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Commit56e19d9

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Don't use on-disk snapshots for exported logical decoding snapshot.
Logical decoding stores historical snapshots on disk, so that logicaldecoding can restart without having to reconstruct a snapshot fromscratch (for which the resources are not guaranteed to be presentanymore). These serialized snapshots were also used when creating anew slot via the walsender interface, which can export a "full"snapshot (i.e. one that can read all tables, not just catalog ones).The problem is that the serialized snapshots are only useful forcatalogs and not for normal user tables. Thus the use of such aserialized snapshot could result in an inconsistent snapshot beingexported, which could lead to queries returning wrong data. Thiswould only happen if logical slots are created while another logicalslot already exists.Author: Petr JelinekReviewed-By: Andres FreundDiscussion:https://postgr.es/m/f37e975c-908f-858e-707f-058d3b1eb214@2ndquadrant.comBackport: 9.4, where logical decoding was introduced.
1 parent7834d20 commit56e19d9

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3 files changed

+22
-9
lines changed

3 files changed

+22
-9
lines changed

‎src/backend/replication/logical/logical.c

Lines changed: 6 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -114,6 +114,7 @@ static LogicalDecodingContext *
114114
StartupDecodingContext(List*output_plugin_options,
115115
XLogRecPtrstart_lsn,
116116
TransactionIdxmin_horizon,
117+
boolneed_full_snapshot,
117118
XLogPageReadCBread_page,
118119
LogicalOutputPluginWriterPrepareWriteprepare_write,
119120
LogicalOutputPluginWriterWritedo_write)
@@ -171,7 +172,8 @@ StartupDecodingContext(List *output_plugin_options,
171172

172173
ctx->reorder=ReorderBufferAllocate();
173174
ctx->snapshot_builder=
174-
AllocateSnapshotBuilder(ctx->reorder,xmin_horizon,start_lsn);
175+
AllocateSnapshotBuilder(ctx->reorder,xmin_horizon,start_lsn,
176+
need_full_snapshot);
175177

176178
ctx->reorder->private_data=ctx;
177179

@@ -297,7 +299,8 @@ CreateInitDecodingContext(char *plugin,
297299
ReplicationSlotSave();
298300

299301
ctx=StartupDecodingContext(NIL,InvalidXLogRecPtr,xmin_horizon,
300-
read_page,prepare_write,do_write);
302+
need_full_snapshot,read_page,prepare_write,
303+
do_write);
301304

302305
/* call output plugin initialization callback */
303306
old_context=MemoryContextSwitchTo(ctx->context);
@@ -386,7 +389,7 @@ CreateDecodingContext(XLogRecPtr start_lsn,
386389
}
387390

388391
ctx=StartupDecodingContext(output_plugin_options,
389-
start_lsn,InvalidTransactionId,
392+
start_lsn,InvalidTransactionId, false,
390393
read_page,prepare_write,do_write);
391394

392395
/* call output plugin initialization callback */

‎src/backend/replication/logical/snapbuild.c

Lines changed: 14 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -165,6 +165,9 @@ struct SnapBuild
165165
*/
166166
TransactionIdinitial_xmin_horizon;
167167

168+
/* Indicates if we are building full snapshot or just catalog one .*/
169+
boolbuilding_full_snapshot;
170+
168171
/*
169172
* Snapshot that's valid to see the catalog state seen at this moment.
170173
*/
@@ -281,7 +284,8 @@ static bool SnapBuildRestore(SnapBuild *builder, XLogRecPtr lsn);
281284
SnapBuild*
282285
AllocateSnapshotBuilder(ReorderBuffer*reorder,
283286
TransactionIdxmin_horizon,
284-
XLogRecPtrstart_lsn)
287+
XLogRecPtrstart_lsn,
288+
boolneed_full_snapshot)
285289
{
286290
MemoryContextcontext;
287291
MemoryContextoldcontext;
@@ -308,6 +312,7 @@ AllocateSnapshotBuilder(ReorderBuffer *reorder,
308312

309313
builder->initial_xmin_horizon=xmin_horizon;
310314
builder->start_decoding_at=start_lsn;
315+
builder->building_full_snapshot=need_full_snapshot;
311316

312317
MemoryContextSwitchTo(oldcontext);
313318

@@ -1245,7 +1250,7 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn
12451250
*
12461251
* a) There were no running transactions when the xl_running_xacts record
12471252
* was inserted, jump to CONSISTENT immediately. We might find such a
1248-
* state we were waiting for b)and c).
1253+
* state we were waiting for b)or c).
12491254
*
12501255
* b) Wait for all toplevel transactions that were running to end. We
12511256
* simply track the number of in-progress toplevel transactions and
@@ -1260,7 +1265,10 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn
12601265
* at all.
12611266
*
12621267
* c) This (in a previous run) or another decoding slot serialized a
1263-
* snapshot to disk that we can use.
1268+
* snapshot to disk that we can use. Can't use this method for the
1269+
* initial snapshot when slot is being created and needs full snapshot
1270+
* for export or direct use, as that snapshot will only contain catalog
1271+
* modifying transactions.
12641272
* ---
12651273
*/
12661274

@@ -1315,8 +1323,9 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn
13151323

13161324
return false;
13171325
}
1318-
/* c) valid on disk state */
1319-
elseif (SnapBuildRestore(builder,lsn))
1326+
/* c) valid on disk state and not building full snapshot */
1327+
elseif (!builder->building_full_snapshot&&
1328+
SnapBuildRestore(builder,lsn))
13201329
{
13211330
/* there won't be any state to cleanup */
13221331
return false;

‎src/include/replication/snapbuild.h

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -54,7 +54,8 @@ struct xl_running_xacts;
5454
externvoidCheckPointSnapBuild(void);
5555

5656
externSnapBuild*AllocateSnapshotBuilder(structReorderBuffer*cache,
57-
TransactionIdxmin_horizon,XLogRecPtrstart_lsn);
57+
TransactionIdxmin_horizon,XLogRecPtrstart_lsn,
58+
boolneed_full_snapshot);
5859
externvoidFreeSnapshotBuilder(SnapBuild*cache);
5960

6061
externvoidSnapBuildSnapDecRefcount(Snapshotsnap);

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