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Don't be so trusting that shm_toc_lookup() will always succeed.
Given the possibility of race conditions and so on, it seems entirely unsafe to just assume that shm_toc_lookup() always finds the key it's looking for --- but that was exactly what all but one call site were doing. To fix, add a "bool noError" argument, similarly to what we have in many other functions, and throw an error on an unexpected lookup failure. Remove now-redundant Asserts that a rather random subset of call sites had. I doubt this will throw any light on buildfarm member lorikeet's recent failures, because if an unnoticed lookup failure were involved, you'd kind of expect a null-pointer-dereference crash rather than the observed symptom. But you never know ... and this is better coding practice even if it never catches anything. Discussion: https://postgr.es/m/9697.1496675981@sss.pgh.pa.us
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+39
-38
lines changed

11 files changed

+39
-38
lines changed

‎src/backend/access/transam/parallel.c

Lines changed: 12 additions & 19 deletions
Original file line numberDiff line numberDiff line change
@@ -392,12 +392,12 @@ ReinitializeParallelDSM(ParallelContext *pcxt)
392392
}
393393

394394
/* Reset a few bits of fixed parallel state to a clean state. */
395-
fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED);
395+
fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED, false);
396396
fps->last_xlog_end = 0;
397397

398398
/* Recreate error queues. */
399399
error_queue_space =
400-
shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE);
400+
shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, false);
401401
for (i = 0; i < pcxt->nworkers; ++i)
402402
{
403403
char *start;
@@ -536,7 +536,7 @@ WaitForParallelWorkersToFinish(ParallelContext *pcxt)
536536
{
537537
FixedParallelState *fps;
538538

539-
fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED);
539+
fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED, false);
540540
if (fps->last_xlog_end > XactLastRecEnd)
541541
XactLastRecEnd = fps->last_xlog_end;
542542
}
@@ -973,8 +973,7 @@ ParallelWorkerMain(Datum main_arg)
973973
errmsg("invalid magic number in dynamic shared memory segment")));
974974

975975
/* Look up fixed parallel state. */
976-
fps = shm_toc_lookup(toc, PARALLEL_KEY_FIXED);
977-
Assert(fps != NULL);
976+
fps = shm_toc_lookup(toc, PARALLEL_KEY_FIXED, false);
978977
MyFixedParallelState = fps;
979978

980979
/*
@@ -983,7 +982,7 @@ ParallelWorkerMain(Datum main_arg)
983982
* errors that happen here will not be reported back to the process that
984983
* requested that this worker be launched.
985984
*/
986-
error_queue_space = shm_toc_lookup(toc, PARALLEL_KEY_ERROR_QUEUE);
985+
error_queue_space = shm_toc_lookup(toc, PARALLEL_KEY_ERROR_QUEUE, false);
987986
mq = (shm_mq *) (error_queue_space +
988987
ParallelWorkerNumber * PARALLEL_ERROR_QUEUE_SIZE);
989988
shm_mq_set_sender(mq, MyProc);
@@ -1027,17 +1026,15 @@ ParallelWorkerMain(Datum main_arg)
10271026
* this before restoring GUCs, because the libraries might define custom
10281027
* variables.
10291028
*/
1030-
libraryspace = shm_toc_lookup(toc, PARALLEL_KEY_LIBRARY);
1031-
Assert(libraryspace != NULL);
1029+
libraryspace = shm_toc_lookup(toc, PARALLEL_KEY_LIBRARY, false);
10321030
RestoreLibraryState(libraryspace);
10331031

10341032
/*
10351033
* Identify the entry point to be called. In theory this could result in
10361034
* loading an additional library, though most likely the entry point is in
10371035
* the core backend or in a library we just loaded.
10381036
*/
1039-
entrypointstate = shm_toc_lookup(toc, PARALLEL_KEY_ENTRYPOINT);
1040-
Assert(entrypointstate != NULL);
1037+
entrypointstate = shm_toc_lookup(toc, PARALLEL_KEY_ENTRYPOINT, false);
10411038
library_name = entrypointstate;
10421039
function_name = entrypointstate + strlen(library_name) + 1;
10431040

@@ -1054,30 +1051,26 @@ ParallelWorkerMain(Datum main_arg)
10541051
SetClientEncoding(GetDatabaseEncoding());
10551052

10561053
/* Restore GUC values from launching backend. */
1057-
gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC);
1058-
Assert(gucspace != NULL);
1054+
gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC, false);
10591055
StartTransactionCommand();
10601056
RestoreGUCState(gucspace);
10611057
CommitTransactionCommand();
10621058

10631059
/* Crank up a transaction state appropriate to a parallel worker. */
1064-
tstatespace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_STATE);
1060+
tstatespace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_STATE, false);
10651061
StartParallelWorkerTransaction(tstatespace);
10661062

10671063
/* Restore combo CID state. */
1068-
combocidspace = shm_toc_lookup(toc, PARALLEL_KEY_COMBO_CID);
1069-
Assert(combocidspace != NULL);
1064+
combocidspace = shm_toc_lookup(toc, PARALLEL_KEY_COMBO_CID, false);
10701065
RestoreComboCIDState(combocidspace);
10711066

10721067
/* Restore transaction snapshot. */
1073-
tsnapspace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT);
1074-
Assert(tsnapspace != NULL);
1068+
tsnapspace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT, false);
10751069
RestoreTransactionSnapshot(RestoreSnapshot(tsnapspace),
10761070
fps->parallel_master_pgproc);
10771071

10781072
/* Restore active snapshot. */
1079-
asnapspace = shm_toc_lookup(toc, PARALLEL_KEY_ACTIVE_SNAPSHOT);
1080-
Assert(asnapspace != NULL);
1073+
asnapspace = shm_toc_lookup(toc, PARALLEL_KEY_ACTIVE_SNAPSHOT, false);
10811074
PushActiveSnapshot(RestoreSnapshot(asnapspace));
10821075

10831076
/*

‎src/backend/executor/execParallel.c

Lines changed: 8 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -341,7 +341,7 @@ ExecParallelSetupTupleQueues(ParallelContext *pcxt, bool reinitialize)
341341
mul_size(PARALLEL_TUPLE_QUEUE_SIZE,
342342
pcxt->nworkers));
343343
else
344-
tqueuespace = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_TUPLE_QUEUE);
344+
tqueuespace = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_TUPLE_QUEUE, false);
345345

346346
/* Create the queues, and become the receiver for each. */
347347
for (i = 0; i < pcxt->nworkers; ++i)
@@ -684,7 +684,7 @@ ExecParallelGetReceiver(dsm_segment *seg, shm_toc *toc)
684684
char *mqspace;
685685
shm_mq *mq;
686686

687-
mqspace = shm_toc_lookup(toc, PARALLEL_KEY_TUPLE_QUEUE);
687+
mqspace = shm_toc_lookup(toc, PARALLEL_KEY_TUPLE_QUEUE, false);
688688
mqspace += ParallelWorkerNumber * PARALLEL_TUPLE_QUEUE_SIZE;
689689
mq = (shm_mq *) mqspace;
690690
shm_mq_set_sender(mq, MyProc);
@@ -705,14 +705,14 @@ ExecParallelGetQueryDesc(shm_toc *toc, DestReceiver *receiver,
705705
char *queryString;
706706

707707
/* Get the query string from shared memory */
708-
queryString = shm_toc_lookup(toc, PARALLEL_KEY_QUERY_TEXT);
708+
queryString = shm_toc_lookup(toc, PARALLEL_KEY_QUERY_TEXT, false);
709709

710710
/* Reconstruct leader-supplied PlannedStmt. */
711-
pstmtspace = shm_toc_lookup(toc, PARALLEL_KEY_PLANNEDSTMT);
711+
pstmtspace = shm_toc_lookup(toc, PARALLEL_KEY_PLANNEDSTMT, false);
712712
pstmt = (PlannedStmt *) stringToNode(pstmtspace);
713713

714714
/* Reconstruct ParamListInfo. */
715-
paramspace = shm_toc_lookup(toc, PARALLEL_KEY_PARAMS);
715+
paramspace = shm_toc_lookup(toc, PARALLEL_KEY_PARAMS, false);
716716
paramLI = RestoreParamList(&paramspace);
717717

718718
/*
@@ -843,7 +843,7 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
843843

844844
/* Set up DestReceiver, SharedExecutorInstrumentation, and QueryDesc. */
845845
receiver = ExecParallelGetReceiver(seg, toc);
846-
instrumentation = shm_toc_lookup(toc, PARALLEL_KEY_INSTRUMENTATION);
846+
instrumentation = shm_toc_lookup(toc, PARALLEL_KEY_INSTRUMENTATION, true);
847847
if (instrumentation != NULL)
848848
instrument_options = instrumentation->instrument_options;
849849
queryDesc = ExecParallelGetQueryDesc(toc, receiver, instrument_options);
@@ -858,7 +858,7 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
858858
InstrStartParallelQuery();
859859

860860
/* Attach to the dynamic shared memory area. */
861-
area_space = shm_toc_lookup(toc, PARALLEL_KEY_DSA);
861+
area_space = shm_toc_lookup(toc, PARALLEL_KEY_DSA, false);
862862
area = dsa_attach_in_place(area_space, seg);
863863

864864
/* Start up the executor */
@@ -875,7 +875,7 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
875875
ExecutorFinish(queryDesc);
876876

877877
/* Report buffer usage during parallel execution. */
878-
buffer_usage = shm_toc_lookup(toc, PARALLEL_KEY_BUFFER_USAGE);
878+
buffer_usage = shm_toc_lookup(toc, PARALLEL_KEY_BUFFER_USAGE, false);
879879
InstrEndParallelQuery(&buffer_usage[ParallelWorkerNumber]);
880880

881881
/* Report instrumentation data if any instrumentation options are set. */

‎src/backend/executor/nodeBitmapHeapscan.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1005,7 +1005,7 @@ ExecBitmapHeapInitializeWorker(BitmapHeapScanState *node, shm_toc *toc)
10051005
ParallelBitmapHeapState *pstate;
10061006
Snapshot snapshot;
10071007

1008-
pstate = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id);
1008+
pstate = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id, false);
10091009
node->pstate = pstate;
10101010

10111011
snapshot = RestoreSnapshot(pstate->phs_snapshot_data);

‎src/backend/executor/nodeCustom.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -194,7 +194,7 @@ ExecCustomScanInitializeWorker(CustomScanState *node, shm_toc *toc)
194194
int plan_node_id = node->ss.ps.plan->plan_node_id;
195195
void *coordinate;
196196

197-
coordinate = shm_toc_lookup(toc, plan_node_id);
197+
coordinate = shm_toc_lookup(toc, plan_node_id, false);
198198
methods->InitializeWorkerCustomScan(node, toc, coordinate);
199199
}
200200
}

‎src/backend/executor/nodeForeignscan.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -344,7 +344,7 @@ ExecForeignScanInitializeWorker(ForeignScanState *node, shm_toc *toc)
344344
int plan_node_id = node->ss.ps.plan->plan_node_id;
345345
void *coordinate;
346346

347-
coordinate = shm_toc_lookup(toc, plan_node_id);
347+
coordinate = shm_toc_lookup(toc, plan_node_id, false);
348348
fdwroutine->InitializeWorkerForeignScan(node, toc, coordinate);
349349
}
350350
}

‎src/backend/executor/nodeIndexonlyscan.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -676,7 +676,7 @@ ExecIndexOnlyScanInitializeWorker(IndexOnlyScanState *node, shm_toc *toc)
676676
{
677677
ParallelIndexScanDesc piscan;
678678

679-
piscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id);
679+
piscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id, false);
680680
node->ioss_ScanDesc =
681681
index_beginscan_parallel(node->ss.ss_currentRelation,
682682
node->ioss_RelationDesc,

‎src/backend/executor/nodeIndexscan.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1714,7 +1714,7 @@ ExecIndexScanInitializeWorker(IndexScanState *node, shm_toc *toc)
17141714
{
17151715
ParallelIndexScanDesc piscan;
17161716

1717-
piscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id);
1717+
piscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id, false);
17181718
node->iss_ScanDesc =
17191719
index_beginscan_parallel(node->ss.ss_currentRelation,
17201720
node->iss_RelationDesc,

‎src/backend/executor/nodeSeqscan.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -332,7 +332,7 @@ ExecSeqScanInitializeWorker(SeqScanState *node, shm_toc *toc)
332332
{
333333
ParallelHeapScanDesc pscan;
334334

335-
pscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id);
335+
pscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id, false);
336336
node->ss.ss_currentScanDesc =
337337
heap_beginscan_parallel(node->ss.ss_currentRelation, pscan);
338338
}

‎src/backend/storage/ipc/shm_toc.c

Lines changed: 9 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -208,14 +208,17 @@ shm_toc_insert(shm_toc *toc, uint64 key, void *address)
208208
/*
209209
* Look up a TOC entry.
210210
*
211+
* If the key is not found, returns NULL if noError is true, otherwise
212+
* throws elog(ERROR).
213+
*
211214
* Unlike the other functions in this file, this operation acquires no lock;
212215
* it uses only barriers. It probably wouldn't hurt concurrency very much even
213216
* if it did get a lock, but since it's reasonably likely that a group of
214217
* worker processes could each read a series of entries from the same TOC
215218
* right around the same time, there seems to be some value in avoiding it.
216219
*/
217220
void *
218-
shm_toc_lookup(shm_toc *toc, uint64 key)
221+
shm_toc_lookup(shm_toc *toc, uint64 key, bool noError)
219222
{
220223
uint64 nentry;
221224
uint64 i;
@@ -226,10 +229,15 @@ shm_toc_lookup(shm_toc *toc, uint64 key)
226229

227230
/* Now search for a matching entry. */
228231
for (i = 0; i < nentry; ++i)
232+
{
229233
if (toc->toc_entry[i].key == key)
230234
return ((char *) toc) + toc->toc_entry[i].offset;
235+
}
231236

232237
/* No matching entry was found. */
238+
if (!noError)
239+
elog(ERROR, "could not find key " UINT64_FORMAT " in shm TOC at %p",
240+
key, toc);
233241
return NULL;
234242
}
235243

‎src/include/storage/shm_toc.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -32,7 +32,7 @@ extern shm_toc *shm_toc_attach(uint64 magic, void *address);
3232
extern void *shm_toc_allocate(shm_toc *toc, Size nbytes);
3333
extern Size shm_toc_freespace(shm_toc *toc);
3434
extern void shm_toc_insert(shm_toc *toc, uint64 key, void *address);
35-
extern void *shm_toc_lookup(shm_toc *toc, uint64 key);
35+
extern void *shm_toc_lookup(shm_toc *toc, uint64 key, bool noError);
3636

3737
/*
3838
* Tools for estimating how large a chunk of shared memory will be needed

‎src/test/modules/test_shm_mq/worker.c

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -95,7 +95,7 @@ test_shm_mq_main(Datum main_arg)
9595
* find it. Our worker number gives our identity: there may be just one
9696
* worker involved in this parallel operation, or there may be many.
9797
*/
98-
hdr = shm_toc_lookup(toc, 0);
98+
hdr = shm_toc_lookup(toc, 0, false);
9999
SpinLockAcquire(&hdr->mutex);
100100
myworkernumber = ++hdr->workers_attached;
101101
SpinLockRelease(&hdr->mutex);
@@ -158,10 +158,10 @@ attach_to_queues(dsm_segment *seg, shm_toc *toc, int myworkernumber,
158158
shm_mq *inq;
159159
shm_mq *outq;
160160

161-
inq = shm_toc_lookup(toc, myworkernumber);
161+
inq = shm_toc_lookup(toc, myworkernumber, false);
162162
shm_mq_set_receiver(inq, MyProc);
163163
*inqhp = shm_mq_attach(inq, seg, NULL);
164-
outq = shm_toc_lookup(toc, myworkernumber + 1);
164+
outq = shm_toc_lookup(toc, myworkernumber + 1, false);
165165
shm_mq_set_sender(outq, MyProc);
166166
*outqhp = shm_mq_attach(outq, seg, NULL);
167167
}

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