEN, which would be wrong if they're paths.)  Also, we need
+		 * a special case for empty lists.
 		 *
 		 * Variables that are not so marked should just be emitted as simple
 		 * string literals.  If the variable is not known to
@@ -13716,6 +13717,9 @@ dumpFunc(Archive *fout, const FuncInfo *finfo)
 			/* this shouldn't fail really */
 			if (SplitGUCList(pos, ',', &namelist))
 			{
+				/* Special case: represent an empty list as '' */
+				if (*namelist == NULL)
+					appendPQExpBufferStr(q, "''");
 				for (nameptr = namelist; *nameptr; nameptr++)
 				{
 					if (nameptr != namelist)
diff --git a/src/test/modules/test_misc/meson.build b/src/test/modules/test_misc/meson.build
index 6b1e730bf4..40d50e245d 100644
--- a/src/test/modules/test_misc/meson.build
+++ b/src/test/modules/test_misc/meson.build
@@ -17,6 +17,7 @@ tests += {
       't/006_signal_autovacuum.pl',
       't/007_catcache_inval.pl',
       't/008_replslot_single_user.pl',
+      't/009_alter_system.pl.pl',
     ],
   },
 }
diff --git a/src/test/modules/test_misc/t/009_alter_system.pl b/src/test/modules/test_misc/t/009_alter_system.pl
new file mode 100644
index 0000000000..1f2d1e28e6
--- /dev/null
+++ b/src/test/modules/test_misc/t/009_alter_system.pl
@@ -0,0 +1,31 @@
+# Copyright (c) 2025, PostgreSQL Global Development Group
+#
+# Test ALTER SYSTEM for list-based GUCs
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Initialize a new node and start it
+my $node = PostgreSQL::Test::Cluster->new('main');
+$node->init;
+$node->start;
+
+# Test setting a list GUC to an empty list
+$node->safe_psql('postgres', "ALTER SYSTEM SET search_path TO ''");
+$node->reload;
+
+my ($ret, $stdout, $stderr) = $node->psql('postgres', "SHOW search_path");
+is($stdout, '', 'ALTER SYSTEM SET search_path TO \'\' results in an empty list');
+
+# Test that ALTER SYSTEM rejects a list with an empty element
+($ret, $stdout, $stderr) = $node->psql('postgres', "ALTER SYSTEM SET search_path TO 'foo', ''");
+isnt($ret, 0, 'ALTER SYSTEM rejects list with empty element');
+like($stderr, qr/does not accept empty-string elements/, 'Correct error message for empty element in list');
+
+# Clean up GUC settings
+$node->safe_psql('postgres', "ALTER SYSTEM RESET search_path");
+$node->reload;
+
+done_testing(); 
diff --git a/src/test/regress/expected/guc.out b/src/test/regress/expected/guc.out
index 7f9e29c765..e49e609415 100644
--- a/src/test/regress/expected/guc.out
+++ b/src/test/regress/expected/guc.out
@@ -31,6 +31,24 @@ SELECT '2006-08-13 12:34:56'::timestamptz;
  2006-08-13 12:34:56-07
 (1 row)
 
+-- Check handling of list GUCs
+SET search_path = 'pg_catalog', Foo, 'Bar';
+SHOW search_path;
+      search_path       
+------------------------
+ pg_catalog, foo, "Bar"
+(1 row)
+
+SET search_path = '';  -- means empty list
+SHOW search_path;
+ search_path 
+-------------
+ 
+(1 row)
+
+SET search_path = '', 'foo';  -- error, empty list elements not OK
+ERROR:  SET search_path does not accept empty-string elements
+RESET search_path;
 -- SET LOCAL has no effect outside of a transaction
 SET LOCAL vacuum_cost_delay TO 50;
 WARNING:  SET LOCAL can only be used in transaction blocks
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 35e8aad770..43d5cf1026 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -3549,21 +3549,23 @@ CREATE FUNCTION func_with_set_params() RETURNS integer
     SET extra_float_digits TO 2
     SET work_mem TO '4MB'
     SET datestyle to iso, mdy
-    SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
+    SET temp_tablespaces to ''
+    SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
     IMMUTABLE STRICT;
 SELECT pg_get_functiondef('func_with_set_params()'::regprocedure);
-                                                                            pg_get_functiondef                                                                            
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
- CREATE OR REPLACE FUNCTION public.func_with_set_params()                                                                                                                +
-  RETURNS integer                                                                                                                                                        +
-  LANGUAGE sql                                                                                                                                                           +
-  IMMUTABLE STRICT                                                                                                                                                       +
-  SET search_path TO 'pg_catalog'                                                                                                                                        +
-  SET extra_float_digits TO '2'                                                                                                                                          +
-  SET work_mem TO '4MB'                                                                                                                                                  +
-  SET "DateStyle" TO 'iso, mdy'                                                                                                                                          +
-  SET local_preload_libraries TO 'Mixed/Case', 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'+
- AS $function$select 1;$function$                                                                                                                                        +
+                                                                          pg_get_functiondef                                                                          
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ CREATE OR REPLACE FUNCTION public.func_with_set_params()                                                                                                            +
+  RETURNS integer                                                                                                                                                    +
+  LANGUAGE sql                                                                                                                                                       +
+  IMMUTABLE STRICT                                                                                                                                                   +
+  SET search_path TO 'pg_catalog'                                                                                                                                    +
+  SET extra_float_digits TO '2'                                                                                                                                      +
+  SET work_mem TO '4MB'                                                                                                                                              +
+  SET "DateStyle" TO 'iso, mdy'                                                                                                                                      +
+  SET temp_tablespaces TO ''                                                                                                                                         +
+  SET local_preload_libraries TO 'Mixed/Case', 'c:/''a"/path', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'+
+ AS $function$select 1;$function$                                                                                                                                    +
  
 (1 row)
 
diff --git a/src/test/regress/sql/guc.sql b/src/test/regress/sql/guc.sql
index f65f84a263..65630135f1 100644
--- a/src/test/regress/sql/guc.sql
+++ b/src/test/regress/sql/guc.sql
@@ -12,6 +12,14 @@ SHOW vacuum_cost_delay;
 SHOW datestyle;
 SELECT '2006-08-13 12:34:56'::timestamptz;
 
+-- Check handling of list GUCs
+SET search_path = 'pg_catalog', Foo, 'Bar';
+SHOW search_path;
+SET search_path = '';  -- means empty list
+SHOW search_path;
+SET search_path = '', 'foo';  -- error, empty list elements not OK
+RESET search_path;
+
 -- SET LOCAL has no effect outside of a transaction
 SET LOCAL vacuum_cost_delay TO 50;
 SHOW vacuum_cost_delay;
diff --git a/src/test/regress/sql/rules.sql b/src/test/regress/sql/rules.sql
index fdd3ff1d16..5c5ff5e9cc 100644
--- a/src/test/regress/sql/rules.sql
+++ b/src/test/regress/sql/rules.sql
@@ -1217,7 +1217,8 @@ CREATE FUNCTION func_with_set_params() RETURNS integer
     SET extra_float_digits TO 2
     SET work_mem TO '4MB'
     SET datestyle to iso, mdy
-    SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
+    SET temp_tablespaces to ''
+    SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
     IMMUTABLE STRICT;
 SELECT pg_get_functiondef('func_with_set_params()'::regprocedure);
 
-- 
2.47.0

diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 9fd48acb1f8..ff31653b6ac 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -1709,6 +1709,26 @@ generic_set:
 					n->location = @3;
 					$$ = n;
 				}
+			| var_name TO NULL_P
+				{
+					VariableSetStmt *n = makeNode(VariableSetStmt);
+
+					n->kind = VAR_SET_VALUE;
+					n->name = $1;
+					n->args = list_make1(makeNullAConst(@3));
+					n->location = @3;
+					$$ = n;
+				}
+			| var_name '=' NULL_P
+				{
+					VariableSetStmt *n = makeNode(VariableSetStmt);
+
+					n->kind = VAR_SET_VALUE;
+					n->name = $1;
+					n->args = list_make1(makeNullAConst(@3));
+					n->location = @3;
+					$$ = n;
+				}
 			| var_name TO DEFAULT
 				{
 					VariableSetStmt *n = makeNode(VariableSetStmt);
diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 3d6e6bdbfd2..f81cfd17fd0 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -3088,7 +3088,8 @@ pg_get_functiondef(PG_FUNCTION_ARGS)
 				 * string literals.  (The elements may be double-quoted as-is,
 				 * but we can't just feed them to the SQL parser; it would do
 				 * the wrong thing with elements that are zero-length or
-				 * longer than NAMEDATALEN.)
+				 * longer than NAMEDATALEN.)  Also, we need a special case for
+				 * empty lists.
 				 *
 				 * Variables that are not so marked should just be emitted as
 				 * simple string literals.  If the variable is not known to
@@ -3106,6 +3107,9 @@ pg_get_functiondef(PG_FUNCTION_ARGS)
 						/* this shouldn't fail really */
 						elog(ERROR, "invalid list syntax in proconfig item");
 					}
+					/* Special case: represent an empty list as NULL */
+					if (namelist == NIL)
+						appendStringInfoString(&buf, "NULL");
 					foreach(lc, namelist)
 					{
 						char	   *curname = (char *) lfirst(lc);
diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index 2c398cd9e5c..1e1f69f7528 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -2753,7 +2753,7 @@ SplitIdentifierString(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
@@ -2880,7 +2880,7 @@ SplitDirectoriesString(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new directory. */
 	do
@@ -3001,7 +3001,7 @@ SplitGUCList(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
diff --git a/src/backend/utils/misc/guc_funcs.c b/src/backend/utils/misc/guc_funcs.c
index b9e26982abd..d0f4e396acc 100644
--- a/src/backend/utils/misc/guc_funcs.c
+++ b/src/backend/utils/misc/guc_funcs.c
@@ -210,12 +210,29 @@ flatten_set_variable_args(const char *name, List *args)
 	else
 		flags = 0;
 
-	/* Complain if list input and non-list variable */
-	if ((flags & GUC_LIST_INPUT) == 0 &&
-		list_length(args) != 1)
-		ereport(ERROR,
-				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("SET %s takes only one argument", name)));
+	/*
+	 * Handle special cases for list input.
+	 */
+	if (flags & GUC_LIST_INPUT)
+	{
+		/* NULL represents an empty list. */
+		if (list_length(args) == 1)
+		{
+			Node	   *arg = (Node *) linitial(args);
+
+			if (IsA(arg, A_Const) &&
+				((A_Const *) arg)->isnull)
+				return pstrdup("");
+		}
+	}
+	else
+	{
+		/* Complain if list input and non-list variable. */
+		if (list_length(args) != 1)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("SET %s takes only one argument", name)));
+	}
 
 	initStringInfo(&buf);
 
@@ -246,6 +263,12 @@ flatten_set_variable_args(const char *name, List *args)
 			elog(ERROR, "unrecognized node type: %d", (int) nodeTag(arg));
 		con = (A_Const *) arg;
 
+		/* Complain if NULL is used with a non-list variable. */
+		if (con->isnull)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("NULL is an invalid value for %s", name)));
+
 		switch (nodeTag(&con->val))
 		{
 			case T_Integer:
@@ -269,6 +292,9 @@ flatten_set_variable_args(const char *name, List *args)
 					Datum		interval;
 					char	   *intervalout;
 
+					/* gram.y ensures this is only reachable for TIME ZONE */
+					Assert(!(flags & GUC_LIST_QUOTE));
+
 					typenameTypeIdAndMod(NULL, typeName, &typoid, &typmod);
 					Assert(typoid == INTERVALOID);
 
diff --git a/src/bin/pg_dump/dumputils.c b/src/bin/pg_dump/dumputils.c
index 05b84c0d6e7..2d22723aa91 100644
--- a/src/bin/pg_dump/dumputils.c
+++ b/src/bin/pg_dump/dumputils.c
@@ -781,7 +781,7 @@ SplitGUCList(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
@@ -893,6 +893,7 @@ makeAlterConfigCommand(PGconn *conn, const char *configitem,
 	 * elements as string literals.  (The elements may be double-quoted as-is,
 	 * but we can't just feed them to the SQL parser; it would do the wrong
 	 * thing with elements that are zero-length or longer than NAMEDATALEN.)
+	 * Also, we need a special case for empty lists.
 	 *
 	 * Variables that are not so marked should just be emitted as simple
 	 * string literals.  If the variable is not known to
@@ -908,6 +909,9 @@ makeAlterConfigCommand(PGconn *conn, const char *configitem,
 		/* this shouldn't fail really */
 		if (SplitGUCList(pos, ',', &namelist))
 		{
+			/* Special case: represent an empty list as NULL */
+			if (*namelist == NULL)
+				appendPQExpBufferStr(buf, "NULL");
 			for (nameptr = namelist; *nameptr; nameptr++)
 			{
 				if (nameptr != namelist)
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index bea793456f9..08fcfcfbdfe 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -13700,7 +13700,8 @@ dumpFunc(Archive *fout, const FuncInfo *finfo)
 		 * and then quote the elements as string literals.  (The elements may
 		 * be double-quoted as-is, but we can't just feed them to the SQL
 		 * parser; it would do the wrong thing with elements that are
-		 * zero-length or longer than NAMEDATALEN.)
+		 * zero-length or longer than NAMEDATALEN.)  Also, we need a special
+		 * case for empty lists.
 		 *
 		 * Variables that are not so marked should just be emitted as simple
 		 * string literals.  If the variable is not known to
@@ -13716,6 +13717,9 @@ dumpFunc(Archive *fout, const FuncInfo *finfo)
 			/* this shouldn't fail really */
 			if (SplitGUCList(pos, ',', &namelist))
 			{
+				/* Special case: represent an empty list as NULL */
+				if (*namelist == NULL)
+					appendPQExpBufferStr(q, "NULL");
 				for (nameptr = namelist; *nameptr; nameptr++)
 				{
 					if (nameptr != namelist)
diff --git a/src/test/regress/expected/guc.out b/src/test/regress/expected/guc.out
index 7f9e29c765c..d6fb879f500 100644
--- a/src/test/regress/expected/guc.out
+++ b/src/test/regress/expected/guc.out
@@ -31,6 +31,28 @@ SELECT '2006-08-13 12:34:56'::timestamptz;
  2006-08-13 12:34:56-07
 (1 row)
 
+-- Check handling of list GUCs
+SET search_path = 'pg_catalog', Foo, 'Bar', '';
+SHOW search_path;
+        search_path         
+----------------------------
+ pg_catalog, foo, "Bar", ""
+(1 row)
+
+SET search_path = null;  -- means empty list
+SHOW search_path;
+ search_path 
+-------------
+ 
+(1 row)
+
+SET search_path = null, null;  -- syntax error
+ERROR:  syntax error at or near ","
+LINE 1: SET search_path = null, null;
+                              ^
+SET enable_seqscan = null;  -- error
+ERROR:  NULL is an invalid value for enable_seqscan
+RESET search_path;
 -- SET LOCAL has no effect outside of a transaction
 SET LOCAL vacuum_cost_delay TO 50;
 WARNING:  SET LOCAL can only be used in transaction blocks
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 35e8aad7701..3cc5f8a77b2 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -3549,6 +3549,7 @@ CREATE FUNCTION func_with_set_params() RETURNS integer
     SET extra_float_digits TO 2
     SET work_mem TO '4MB'
     SET datestyle to iso, mdy
+    SET temp_tablespaces to NULL
     SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
     IMMUTABLE STRICT;
 SELECT pg_get_functiondef('func_with_set_params()'::regprocedure);
@@ -3562,6 +3563,7 @@ SELECT pg_get_functiondef('func_with_set_params()'::regprocedure);
   SET extra_float_digits TO '2'                                                                                                                                          +
   SET work_mem TO '4MB'                                                                                                                                                  +
   SET "DateStyle" TO 'iso, mdy'                                                                                                                                          +
+  SET temp_tablespaces TO NULL                                                                                                                                           +
   SET local_preload_libraries TO 'Mixed/Case', 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'+
  AS $function$select 1;$function$                                                                                                                                        +
  
diff --git a/src/test/regress/sql/guc.sql b/src/test/regress/sql/guc.sql
index f65f84a2632..bafaf067e82 100644
--- a/src/test/regress/sql/guc.sql
+++ b/src/test/regress/sql/guc.sql
@@ -12,6 +12,15 @@ SHOW vacuum_cost_delay;
 SHOW datestyle;
 SELECT '2006-08-13 12:34:56'::timestamptz;
 
+-- Check handling of list GUCs
+SET search_path = 'pg_catalog', Foo, 'Bar', '';
+SHOW search_path;
+SET search_path = null;  -- means empty list
+SHOW search_path;
+SET search_path = null, null;  -- syntax error
+SET enable_seqscan = null;  -- error
+RESET search_path;
+
 -- SET LOCAL has no effect outside of a transaction
 SET LOCAL vacuum_cost_delay TO 50;
 SHOW vacuum_cost_delay;
diff --git a/src/test/regress/sql/rules.sql b/src/test/regress/sql/rules.sql
index fdd3ff1d161..3f240bec7b0 100644
--- a/src/test/regress/sql/rules.sql
+++ b/src/test/regress/sql/rules.sql
@@ -1217,6 +1217,7 @@ CREATE FUNCTION func_with_set_params() RETURNS integer
     SET extra_float_digits TO 2
     SET work_mem TO '4MB'
     SET datestyle to iso, mdy
+    SET temp_tablespaces to NULL
     SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
     IMMUTABLE STRICT;
 SELECT pg_get_functiondef('func_with_set_params()'::regprocedure);

From 78044b38eda13ee6abe49ac7cd4c689ad7f58b9c Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sat, 1 Nov 2025 15:59:00 -0400
Subject: [PATCH v6] Allow "SET list_guc TO NULL" to specify setting the GUC to
 empty.

We have never had a SET syntax that allows setting a GUC_LIST_INPUT
parameter to be an empty list.  A locution such as
	SET search_path = '';
doesn't mean that; it means setting the GUC to contain a single item
that is an empty string.  (For search_path the net effect is much the
same, because search_path ignores invalid schema names and '' must be
invalid.)  This is confusing, not least because configuration-file
entries and the set_config() function can easily produce empty-list
values.

We debated making the above syntax work, but that would foreclose
ever allowing empty-string items to be valid in list GUCs.  While
there isn't any obvious use-case for that today, it feels like the
kind of restriction that might hurt someday.  Instead, let's accept
the forbidden-up-to-now value NULL and treat that as meaning an
empty list.  (An objection to this could be "what if we someday want
to allow NULL as a GUC value?".  That seems unlikely though, and even
if we did allow it for scalar GUCs, we could continue to treat it as
meaning an empty list for list GUCs.)
---
 doc/src/sgml/ref/alter_system.sgml  |  2 ++
 doc/src/sgml/ref/set.sgml           | 24 +++++++++++++++---
 src/backend/parser/gram.y           | 20 +++++++++++++++
 src/backend/utils/adt/ruleutils.c   |  6 ++++-
 src/backend/utils/adt/varlena.c     |  6 ++---
 src/backend/utils/misc/guc_funcs.c  | 38 ++++++++++++++++++++++++-----
 src/bin/pg_dump/dumputils.c         |  6 ++++-
 src/bin/pg_dump/pg_dump.c           |  6 ++++-
 src/test/regress/expected/guc.out   | 22 +++++++++++++++++
 src/test/regress/expected/rules.out |  2 ++
 src/test/regress/sql/guc.sql        |  9 +++++++
 src/test/regress/sql/rules.sql      |  1 +
 12 files changed, 126 insertions(+), 16 deletions(-)

diff --git a/doc/src/sgml/ref/alter_system.sgml b/doc/src/sgml/ref/alter_system.sgml
index 1bde66d6ad2..65676e5c1c5 100644
--- a/doc/src/sgml/ref/alter_system.sgml
+++ b/doc/src/sgml/ref/alter_system.sgml
@@ -84,6 +84,8 @@ ALTER SYSTEM RESET ALL
       constants, identifiers, numbers, or comma-separated lists of
       these, as appropriate for the particular parameter.
       Values that are neither numbers nor valid identifiers must be quoted.
+      If the parameter accepts a list of values, <literal>NULL</literal>
+      can be written to specify an empty list.
       <literal>DEFAULT</literal> can be written to specify removing the
       parameter and its value from <filename>postgresql.auto.conf</filename>.
      </para>
diff --git a/doc/src/sgml/ref/set.sgml b/doc/src/sgml/ref/set.sgml
index 2218f54682e..16c7e9a7b26 100644
--- a/doc/src/sgml/ref/set.sgml
+++ b/doc/src/sgml/ref/set.sgml
@@ -21,8 +21,8 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-SET [ SESSION | LOCAL ] <replaceable class="parameter">configuration_parameter</replaceable> { TO | = } { <replaceable class="parameter">value</replaceable> | '<replaceable class="parameter">value</replaceable>' | DEFAULT }
-SET [ SESSION | LOCAL ] TIME ZONE { <replaceable class="parameter">value</replaceable> | '<replaceable class="parameter">value</replaceable>' | LOCAL | DEFAULT }
+SET [ SESSION | LOCAL ] <replaceable class="parameter">configuration_parameter</replaceable> { TO | = } { <replaceable class="parameter">value</replaceable> [, ...] | DEFAULT }
+SET [ SESSION | LOCAL ] TIME ZONE { <replaceable class="parameter">value</replaceable> | LOCAL | DEFAULT }
 </synopsis>
  </refsynopsisdiv>
 
@@ -123,7 +123,7 @@ SET [ SESSION | LOCAL ] TIME ZONE { <replaceable class="parameter">value</replac
     <term><replaceable class="parameter">configuration_parameter</replaceable></term>
     <listitem>
      <para>
-      Name of a settable run-time parameter.  Available parameters are
+      Name of a settable configuration parameter.  Available parameters are
       documented in <xref linkend="runtime-config"/> and below.
      </para>
     </listitem>
@@ -133,9 +133,12 @@ SET [ SESSION | LOCAL ] TIME ZONE { <replaceable class="parameter">value</replac
     <term><replaceable class="parameter">value</replaceable></term>
     <listitem>
      <para>
-      New value of parameter.  Values can be specified as string
+      New value of the parameter.  Values can be specified as string
       constants, identifiers, numbers, or comma-separated lists of
       these, as appropriate for the particular parameter.
+      Values that are neither numbers nor valid identifiers must be quoted.
+      If the parameter accepts a list of values, <literal>NULL</literal>
+      can be written to specify an empty list.
       <literal>DEFAULT</literal> can be written to specify
       resetting the parameter to its default value (that is, whatever
       value it would have had if no <command>SET</command> had been executed
@@ -283,6 +286,19 @@ SELECT setseed(<replaceable>value</replaceable>);
    Set the schema search path:
 <programlisting>
 SET search_path TO my_schema, public;
+</programlisting>
+   Note that this is not the same as
+<programlisting>
+SET search_path TO 'my_schema, public';
+</programlisting>
+   which would have the effect of setting <varname>search_path</varname>
+   to contain a single, probably-nonexistent schema name.
+  </para>
+
+  <para>
+   Set the list of temporary tablespace names to be empty:
+<programlisting>
+SET temp_tablespaces TO NULL;
 </programlisting>
   </para>
 
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index a4b29c822e8..50828a27053 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -1716,6 +1716,26 @@ generic_set:
 					n->location = @3;
 					$$ = n;
 				}
+			| var_name TO NULL_P
+				{
+					VariableSetStmt *n = makeNode(VariableSetStmt);
+
+					n->kind = VAR_SET_VALUE;
+					n->name = $1;
+					n->args = list_make1(makeNullAConst(@3));
+					n->location = @3;
+					$$ = n;
+				}
+			| var_name '=' NULL_P
+				{
+					VariableSetStmt *n = makeNode(VariableSetStmt);
+
+					n->kind = VAR_SET_VALUE;
+					n->name = $1;
+					n->args = list_make1(makeNullAConst(@3));
+					n->location = @3;
+					$$ = n;
+				}
 			| var_name TO DEFAULT
 				{
 					VariableSetStmt *n = makeNode(VariableSetStmt);
diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 79ec136231b..5398679cce2 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -3087,7 +3087,8 @@ pg_get_functiondef(PG_FUNCTION_ARGS)
 				 * string literals.  (The elements may be double-quoted as-is,
 				 * but we can't just feed them to the SQL parser; it would do
 				 * the wrong thing with elements that are zero-length or
-				 * longer than NAMEDATALEN.)
+				 * longer than NAMEDATALEN.)  Also, we need a special case for
+				 * empty lists.
 				 *
 				 * Variables that are not so marked should just be emitted as
 				 * simple string literals.  If the variable is not known to
@@ -3105,6 +3106,9 @@ pg_get_functiondef(PG_FUNCTION_ARGS)
 						/* this shouldn't fail really */
 						elog(ERROR, "invalid list syntax in proconfig item");
 					}
+					/* Special case: represent an empty list as NULL */
+					if (namelist == NIL)
+						appendStringInfoString(&buf, "NULL");
 					foreach(lc, namelist)
 					{
 						char	   *curname = (char *) lfirst(lc);
diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index 8d735786e51..3894457ab40 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -2753,7 +2753,7 @@ SplitIdentifierString(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
@@ -2880,7 +2880,7 @@ SplitDirectoriesString(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new directory. */
 	do
@@ -3001,7 +3001,7 @@ SplitGUCList(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
diff --git a/src/backend/utils/misc/guc_funcs.c b/src/backend/utils/misc/guc_funcs.c
index 4f58fa3d4e0..9dbc5d3aeb9 100644
--- a/src/backend/utils/misc/guc_funcs.c
+++ b/src/backend/utils/misc/guc_funcs.c
@@ -210,12 +210,29 @@ flatten_set_variable_args(const char *name, List *args)
 	else
 		flags = 0;
 
-	/* Complain if list input and non-list variable */
-	if ((flags & GUC_LIST_INPUT) == 0 &&
-		list_length(args) != 1)
-		ereport(ERROR,
-				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("SET %s takes only one argument", name)));
+	/*
+	 * Handle special cases for list input.
+	 */
+	if (flags & GUC_LIST_INPUT)
+	{
+		/* NULL represents an empty list. */
+		if (list_length(args) == 1)
+		{
+			Node	   *arg = (Node *) linitial(args);
+
+			if (IsA(arg, A_Const) &&
+				((A_Const *) arg)->isnull)
+				return pstrdup("");
+		}
+	}
+	else
+	{
+		/* Complain if list input and non-list variable. */
+		if (list_length(args) != 1)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("SET %s takes only one argument", name)));
+	}
 
 	initStringInfo(&buf);
 
@@ -246,6 +263,12 @@ flatten_set_variable_args(const char *name, List *args)
 			elog(ERROR, "unrecognized node type: %d", (int) nodeTag(arg));
 		con = (A_Const *) arg;
 
+		/* Complain if NULL is used with a non-list variable. */
+		if (con->isnull)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("NULL is an invalid value for %s", name)));
+
 		switch (nodeTag(&con->val))
 		{
 			case T_Integer:
@@ -269,6 +292,9 @@ flatten_set_variable_args(const char *name, List *args)
 					Datum		interval;
 					char	   *intervalout;
 
+					/* gram.y ensures this is only reachable for TIME ZONE */
+					Assert(!(flags & GUC_LIST_QUOTE));
+
 					typenameTypeIdAndMod(NULL, typeName, &typoid, &typmod);
 					Assert(typoid == INTERVALOID);
 
diff --git a/src/bin/pg_dump/dumputils.c b/src/bin/pg_dump/dumputils.c
index 05b84c0d6e7..2d22723aa91 100644
--- a/src/bin/pg_dump/dumputils.c
+++ b/src/bin/pg_dump/dumputils.c
@@ -781,7 +781,7 @@ SplitGUCList(char *rawstring, char separator,
 		nextp++;				/* skip leading whitespace */
 
 	if (*nextp == '\0')
-		return true;			/* allow empty string */
+		return true;			/* empty string represents empty list */
 
 	/* At the top of the loop, we are at start of a new identifier. */
 	do
@@ -893,6 +893,7 @@ makeAlterConfigCommand(PGconn *conn, const char *configitem,
 	 * elements as string literals.  (The elements may be double-quoted as-is,
 	 * but we can't just feed them to the SQL parser; it would do the wrong
 	 * thing with elements that are zero-length or longer than NAMEDATALEN.)
+	 * Also, we need a special case for empty lists.
 	 *
 	 * Variables that are not so marked should just be emitted as simple
 	 * string literals.  If the variable is not known to
@@ -908,6 +909,9 @@ makeAlterConfigCommand(PGconn *conn, const char *configitem,
 		/* this shouldn't fail really */
 		if (SplitGUCList(pos, ',', &namelist))
 		{
+			/* Special case: represent an empty list as NULL */
+			if (*namelist == NULL)
+				appendPQExpBufferStr(buf, "NULL");
 			for (nameptr = namelist; *nameptr; nameptr++)
 			{
 				if (nameptr != namelist)
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index 47913178a93..a00918bacb4 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -13764,7 +13764,8 @@ dumpFunc(Archive *fout, const FuncInfo *finfo)
 		 * and then quote the elements as string literals.  (The elements may
 		 * be double-quoted as-is, but we can't just feed them to the SQL
 		 * parser; it would do the wrong thing with elements that are
-		 * zero-length or longer than NAMEDATALEN.)
+		 * zero-length or longer than NAMEDATALEN.)  Also, we need a special
+		 * case for empty lists.
 		 *
 		 * Variables that are not so marked should just be emitted as simple
 		 * string literals.  If the variable is not known to
@@ -13780,6 +13781,9 @@ dumpFunc(Archive *fout, const FuncInfo *finfo)
 			/* this shouldn't fail really */
 			if (SplitGUCList(pos, ',', &namelist))
 			{
+				/* Special case: represent an empty list as NULL */
+				if (*namelist == NULL)
+					appendPQExpBufferStr(q, "NULL");
 				for (nameptr = namelist; *nameptr; nameptr++)
 				{
 					if (nameptr != namelist)
diff --git a/src/test/regress/expected/guc.out b/src/test/regress/expected/guc.out
index 7f9e29c765c..d6fb879f500 100644
--- a/src/test/regress/expected/guc.out
+++ b/src/test/regress/expected/guc.out
@@ -31,6 +31,28 @@ SELECT '2006-08-13 12:34:56'::timestamptz;
  2006-08-13 12:34:56-07
 (1 row)
 
+-- Check handling of list GUCs
+SET search_path = 'pg_catalog', Foo, 'Bar', '';
+SHOW search_path;
+        search_path         
+----------------------------
+ pg_catalog, foo, "Bar", ""
+(1 row)
+
+SET search_path = null;  -- means empty list
+SHOW search_path;
+ search_path 
+-------------
+ 
+(1 row)
+
+SET search_path = null, null;  -- syntax error
+ERROR:  syntax error at or near ","
+LINE 1: SET search_path = null, null;
+                              ^
+SET enable_seqscan = null;  -- error
+ERROR:  NULL is an invalid value for enable_seqscan
+RESET search_path;
 -- SET LOCAL has no effect outside of a transaction
 SET LOCAL vacuum_cost_delay TO 50;
 WARNING:  SET LOCAL can only be used in transaction blocks
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 77e25ca029e..2bf968ae3d3 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -3572,6 +3572,7 @@ CREATE FUNCTION func_with_set_params() RETURNS integer
     SET extra_float_digits TO 2
     SET work_mem TO '4MB'
     SET datestyle to iso, mdy
+    SET temp_tablespaces to NULL
     SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
     IMMUTABLE STRICT;
 SELECT pg_get_functiondef('func_with_set_params()'::regprocedure);
@@ -3585,6 +3586,7 @@ SELECT pg_get_functiondef('func_with_set_params()'::regprocedure);
   SET extra_float_digits TO '2'                                                                                                                                          +
   SET work_mem TO '4MB'                                                                                                                                                  +
   SET "DateStyle" TO 'iso, mdy'                                                                                                                                          +
+  SET temp_tablespaces TO NULL                                                                                                                                           +
   SET local_preload_libraries TO 'Mixed/Case', 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'+
  AS $function$select 1;$function$                                                                                                                                        +
  
diff --git a/src/test/regress/sql/guc.sql b/src/test/regress/sql/guc.sql
index f65f84a2632..bafaf067e82 100644
--- a/src/test/regress/sql/guc.sql
+++ b/src/test/regress/sql/guc.sql
@@ -12,6 +12,15 @@ SHOW vacuum_cost_delay;
 SHOW datestyle;
 SELECT '2006-08-13 12:34:56'::timestamptz;
 
+-- Check handling of list GUCs
+SET search_path = 'pg_catalog', Foo, 'Bar', '';
+SHOW search_path;
+SET search_path = null;  -- means empty list
+SHOW search_path;
+SET search_path = null, null;  -- syntax error
+SET enable_seqscan = null;  -- error
+RESET search_path;
+
 -- SET LOCAL has no effect outside of a transaction
 SET LOCAL vacuum_cost_delay TO 50;
 SHOW vacuum_cost_delay;
diff --git a/src/test/regress/sql/rules.sql b/src/test/regress/sql/rules.sql
index fdd3ff1d161..3f240bec7b0 100644
--- a/src/test/regress/sql/rules.sql
+++ b/src/test/regress/sql/rules.sql
@@ -1217,6 +1217,7 @@ CREATE FUNCTION func_with_set_params() RETURNS integer
     SET extra_float_digits TO 2
     SET work_mem TO '4MB'
     SET datestyle to iso, mdy
+    SET temp_tablespaces to NULL
     SET local_preload_libraries TO "Mixed/Case", 'c:/''a"/path', '', '0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789'
     IMMUTABLE STRICT;
 SELECT pg_get_functiondef('func_with_set_params()'::regprocedure);
-- 
2.43.7