Introduce GUC_NO_RESET flag.

Previously, the transaction-property GUCs such as transaction_isolation
could be reset after starting a transaction, because we marked them
as GUC_NO_RESET_ALL but still allowed a targeted RESET.  That leads to
assertion failures or worse, because those properties aren't supposed
to change after we've acquired a transaction snapshot.

There are some NO_RESET_ALL variables for which RESET is okay, so
we can't just redefine the semantics of that flag.  Instead introduce
a separate GUC_NO_RESET flag.  Mark "seed", as well as the transaction
property GUCs, as GUC_NO_RESET.

We have to disallow GUC_ACTION_SAVE as well as straight RESET, because
otherwise a function having a "SET transaction_isolation" clause can
still break things: the end-of-function restore action is equivalent
to a RESET.

No back-patch, as it's conceivable that someone is doing something
this patch will forbid (like resetting one of these GUCs at transaction
start, or "CREATE FUNCTION ... SET transaction_read_only = 1") and not
running into problems with it today.  Given how long we've had this
issue and not noticed, the side effects in non-assert builds can't be
too serious.

Per bug #17385 from Andrew Bille.

Masahiko Sawada

Discussion: https://postgr.es/m/17385-9ee529fb091f0ce5@postgresql.org

1 files changed, 20 insertions, 0 deletions

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 8c2e08fad6a..ff082fc3d95 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -3243,6 +3243,26 @@ set_config_option_ext(const char *name, const char *value,
 		}
 	}
 
+	/* Disallow resetting and saving GUC_NO_RESET values */
+	if (record->flags & GUC_NO_RESET)
+	{
+		if (value == NULL)
+		{
+			ereport(elevel,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("parameter \"%s\" cannot be reset", name)));
+			return 0;
+		}
+		if (action == GUC_ACTION_SAVE)
+		{
+			ereport(elevel,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("parameter \"%s\" cannot be set locally in functions",
+							name)));
+			return 0;
+		}
+	}
+
 	/*
 	 * Should we set reset/stacked values?	(If so, the behavior is not
 	 * transactional.)	This is done either when we get a default value from