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

The sole caller expects NULL to be returned in such a case, so makeit so and document it.Per reports from Andreas Seltenreich and Regina Obe. This doesn'treally fix their problem, as now their RETURNING queries will say"ERROR: no known snapshots", but in any case this function shouldnot dump core in a reasonably-foreseeable situation.Report: <87vazemeda.fsf@credativ.de>Report: <20160807051854.1427.32414@wrigleys.postgresql.org>

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src/backend/utils/time
- snapmgr.c

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-5

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`‎src/backend/utils/time/snapmgr.c`

Lines changed: 7 additions & 5 deletions

Original file line number	Diff line number	Diff line change
`@@ -399,14 +399,14 @@ GetLatestSnapshot(void)`
`399`	`399`	`/*`
`400`	`400`	`* GetOldestSnapshot`
`401`	`401`	`*`
`402`		`- *Get the oldest known snapshot, as judged by the LSN.`
	`402`	`+ *Get the transaction's oldest known snapshot, as judged by the LSN.`
	`403`	`+ *Will return NULL if there are no active or registered snapshots.`
`403`	`404`	`*/`
`404`	`405`	`Snapshot`
`405`	`406`	`GetOldestSnapshot(void)`
`406`	`407`	`{`
`407`	`408`	`SnapshotOldestRegisteredSnapshot=NULL;`
`408`	`409`	`XLogRecPtrRegisteredLSN=InvalidXLogRecPtr;`
`409`		`-XLogRecPtrActiveLSN=InvalidXLogRecPtr;`
`410`	`410`
`411`	`411`	`if (!pairingheap_is_empty(&RegisteredSnapshots))`
`412`	`412`	`{`
`@@ -416,10 +416,12 @@ GetOldestSnapshot(void)`
`416`	`416`	`}`
`417`	`417`
`418`	`418`	`if (OldestActiveSnapshot!=NULL)`
`419`		`-ActiveLSN=OldestActiveSnapshot->as_snap->lsn;`
	`419`	`+{`
	`420`	`+XLogRecPtrActiveLSN=OldestActiveSnapshot->as_snap->lsn;`
`420`	`421`
`421`		`-if (XLogRecPtrIsInvalid(RegisteredLSN)\|\|RegisteredLSN>ActiveLSN)`
`422`		`-returnOldestActiveSnapshot->as_snap;`
	`422`	`+if (XLogRecPtrIsInvalid(RegisteredLSN)\|\|RegisteredLSN>ActiveLSN)`
	`423`	`+returnOldestActiveSnapshot->as_snap;`
	`424`	`+}`
`423`	`425`
`424`	`426`	`returnOldestRegisteredSnapshot;`
`425`	`427`	`}`

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Commit07a601e

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`‎src/backend/utils/time/snapmgr.c`

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