Re-add default_with_oids GUC to avoid breaking old dump files.

After 578b2297 / the removal of WITH OIDS support, older dump files
containing
    SET default_with_oids = false;
either report unnecessary errors (as the subsequent tables have no
oids) or even fail to restore entirely (when using transaction mode).
To avoid that, re-add the GUC, but don't allow setting it to true.

Per complaint from Tom Lane.

Author: Amit Khandekar, editorialized by me
Discussion: https://postgr.es/m/CAJ3gD9dZyxrtL0rJfoNoOj6v7fJSDaXBngi9wy5XU8m-ioXhAA@mail.gmail.com

src/backend/utils/misc/guc.c

@@ -209,6 +209,7 @@ static void assign_recovery_target_name(const char *newval, void *extra);
 static bool check_recovery_target_lsn(char **newval, void **extra, GucSource source);
 static void assign_recovery_target_lsn(const char *newval, void *extra);
 static bool check_primary_slot_name(char **newval, void **extra, GucSource source);
+static bool check_default_with_oids(bool *newval, void **extra, GucSource source);
 
 /* Private functions in guc-file.l that need to be called from guc.c */
 static ConfigVariable *ProcessConfigFileInternal(GucContext context,
@@ -479,6 +480,12 @@ char	   *event_source;
 
 bool		row_security;
 bool		check_function_bodies = true;
+
+/*
+ * This GUC exists solely for backward compatibility, check its definition for
+ * details.
+ */
+bool		default_with_oids = false;
 bool		session_auth_is_superuser;
 
 int			log_min_error_statement = ERROR;
@@ -1538,6 +1545,21 @@ static struct config_bool ConfigureNamesBool[] =
 		true,
 		NULL, NULL, NULL
 	},
+	/*
+	 * WITH OIDS support, and consequently default_with_oids, was removed in
+	 * PostgreSQL 12, but we tolerate the parameter being set to false to
+	 * avoid unnecessarily breaking older dump files.
+	 */
+	{
+		{"default_with_oids", PGC_USERSET, COMPAT_OPTIONS_PREVIOUS,
+			gettext_noop("WITH OIDS is no longer supported; this can only be false."),
+			NULL,
+			GUC_NO_SHOW_ALL | GUC_NOT_IN_SAMPLE
+		},
+		&default_with_oids,
+		false,
+		check_default_with_oids, NULL, NULL
+	},
 	{
 		{"logging_collector", PGC_POSTMASTER, LOGGING_WHERE,
 			gettext_noop("Start a subprocess to capture stderr output and/or csvlogs into log files."),
@@ -11311,4 +11333,19 @@ check_primary_slot_name(char **newval, void **extra, GucSource source)
 	return true;
 }
 
+static bool
+check_default_with_oids(bool *newval, void **extra, GucSource source)
+{
+	if (*newval)
+	{
+		/* check the GUC's definition for an explanation */
+		GUC_check_errcode(ERRCODE_FEATURE_NOT_SUPPORTED);
+		GUC_check_errmsg("tables declared WITH OIDS are not supported");
+
+		return false;
+	}
+
+	return true;
+}
+
 #include "guc-file.c"

src/test/regress/expected/guc.out

@@ -767,3 +767,7 @@ NOTICE:  text search configuration "no_such_config" does not exist
 select func_with_bad_set();
 ERROR:  invalid value for parameter "default_text_search_config": "no_such_config"
 reset check_function_bodies;
+set default_with_oids to f;
+-- Should not allow to set it to true.
+set default_with_oids to t;
+ERROR:  tables declared WITH OIDS are not supported

src/test/regress/sql/guc.sql

@@ -288,3 +288,7 @@ set default_text_search_config = no_such_config;
 select func_with_bad_set();
 
 reset check_function_bodies;
+
+set default_with_oids to f;
+-- Should not allow to set it to true.
+set default_with_oids to t;