Patch for bug #17056 fast default on non-plain table

From 06b073a01da5f3e69417ffb88fea1320c0e0a08b Mon Sep 17 00:00:00 2001
From: Andrew Dunstan <andrew@dunslane.net>
Date: Thu, 17 Jun 2021 11:49:33 -0400
Subject: [PATCH] Don't set a fast default for anything but a plain table
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The fast default code added in Release 11 omitted to check that the
table a fast default was being added to was a plain table. Thus one
could be added to a foreign table, which predicably blows up. Here we
perform that check, and in addition if we encounter a missing value for
an attribute of something other than a plain table we ignore it.

Fixes bug #17056

Backpatch to release 11,

Reviewed by: Andres Freund, Álvaro Herrera and Tom Lane
---
 src/backend/catalog/heap.c                 | 10 +++++++++-
 src/backend/commands/tablecmds.c           |  5 +++--
 src/backend/utils/cache/relcache.c         | 19 ++++++++++++++++++-
 src/test/regress/expected/fast_default.out | 19 +++++++++++++++++++
 src/test/regress/sql/fast_default.sql      | 14 ++++++++++++++
 5 files changed, 63 insertions(+), 4 deletions(-)

diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c
index afa830d924..1293dc04ca 100644
--- a/src/backend/catalog/heap.c
+++ b/src/backend/catalog/heap.c
@@ -2161,6 +2161,13 @@ SetAttrMissing(Oid relid, char *attname, char *value)
 	/* lock the table the attribute belongs to */
 	tablerel = table_open(relid, AccessExclusiveLock);
 
+	/* Don't do anything unless it's a plain table */
+	if (tablerel->rd_rel->relkind != RELKIND_RELATION)
+	{
+		table_close(tablerel, AccessExclusiveLock);
+		return;
+	}
+
 	/* Lock the attribute row and get the data */
 	attrrel = table_open(AttributeRelationId, RowExclusiveLock);
 	atttup = SearchSysCacheAttName(relid, attname);
@@ -2287,7 +2294,8 @@ StoreAttrDefault(Relation rel, AttrNumber attnum,
 		valuesAtt[Anum_pg_attribute_atthasdef - 1] = true;
 		replacesAtt[Anum_pg_attribute_atthasdef - 1] = true;
 
-		if (add_column_mode && !attgenerated)
+		if (rel->rd_rel->relkind == RELKIND_RELATION  && add_column_mode &&
+			!attgenerated)
 		{
 			expr2 = expression_planner(expr2);
 			estate = CreateExecutorState();
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 028e8ac46b..2eef07e452 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -12215,9 +12215,10 @@ ATExecAlterColumnType(AlteredTableInfo *tab, Relation rel,
 	/*
 	 * Here we go --- change the recorded column type and collation.  (Note
 	 * heapTup is a copy of the syscache entry, so okay to scribble on.) First
-	 * fix up the missing value if any.
+	 * fix up the missing value if any. There shouldn't be any missing values
+	 * for anything except plain tables, but if there are, ignore them.
 	 */
-	if (attTup->atthasmissing)
+	if (rel->rd_rel->relkind == RELKIND_RELATION  && attTup->atthasmissing)
 	{
 		Datum		missingval;
 		bool		missingNull;
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index fd05615e76..446cfcc4e8 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -551,6 +551,7 @@ RelationBuildTupleDesc(Relation relation)
 	{
 		Form_pg_attribute attp;
 		int			attnum;
+		bool        atthasmissing;
 
 		attp = (Form_pg_attribute) GETSTRUCT(pg_attribute_tuple);
 
@@ -563,6 +564,22 @@ RelationBuildTupleDesc(Relation relation)
 			   attp,
 			   ATTRIBUTE_FIXED_PART_SIZE);
 
+		/*
+		 * Fix atthasmissing flag - it's only for plain tables. Others
+		 * should not have missing values set, but there may be some left from
+		 * before when we placed that check, so this code defensively ignores
+		 * such values.
+		 */
+		atthasmissing = attp->atthasmissing;
+		if (relation->rd_rel->relkind != RELKIND_RELATION && atthasmissing)
+		{
+			Form_pg_attribute nattp;
+
+			atthasmissing = false;
+			nattp = TupleDescAttr(relation->rd_att, attnum - 1);
+			nattp->atthasmissing = false;
+		}
+
 		/* Update constraint/default info */
 		if (attp->attnotnull)
 			constr->has_not_null = true;
@@ -572,7 +589,7 @@ RelationBuildTupleDesc(Relation relation)
 			ndef++;
 
 		/* If the column has a "missing" value, put it in the attrmiss array */
-		if (attp->atthasmissing)
+		if (atthasmissing)
 		{
 			Datum		missingval;
 			bool		missingNull;
diff --git a/src/test/regress/expected/fast_default.out b/src/test/regress/expected/fast_default.out
index 10bc5ff757..91f25717b5 100644
--- a/src/test/regress/expected/fast_default.out
+++ b/src/test/regress/expected/fast_default.out
@@ -797,7 +797,26 @@ SELECT * FROM t WHERE a IS NULL;
 (1 row)
 
 ROLLBACK;
+-- verify that a default set on a non-plain table doesn't set a missing
+-- value on the attribute
+CREATE FOREIGN DATA WRAPPER dummy;
+CREATE SERVER s0 FOREIGN DATA WRAPPER dummy;
+CREATE FOREIGN TABLE ft1 (c1 integer NOT NULL) SERVER s0;
+ALTER FOREIGN TABLE ft1 ADD COLUMN c8 integer DEFAULT 0;
+ALTER FOREIGN TABLE ft1 ALTER COLUMN c8 TYPE char(10);
+SELECT count(*)
+  FROM pg_attribute
+  WHERE attrelid = 'ft1'::regclass AND
+    (attmissingval IS NOT NULL OR atthasmissing);
+ count 
+-------
+     0
+(1 row)
+
 -- cleanup
+DROP FOREIGN TABLE ft1;
+DROP SERVER s0;
+DROP FOREIGN DATA WRAPPER dummy;
 DROP TABLE vtype;
 DROP TABLE vtype2;
 DROP TABLE follower;
diff --git a/src/test/regress/sql/fast_default.sql b/src/test/regress/sql/fast_default.sql
index 4589b9e58d..16a3b7ca51 100644
--- a/src/test/regress/sql/fast_default.sql
+++ b/src/test/regress/sql/fast_default.sql
@@ -524,8 +524,22 @@ SET LOCAL enable_seqscan = false;
 SELECT * FROM t WHERE a IS NULL;
 ROLLBACK;
 
+-- verify that a default set on a non-plain table doesn't set a missing
+-- value on the attribute
+CREATE FOREIGN DATA WRAPPER dummy;
+CREATE SERVER s0 FOREIGN DATA WRAPPER dummy;
+CREATE FOREIGN TABLE ft1 (c1 integer NOT NULL) SERVER s0;
+ALTER FOREIGN TABLE ft1 ADD COLUMN c8 integer DEFAULT 0;
+ALTER FOREIGN TABLE ft1 ALTER COLUMN c8 TYPE char(10);
+SELECT count(*)
+  FROM pg_attribute
+  WHERE attrelid = 'ft1'::regclass AND
+    (attmissingval IS NOT NULL OR atthasmissing);
 
 -- cleanup
+DROP FOREIGN TABLE ft1;
+DROP SERVER s0;
+DROP FOREIGN DATA WRAPPER dummy;
 DROP TABLE vtype;
 DROP TABLE vtype2;
 DROP TABLE follower;
-- 
2.25.4