Prohibit slot manipulation while in single-user mode

Replication-related commands are rarely used in single-user mode and have been
broken for years. This commit prohibits calling slot manipulation SQL functions
to avoid additional risk of failures. One exception is pg_drop_replication_slot.
It is still allowed because users may want to clean up their mistakes in the mode.

Author: HUUTFJ

doc/src/sgml/func/func-admin.sgml

@@ -1012,6 +1012,11 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
     are also relevant for replication.
    </para>
 
+   <para>
+    Note that slot manipulation functions except <function>pg_drop_replication_slot</function>
+    cannot be used in single-user mode.
+   </para>
+
    <table id="functions-replication-table">
     <title>Replication Management Functions</title>
     <tgroup cols="1">

src/backend/replication/logical/logicalfuncs.c

@@ -113,6 +113,9 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 	List	   *options = NIL;
 	DecodingOutputState *p;
 
+	/* Slot manipulation is not allowed in single-user mode */
+	CheckSlotIsInSingleUserMode();
+
 	CheckSlotPermissions();
 
 	CheckLogicalDecodingRequirements();

src/backend/replication/slot.c

@@ -1529,6 +1529,18 @@ CheckSlotPermissions(void)
 						   "REPLICATION")));
 }
 
+/*
+ * Check whether the instance is in single-user mode.
+ */
+void
+CheckSlotIsInSingleUserMode(void)
+{
+	if (!IsUnderPostmaster)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("replication slots cannot be used in single-user mode")));
+}
+
 /*
  * Reserve WAL for the currently active slot.
  *

src/backend/replication/slotfuncs.c

@@ -17,6 +17,7 @@
 #include "access/xlogrecovery.h"
 #include "access/xlogutils.h"
 #include "funcapi.h"
+#include "miscadmin.h"
 #include "replication/logical.h"
 #include "replication/slot.h"
 #include "replication/slotsync.h"
@@ -76,6 +77,9 @@ pg_create_physical_replication_slot(PG_FUNCTION_ARGS)
 	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
 		elog(ERROR, "return type must be a row type");
 
+	/* Slot manipulation is not allowed in single-user mode */
+	CheckSlotIsInSingleUserMode();
+
 	CheckSlotPermissions();
 
 	CheckSlotRequirements();
@@ -182,6 +186,9 @@ pg_create_logical_replication_slot(PG_FUNCTION_ARGS)
 	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
 		elog(ERROR, "return type must be a row type");
 
+	/* Slot manipulation is not allowed in single-user mode */
+	CheckSlotIsInSingleUserMode();
+
 	CheckSlotPermissions();
 
 	CheckLogicalDecodingRequirements();
@@ -521,6 +528,9 @@ pg_replication_slot_advance(PG_FUNCTION_ARGS)
 
 	Assert(!MyReplicationSlot);
 
+	/* Slot manipulation is not allowed in single-user mode */
+	CheckSlotIsInSingleUserMode();
+
 	CheckSlotPermissions();
 
 	if (XLogRecPtrIsInvalid(moveto))
@@ -618,9 +628,13 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
 	TupleDesc	tupdesc;
 	HeapTuple	tuple;
 
+
 	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
 		elog(ERROR, "return type must be a row type");
 
+	/* Slot manipulation is not allowed in single-user mode */
+	CheckSlotIsInSingleUserMode();
+
 	CheckSlotPermissions();
 
 	if (logical_slot)

src/backend/utils/adt/pg_upgrade_support.c

@@ -296,6 +296,9 @@ binary_upgrade_logical_slot_has_caught_up(PG_FUNCTION_ARGS)
 	 */
 	Assert(has_rolreplication(GetUserId()));
 
+	/* Slot manipulation is not allowed in single-user mode */
+	CheckSlotIsInSingleUserMode();
+
 	slot_name = PG_GETARG_NAME(0);
 
 	/* Acquire the given slot */

src/include/replication/slot.h

@@ -342,6 +342,7 @@ extern void CheckPointReplicationSlots(bool is_shutdown);
 
 extern void CheckSlotRequirements(void);
 extern void CheckSlotPermissions(void);
+extern void CheckSlotIsInSingleUserMode(void);
 extern ReplicationSlotInvalidationCause
 			GetSlotInvalidationCause(const char *cause_name);
 extern const char *GetSlotInvalidationCauseName(ReplicationSlotInvalidationCause cause);

src/test/modules/test_misc/t/008_slots_in_single_user_mode.pl

@@ -27,9 +27,31 @@ sub run_test_in_single_user_mode
 		],
 		\$commands);
 
+	return $result;
+}
+
+# Wrapper function for run_test_in_single_user_mode. This would be used when
+# the input command succeeds.
+sub run_test_in_single_user_mode_success
+{
+	my ($node, $commands, $testname) = @_;
+
+	my $result = run_test_in_single_user_mode($node, $commands, $testname);
+
 	ok($result, $testname);
 }
 
+# Wrapper function for run_test_in_single_user_mode. This would be used when
+# the input command fails.
+sub run_test_in_single_user_mode_fail
+{
+	my ($node, $commands, $testname) = @_;
+
+	my $result = run_test_in_single_user_mode($node, $commands, $testname);
+
+	ok(!$result, $testname);
+}
+
 my $slotname = 'test_slot';
 
 # Initialize a node
@@ -38,34 +60,38 @@ sub run_test_in_single_user_mode
 $node->start;
 
 # Define initial table
-$node->safe_psql('postgres', "CREATE TABLE foo (id int)");
+$node->safe_psql(
+	'postgres', qq(
+CREATE TABLE foo (id int);
+SELECT pg_create_logical_replication_slot('$slotname', 'test_decoding');
+));
 
 # Stop the node to run and test in single-user mode
 $node->stop;
 
-run_test_in_single_user_mode(
+run_test_in_single_user_mode_fail(
 	$node,
-	"SELECT pg_create_logical_replication_slot('$slotname', 'test_decoding')",
-	"replication slot can be created in single-user mode");
+	"SELECT pg_create_logical_replication_slot('another_slot', 'test_decoding')",
+	"replication slot cannot be created in single-user mode");
 
-run_test_in_single_user_mode(
+run_test_in_single_user_mode_fail(
 	$node, qq(
 INSERT INTO foo VALUES (1);
 SELECT count(1) FROM pg_logical_slot_get_changes('$slotname', NULL, NULL);
 ),
-	"logical decoding be done in single-user mode");
+	"logical decoding cannot be done in single-user mode");
 
-run_test_in_single_user_mode(
+run_test_in_single_user_mode_fail(
 	$node,
 	"SELECT pg_replication_slot_advance('$slotname', pg_current_wal_lsn())",
-	"replication slot can be advanced in single-user mode");
+	"replication slot cannot be advanced in single-user mode");
 
-run_test_in_single_user_mode(
+run_test_in_single_user_mode_fail(
 	$node,
 	"SELECT pg_copy_logical_replication_slot('$slotname', 'dest_slot')",
-	"replication slot can be copied in single-user mode");
+	"replication slot cannot be copied in single-user mode");
 
-run_test_in_single_user_mode(
+run_test_in_single_user_mode_success(
 	$node,
 	"SELECT pg_drop_replication_slot('$slotname')",
 	"replication slot can be dropped in single-user mode");