Libpq support to connect to standby server as priority

diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 8d54333..6d37428 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -327,7 +327,7 @@ static const internalPQconninfoOption PQconninfoOptions[] = {
 
 	{"target_session_attrs", "PGTARGETSESSIONATTRS",
 		DefaultTargetSessionAttrs, NULL,
-		"Target-Session-Attrs", "", 11, /* sizeof("read-write") = 11 */
+		"Target-Session-Attrs", "", 12, /* sizeof("prefer-read") = 12 */
 	offsetof(struct pg_conn, target_session_attrs)},
 
 	/* Terminating entry --- MUST BE LAST */
@@ -1184,7 +1184,8 @@ connectOptions2(PGconn *conn)
 	if (conn->target_session_attrs)
 	{
 		if (strcmp(conn->target_session_attrs, "any") != 0
-			&& strcmp(conn->target_session_attrs, "read-write") != 0)
+			&& strcmp(conn->target_session_attrs, "read-write") != 0
+			&& strcmp(conn->target_session_attrs, "prefer-read") != 0)
 		{
 			conn->status = CONNECTION_BAD;
 			printfPQExpBuffer(&conn->errorMessage,
@@ -2086,8 +2087,22 @@ keep_going:						/* We will come back to here until there is
 					{
 						if (++conn->whichhost >= conn->nconnhost)
 						{
-							conn->whichhost = 0;
-							break;
+							if (conn->primary_host_index > 0 &&
+									strcmp(conn->target_session_attrs,"prefer-read") ==0 )
+							{
+								/*
+								 * Go to here means failed to connect to read-only servers
+								 * and now try connect to read-write server again.
+								 * Only under the 'prefer-read' scenario will go to here.
+								 */
+								conn->addr_cur = conn->connhost[conn->primary_host_index].addrlist;
+								conn->whichhost = conn->primary_host_index;
+							}
+							else
+							{
+								conn->whichhost = 0;
+								break;
+							}
 						}
 						conn->addr_cur =
 							conn->connhost[conn->whichhost].addrlist;
@@ -2341,6 +2356,14 @@ keep_going:						/* We will come back to here until there is
 						conn->status = CONNECTION_NEEDED;
 						goto keep_going;
 					}
+					else if (conn->primary_host_index >= 0  &&
+							strcmp(conn->target_session_attrs, "prefer-read") == 0)
+					{
+						conn->addr_cur = conn->connhost[conn->primary_host_index].addrlist;
+						conn->whichhost = conn->primary_host_index;
+						conn->status = CONNECTION_NEEDED;
+						goto keep_going;
+					}
 					goto error_return;
 				}
 
@@ -2978,10 +3001,12 @@ keep_going:						/* We will come back to here until there is
 				}
 
 				/*
-				 * If a read-write connection is required, see if we have one.
+				 * If a read-write or prefer-read connection is required,
+				 * see if we have one.
 				 */
 				if (conn->target_session_attrs != NULL &&
-					strcmp(conn->target_session_attrs, "read-write") == 0)
+					(strcmp(conn->target_session_attrs, "read-write") == 0 ||
+					 strcmp(conn->target_session_attrs, "prefer-read") == 0))
 				{
 					/*
 					 * We are yet to make a connection. Save all existing
@@ -3042,10 +3067,12 @@ keep_going:						/* We will come back to here until there is
 			}
 
 			/*
-			 * If a read-write connection is requested check for same.
+			 * If a read-write or prefer-read connection is requested
+			 * check for same.
 			 */
 			if (conn->target_session_attrs != NULL &&
-				strcmp(conn->target_session_attrs, "read-write") == 0)
+				(strcmp(conn->target_session_attrs, "read-write") == 0 ||
+				 strcmp(conn->target_session_attrs, "prefer-read") == 0))
 			{
 				if (!saveErrorMessage(conn, &savedMessage))
 					goto error_return;
@@ -3124,57 +3151,130 @@ keep_going:						/* We will come back to here until there is
 					PQntuples(res) == 1)
 				{
 					char	   *val;
+					bool		readonly_server = false;
 
 					val = PQgetvalue(res, 0, 0);
 					if (strncmp(val, "on", 2) == 0)
+						readonly_server = true;
+
+					if (strcmp(conn->target_session_attrs, "read-write") == 0)
 					{
-						const char *displayed_host;
-						const char *displayed_port;
+						if(readonly_server)
+						{
+							if (conn->connhost[conn->whichhost].type == CHT_HOST_ADDRESS)
+								displayed_host = conn->connhost[conn->whichhost].hostaddr;
+							else
+								displayed_host = conn->connhost[conn->whichhost].host;
+							displayed_port = conn->connhost[conn->whichhost].port;
+							if (displayed_port == NULL || displayed_port[0] == '\0')
+								displayed_port = DEF_PGPORT_STR;
 
-						if (conn->connhost[conn->whichhost].type == CHT_HOST_ADDRESS)
-							displayed_host = conn->connhost[conn->whichhost].hostaddr;
-						else
-							displayed_host = conn->connhost[conn->whichhost].host;
-						displayed_port = conn->connhost[conn->whichhost].port;
-						if (displayed_port == NULL || displayed_port[0] == '\0')
-							displayed_port = DEF_PGPORT_STR;
+							PQclear(res);
+							restoreErrorMessage(conn, &savedMessage);
 
-						PQclear(res);
-						restoreErrorMessage(conn, &savedMessage);
+							/* Not writable; close connection. */
+							appendPQExpBuffer(&conn->errorMessage,
+											  libpq_gettext("could not make a writable "
+															"connection to server "
+															"\"%s:%s\"\n"),
+											  displayed_host, displayed_port);
+							conn->status = CONNECTION_OK;
+							sendTerminateConn(conn);
+							pqDropConnection(conn, true);
 
-						/* Not writable; close connection. */
-						appendPQExpBuffer(&conn->errorMessage,
-										  libpq_gettext("could not make a writable "
-														"connection to server "
-														"\"%s:%s\"\n"),
-										  displayed_host, displayed_port);
-						conn->status = CONNECTION_OK;
-						sendTerminateConn(conn);
-						pqDropConnection(conn, true);
+							/* Skip any remaining addresses for this host. */
+							conn->addr_cur = NULL;
+							if (conn->whichhost + 1 < conn->nconnhost)
+							{
+								conn->status = CONNECTION_NEEDED;
+								goto keep_going;
+							}
 
-						/* Skip any remaining addresses for this host. */
-						conn->addr_cur = NULL;
-						if (conn->whichhost + 1 < conn->nconnhost)
+							/* No more addresses to try. So we fail. */
+							goto error_return;
+						}
+						else /* server support read-write */
 						{
-							conn->status = CONNECTION_NEEDED;
+							PQclear(res);
+							termPQExpBuffer(&savedMessage);
+
+							/* We can release the address lists now. */
+							release_all_addrinfo(conn);
+
+							/*
+							 * Finish reading any remaining messages before being
+							 * considered as ready.
+							 */
+							conn->status = CONNECTION_CONSUME;
 							goto keep_going;
 						}
-
-						/* No more addresses to try. So we fail. */
-						goto error_return;
 					}
-					PQclear(res);
-					termPQExpBuffer(&savedMessage);
+					else /* conn->target_session_attrs is prefer-read */
+					{
+						if(readonly_server)
+						{
+							PQclear(res);
+							termPQExpBuffer(&savedMessage);
 
-					/* We can release the address lists now. */
-					release_all_addrinfo(conn);
+							/* We can release the address lists now. */
+							release_all_addrinfo(conn);
 
-					/*
-					 * Finish reading any remaining messages before being
-					 * considered as ready.
-					 */
-					conn->status = CONNECTION_CONSUME;
-					goto keep_going;
+							/*
+							 * Finish reading any remaining messages before being
+							 * considered as ready.
+							 */
+							conn->status = CONNECTION_CONSUME;
+							goto keep_going;
+						}
+						else /* server support read-write */
+						{
+							if ((conn->primary_host_index < 0) && (conn->whichhost + 1 < conn->nconnhost))
+							{
+								if (conn->connhost[conn->whichhost].type == CHT_HOST_ADDRESS)
+									displayed_host = conn->connhost[conn->whichhost].hostaddr;
+								else
+									displayed_host = conn->connhost[conn->whichhost].host;
+								displayed_port = conn->connhost[conn->whichhost].port;
+								if (displayed_port == NULL || displayed_port[0] == '\0')
+									displayed_port = DEF_PGPORT_STR;
+
+								PQclear(res);
+								restoreErrorMessage(conn, &savedMessage);
+
+								/*
+								 * Connecting to a writable server, close it
+								 * and try to connect to another one.
+								 */
+								conn->status = CONNECTION_OK;
+								sendTerminateConn(conn);
+								pqDropConnection(conn, true);
+
+								/* Skip any remaining addresses for this host. */
+								conn->addr_cur = NULL;
+
+								conn->status = CONNECTION_NEEDED;
+
+								/* Record primary host index */
+								conn->primary_host_index = conn->whichhost;
+								goto keep_going;
+							}
+							else /* No more host to connect, keep this connection */
+							{
+								PQclear(res);
+								termPQExpBuffer(&savedMessage);
+
+								/* We can release the address lists now. */
+								release_all_addrinfo(conn);
+
+								/*
+								 * Finish reading any remaining messages before being
+								 * considered as ready.
+								 */
+								conn->status = CONNECTION_CONSUME;
+								goto keep_going;
+							}
+						}
+					}
 				}
 
 				/*
@@ -3393,6 +3493,8 @@ makeEmptyPGconn(void)
 		conn = NULL;
 	}
 
+	conn->primary_host_index = -1;
+
 	return conn;
 }
 
diff --git a/src/interfaces/libpq/libpq-int.h b/src/interfaces/libpq/libpq-int.h
index 516039e..3241246 100644
--- a/src/interfaces/libpq/libpq-int.h
+++ b/src/interfaces/libpq/libpq-int.h
@@ -362,7 +362,7 @@ struct pg_conn
 	char	   *krbsrvname;		/* Kerberos service name */
 #endif
 
-	/* Type of connection to make.  Possible values: any, read-write. */
+	/* Type of connection to make.  Possible values: any, read-write, perfer-read. */
 	char	   *target_session_attrs;
 
 	/* Optional file to write trace info to */
@@ -396,6 +396,7 @@ struct pg_conn
 	int			nconnhost;		/* # of possible hosts */
 	int			whichhost;		/* host we're currently considering */
 	pg_conn_host *connhost;		/* details about each possible host */
+	int 		primary_host_index; /* index for primary host in connhost */
 
 	/* Connection data */
 	pgsocket	sock;			/* FD for socket, PGINVALID_SOCKET if