Allow callback functions to deregister themselves during a call.

Fetch the next-item pointer before the call not after, so that
we aren't dereferencing a dangling pointer if the callback
deregistered itself during the call.  The risky coding pattern
appears in CallXactCallbacks, CallSubXactCallbacks, and
ResourceOwnerReleaseInternal.  (There are some other places that
might be at hazard if they offered deregistration functionality,
but they don't.)

I (tgl) considered back-patching this, but desisted because it
wouldn't be very safe for extensions to rely on this working in
pre-v16 branches.

Hao Wu

Discussion: https://postgr.es/m/CAH+9SWXTiERkmhRke+QCcc+jRH8d5fFHTxh8ZK0-Yn4BSpyaAg@mail.gmail.com

1 files changed, 12 insertions, 2 deletions

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 2bb975943cf..c1ffbd89b88 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -3656,9 +3656,14 @@ static void
 CallXactCallbacks(XactEvent event)
 {
 	XactCallbackItem *item;
+	XactCallbackItem *next;
 
-	for (item = Xact_callbacks; item; item = item->next)
+	for (item = Xact_callbacks; item; item = next)
+	{
+		/* allow callbacks to unregister themselves when called */
+		next = item->next;
 		item->callback(event, item->arg);
+	}
 }
 
 
@@ -3713,9 +3718,14 @@ CallSubXactCallbacks(SubXactEvent event,
 					 SubTransactionId parentSubid)
 {
 	SubXactCallbackItem *item;
+	SubXactCallbackItem *next;
 
-	for (item = SubXact_callbacks; item; item = item->next)
+	for (item = SubXact_callbacks; item; item = next)
+	{
+		/* allow callbacks to unregister themselves when called */
+		next = item->next;
 		item->callback(event, mySubid, parentSubid, item->arg);
+	}
 }