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Commitc2a14bc

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Protect against SnapshotNow race conditions in pg_tablespace scans.
Use of SnapshotNow is known to expose us to race conditions if the tuple(s)being sought could be updated by concurrently-committing transactions.CREATE DATABASE and DROP DATABASE are particularly exposed because they doheavyweight filesystem operations during their scans of pg_tablespace,so that the scans run for a very long time compared to most. Furthermore,the potential consequences of a missed or twice-visited row are nastierthan average:* createdb() could fail with a bogus "file already exists" error, or silently fail to copy one or more tablespace's worth of files into the new database.* remove_dbtablespaces() could miss one or more tablespaces, thus failing to free filesystem space for the dropped database.* check_db_file_conflict() could likewise miss a tablespace, leading to an OID conflict that could result in data loss either immediately or in future operations. (This seems of very low probability, though, since a duplicate database OID would be unlikely to start with.)Hence, it seems worth fixing these three places to use MVCC snapshots, eventhough this will someday be superseded by a generic solution to SnapshotNowrace conditions.Back-patch to all active branches.Stephen Frost and Tom Lane
1 parent530bbfa commitc2a14bc

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‎src/backend/commands/dbcommands.c

Lines changed: 54 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -131,6 +131,7 @@ createdb(const CreatedbStmt *stmt)
131131
intnotherbackends;
132132
intnpreparedxacts;
133133
createdb_failure_paramsfparms;
134+
Snapshotsnapshot;
134135

135136
/* Extract options from the statement node tree */
136137
foreach(option,stmt->options)
@@ -534,6 +535,29 @@ createdb(const CreatedbStmt *stmt)
534535
*/
535536
RequestCheckpoint(CHECKPOINT_IMMEDIATE |CHECKPOINT_FORCE |CHECKPOINT_WAIT);
536537

538+
/*
539+
* Take an MVCC snapshot to use while scanning through pg_tablespace. For
540+
* safety, register the snapshot (this prevents it from changing if
541+
* something else were to request a snapshot during the loop).
542+
*
543+
* Traversing pg_tablespace with an MVCC snapshot is necessary to provide
544+
* us with a consistent view of the tablespaces that exist. Using
545+
* SnapshotNow here would risk seeing the same tablespace multiple times,
546+
* or worse not seeing a tablespace at all, if its tuple is moved around
547+
* by a concurrent update (eg an ACL change).
548+
*
549+
* Inconsistency of this sort is inherent to all SnapshotNow scans, unless
550+
* some lock is held to prevent concurrent updates of the rows being
551+
* sought.There should be a generic fix for that, but in the meantime
552+
* it's worth fixing this case in particular because we are doing very
553+
* heavyweight operations within the scan, so that the elapsed time for
554+
* the scan is vastly longer than for most other catalog scans. That
555+
* means there's a much wider window for concurrent updates to cause
556+
* trouble here than anywhere else. XXX this code should be changed
557+
* whenever a generic fix is implemented.
558+
*/
559+
snapshot=RegisterSnapshot(GetLatestSnapshot());
560+
537561
/*
538562
* Once we start copying subdirectories, we need to be able to clean 'em
539563
* up if we fail. Use an ENSURE block to make sure this happens. (This
@@ -551,7 +575,7 @@ createdb(const CreatedbStmt *stmt)
551575
* each one to the new database.
552576
*/
553577
rel=heap_open(TableSpaceRelationId,AccessShareLock);
554-
scan=heap_beginscan(rel,SnapshotNow,0,NULL);
578+
scan=heap_beginscan(rel,snapshot,0,NULL);
555579
while ((tuple=heap_getnext(scan,ForwardScanDirection))!=NULL)
556580
{
557581
Oidsrctablespace=HeapTupleGetOid(tuple);
@@ -656,6 +680,9 @@ createdb(const CreatedbStmt *stmt)
656680
PG_END_ENSURE_ERROR_CLEANUP(createdb_failure_callback,
657681
PointerGetDatum(&fparms));
658682

683+
/* Free our snapshot */
684+
UnregisterSnapshot(snapshot);
685+
659686
returndboid;
660687
}
661688

@@ -1715,9 +1742,20 @@ remove_dbtablespaces(Oid db_id)
17151742
Relationrel;
17161743
HeapScanDescscan;
17171744
HeapTupletuple;
1745+
Snapshotsnapshot;
1746+
1747+
/*
1748+
* As in createdb(), we'd better use an MVCC snapshot here, since this
1749+
* scan can run for a long time. Duplicate visits to tablespaces would be
1750+
* harmless, but missing a tablespace could result in permanently leaked
1751+
* files.
1752+
*
1753+
* XXX change this when a generic fix for SnapshotNow races is implemented
1754+
*/
1755+
snapshot=RegisterSnapshot(GetLatestSnapshot());
17181756

17191757
rel=heap_open(TableSpaceRelationId,AccessShareLock);
1720-
scan=heap_beginscan(rel,SnapshotNow,0,NULL);
1758+
scan=heap_beginscan(rel,snapshot,0,NULL);
17211759
while ((tuple=heap_getnext(scan,ForwardScanDirection))!=NULL)
17221760
{
17231761
Oiddsttablespace=HeapTupleGetOid(tuple);
@@ -1763,6 +1801,7 @@ remove_dbtablespaces(Oid db_id)
17631801

17641802
heap_endscan(scan);
17651803
heap_close(rel,AccessShareLock);
1804+
UnregisterSnapshot(snapshot);
17661805
}
17671806

17681807
/*
@@ -1784,9 +1823,19 @@ check_db_file_conflict(Oid db_id)
17841823
Relationrel;
17851824
HeapScanDescscan;
17861825
HeapTupletuple;
1826+
Snapshotsnapshot;
1827+
1828+
/*
1829+
* As in createdb(), we'd better use an MVCC snapshot here; missing a
1830+
* tablespace could result in falsely reporting the OID is unique, with
1831+
* disastrous future consequences per the comment above.
1832+
*
1833+
* XXX change this when a generic fix for SnapshotNow races is implemented
1834+
*/
1835+
snapshot=RegisterSnapshot(GetLatestSnapshot());
17871836

17881837
rel=heap_open(TableSpaceRelationId,AccessShareLock);
1789-
scan=heap_beginscan(rel,SnapshotNow,0,NULL);
1838+
scan=heap_beginscan(rel,snapshot,0,NULL);
17901839
while ((tuple=heap_getnext(scan,ForwardScanDirection))!=NULL)
17911840
{
17921841
Oiddsttablespace=HeapTupleGetOid(tuple);
@@ -1812,6 +1861,8 @@ check_db_file_conflict(Oid db_id)
18121861

18131862
heap_endscan(scan);
18141863
heap_close(rel,AccessShareLock);
1864+
UnregisterSnapshot(snapshot);
1865+
18151866
returnresult;
18161867
}
18171868

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