Avoid crashing in GetOldestSnapshot() if there are no known snapshots.

The sole caller expects NULL to be returned in such a case, so make
it so and document it.

Per reports from Andreas Seltenreich and Regina Obe.  This doesn't
really fix their problem, as now their RETURNING queries will say
"ERROR: no known snapshots", but in any case this function should
not dump core in a reasonably-foreseeable situation.

Report: <[email protected]>
Report: <20160807051854[email protected]>

diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 4bddaed33ba91653df87a471332c444fd0597fef..1ec9f70f0eeff3dfe47b4dea17faa32d271f5472 100644 (file)
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -399,14 +399,14 @@ GetLatestSnapshot(void)
 /*
  * GetOldestSnapshot
  *
- *     Get the oldest known snapshot, as judged by the LSN.
+ *     Get the transaction's oldest known snapshot, as judged by the LSN.
+ *     Will return NULL if there are no active or registered snapshots.
  */
 Snapshot
 GetOldestSnapshot(void)
 {
    Snapshot    OldestRegisteredSnapshot = NULL;
    XLogRecPtr  RegisteredLSN = InvalidXLogRecPtr;
-   XLogRecPtr  ActiveLSN = InvalidXLogRecPtr;
 
    if (!pairingheap_is_empty(&RegisteredSnapshots))
    {
@@ -416,10 +416,12 @@ GetOldestSnapshot(void)
    }
 
    if (OldestActiveSnapshot != NULL)
-       ActiveLSN = OldestActiveSnapshot->as_snap->lsn;
+   {
+       XLogRecPtr  ActiveLSN = OldestActiveSnapshot->as_snap->lsn;
 
-   if (XLogRecPtrIsInvalid(RegisteredLSN) || RegisteredLSN > ActiveLSN)
-       return OldestActiveSnapshot->as_snap;
+       if (XLogRecPtrIsInvalid(RegisteredLSN) || RegisteredLSN > ActiveLSN)
+           return OldestActiveSnapshot->as_snap;
+   }
 
    return OldestRegisteredSnapshot;
 }