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

src/backend/access/transam/parallel.c

@@ -392,12 +392,12 @@ ReinitializeParallelDSM(ParallelContext *pcxt)
 	}
 
 	/* Reset a few bits of fixed parallel state to a clean state. */
-	fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED);
+	fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED, false);
 	fps->last_xlog_end = 0;
 
 	/* Recreate error queues. */
 	error_queue_space =
-		shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE);
+		shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, false);
 	for (i = 0; i < pcxt->nworkers; ++i)
 	{
 		char	   *start;
@@ -536,7 +536,7 @@ WaitForParallelWorkersToFinish(ParallelContext *pcxt)
 	{
 		FixedParallelState *fps;
 
-		fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED);
+		fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED, false);
 		if (fps->last_xlog_end > XactLastRecEnd)
 			XactLastRecEnd = fps->last_xlog_end;
 	}
@@ -973,8 +973,7 @@ ParallelWorkerMain(Datum main_arg)
 		   errmsg("invalid magic number in dynamic shared memory segment")));
 
 	/* Look up fixed parallel state. */
-	fps = shm_toc_lookup(toc, PARALLEL_KEY_FIXED);
+	fps = shm_toc_lookup(toc, PARALLEL_KEY_FIXED, false);
-	Assert(fps != NULL);
 	MyFixedParallelState = fps;
 
 	/*
@@ -983,7 +982,7 @@ ParallelWorkerMain(Datum main_arg)
 	 * errors that happen here will not be reported back to the process that
 	 * requested that this worker be launched.
 	 */
-	error_queue_space = shm_toc_lookup(toc, PARALLEL_KEY_ERROR_QUEUE);
+	error_queue_space = shm_toc_lookup(toc, PARALLEL_KEY_ERROR_QUEUE, false);
 	mq = (shm_mq *) (error_queue_space +
 					 ParallelWorkerNumber * PARALLEL_ERROR_QUEUE_SIZE);
 	shm_mq_set_sender(mq, MyProc);
@@ -1027,8 +1026,7 @@ ParallelWorkerMain(Datum main_arg)
 	 * this before restoring GUCs, because the libraries might define custom
 	 * variables.
 	 */
-	libraryspace = shm_toc_lookup(toc, PARALLEL_KEY_LIBRARY);
+	libraryspace = shm_toc_lookup(toc, PARALLEL_KEY_LIBRARY, false);
-	Assert(libraryspace != NULL);
 	RestoreLibraryState(libraryspace);
 
 	/*
@@ -1036,8 +1034,7 @@ ParallelWorkerMain(Datum main_arg)
 	 * loading an additional library, though most likely the entry point is in
 	 * the core backend or in a library we just loaded.
 	 */
-	entrypointstate = shm_toc_lookup(toc, PARALLEL_KEY_ENTRYPOINT);
+	entrypointstate = shm_toc_lookup(toc, PARALLEL_KEY_ENTRYPOINT, false);
-	Assert(entrypointstate != NULL);
 	library_name = entrypointstate;
 	function_name = entrypointstate + strlen(library_name) + 1;
 
@@ -1054,30 +1051,26 @@ ParallelWorkerMain(Datum main_arg)
 	SetClientEncoding(GetDatabaseEncoding());
 
 	/* Restore GUC values from launching backend. */
-	gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC);
+	gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC, false);
-	Assert(gucspace != NULL);
 	StartTransactionCommand();
 	RestoreGUCState(gucspace);
 	CommitTransactionCommand();
 
 	/* Crank up a transaction state appropriate to a parallel worker. */
-	tstatespace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_STATE);
+	tstatespace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_STATE, false);
 	StartParallelWorkerTransaction(tstatespace);
 
 	/* Restore combo CID state. */
-	combocidspace = shm_toc_lookup(toc, PARALLEL_KEY_COMBO_CID);
+	combocidspace = shm_toc_lookup(toc, PARALLEL_KEY_COMBO_CID, false);
-	Assert(combocidspace != NULL);
 	RestoreComboCIDState(combocidspace);
 
 	/* Restore transaction snapshot. */
-	tsnapspace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT);
+	tsnapspace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT, false);
-	Assert(tsnapspace != NULL);
 	RestoreTransactionSnapshot(RestoreSnapshot(tsnapspace),
 							   fps->parallel_master_pgproc);
 
 	/* Restore active snapshot. */
-	asnapspace = shm_toc_lookup(toc, PARALLEL_KEY_ACTIVE_SNAPSHOT);
+	asnapspace = shm_toc_lookup(toc, PARALLEL_KEY_ACTIVE_SNAPSHOT, false);
-	Assert(asnapspace != NULL);
 	PushActiveSnapshot(RestoreSnapshot(asnapspace));
 
 	/*

src/backend/executor/execParallel.c

@@ -341,7 +341,7 @@ ExecParallelSetupTupleQueues(ParallelContext *pcxt, bool reinitialize)
 							 mul_size(PARALLEL_TUPLE_QUEUE_SIZE,
 									  pcxt->nworkers));
 	else
-		tqueuespace = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_TUPLE_QUEUE);
+		tqueuespace = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_TUPLE_QUEUE, false);
 
 	/* Create the queues, and become the receiver for each. */
 	for (i = 0; i < pcxt->nworkers; ++i)
@@ -684,7 +684,7 @@ ExecParallelGetReceiver(dsm_segment *seg, shm_toc *toc)
 	char	   *mqspace;
 	shm_mq	   *mq;
 
-	mqspace = shm_toc_lookup(toc, PARALLEL_KEY_TUPLE_QUEUE);
+	mqspace = shm_toc_lookup(toc, PARALLEL_KEY_TUPLE_QUEUE, false);
 	mqspace += ParallelWorkerNumber * PARALLEL_TUPLE_QUEUE_SIZE;
 	mq = (shm_mq *) mqspace;
 	shm_mq_set_sender(mq, MyProc);
@@ -705,14 +705,14 @@ ExecParallelGetQueryDesc(shm_toc *toc, DestReceiver *receiver,
 	char	   *queryString;
 
 	/* Get the query string from shared memory */
-	queryString = shm_toc_lookup(toc, PARALLEL_KEY_QUERY_TEXT);
+	queryString = shm_toc_lookup(toc, PARALLEL_KEY_QUERY_TEXT, false);
 
 	/* Reconstruct leader-supplied PlannedStmt. */
-	pstmtspace = shm_toc_lookup(toc, PARALLEL_KEY_PLANNEDSTMT);
+	pstmtspace = shm_toc_lookup(toc, PARALLEL_KEY_PLANNEDSTMT, false);
 	pstmt = (PlannedStmt *) stringToNode(pstmtspace);
 
 	/* Reconstruct ParamListInfo. */
-	paramspace = shm_toc_lookup(toc, PARALLEL_KEY_PARAMS);
+	paramspace = shm_toc_lookup(toc, PARALLEL_KEY_PARAMS, false);
 	paramLI = RestoreParamList(&paramspace);
 
 	/*
@@ -843,7 +843,7 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 
 	/* Set up DestReceiver, SharedExecutorInstrumentation, and QueryDesc. */
 	receiver = ExecParallelGetReceiver(seg, toc);
-	instrumentation = shm_toc_lookup(toc, PARALLEL_KEY_INSTRUMENTATION);
+	instrumentation = shm_toc_lookup(toc, PARALLEL_KEY_INSTRUMENTATION, true);
 	if (instrumentation != NULL)
 		instrument_options = instrumentation->instrument_options;
 	queryDesc = ExecParallelGetQueryDesc(toc, receiver, instrument_options);
@@ -858,7 +858,7 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 	InstrStartParallelQuery();
 
 	/* Attach to the dynamic shared memory area. */
-	area_space = shm_toc_lookup(toc, PARALLEL_KEY_DSA);
+	area_space = shm_toc_lookup(toc, PARALLEL_KEY_DSA, false);
 	area = dsa_attach_in_place(area_space, seg);
 
 	/* Start up the executor */
@@ -875,7 +875,7 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 	ExecutorFinish(queryDesc);
 
 	/* Report buffer usage during parallel execution. */
-	buffer_usage = shm_toc_lookup(toc, PARALLEL_KEY_BUFFER_USAGE);
+	buffer_usage = shm_toc_lookup(toc, PARALLEL_KEY_BUFFER_USAGE, false);
 	InstrEndParallelQuery(&buffer_usage[ParallelWorkerNumber]);
 
 	/* Report instrumentation data if any instrumentation options are set. */

src/backend/executor/nodeBitmapHeapscan.c

@@ -1005,7 +1005,7 @@ ExecBitmapHeapInitializeWorker(BitmapHeapScanState *node, shm_toc *toc)
 	ParallelBitmapHeapState *pstate;
 	Snapshot	snapshot;
 
-	pstate = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id);
+	pstate = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id, false);
 	node->pstate = pstate;
 
 	snapshot = RestoreSnapshot(pstate->phs_snapshot_data);

src/backend/executor/nodeCustom.c

@@ -194,7 +194,7 @@ ExecCustomScanInitializeWorker(CustomScanState *node, shm_toc *toc)
 		int			plan_node_id = node->ss.ps.plan->plan_node_id;
 		void	   *coordinate;
 
-		coordinate = shm_toc_lookup(toc, plan_node_id);
+		coordinate = shm_toc_lookup(toc, plan_node_id, false);
 		methods->InitializeWorkerCustomScan(node, toc, coordinate);
 	}
 }

src/backend/executor/nodeForeignscan.c

@@ -344,7 +344,7 @@ ExecForeignScanInitializeWorker(ForeignScanState *node, shm_toc *toc)
 		int			plan_node_id = node->ss.ps.plan->plan_node_id;
 		void	   *coordinate;
 
-		coordinate = shm_toc_lookup(toc, plan_node_id);
+		coordinate = shm_toc_lookup(toc, plan_node_id, false);
 		fdwroutine->InitializeWorkerForeignScan(node, toc, coordinate);
 	}
 }

src/backend/executor/nodeIndexonlyscan.c

@@ -676,7 +676,7 @@ ExecIndexOnlyScanInitializeWorker(IndexOnlyScanState *node, shm_toc *toc)
 {
 	ParallelIndexScanDesc piscan;
 
-	piscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id);
+	piscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id, false);
 	node->ioss_ScanDesc =
 		index_beginscan_parallel(node->ss.ss_currentRelation,
 								 node->ioss_RelationDesc,

src/backend/executor/nodeIndexscan.c

@@ -1714,7 +1714,7 @@ ExecIndexScanInitializeWorker(IndexScanState *node, shm_toc *toc)
 {
 	ParallelIndexScanDesc piscan;
 
-	piscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id);
+	piscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id, false);
 	node->iss_ScanDesc =
 		index_beginscan_parallel(node->ss.ss_currentRelation,
 								 node->iss_RelationDesc,

src/backend/executor/nodeSeqscan.c

@@ -332,7 +332,7 @@ ExecSeqScanInitializeWorker(SeqScanState *node, shm_toc *toc)
 {
 	ParallelHeapScanDesc pscan;
 
-	pscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id);
+	pscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id, false);
 	node->ss.ss_currentScanDesc =
 		heap_beginscan_parallel(node->ss.ss_currentRelation, pscan);
 }

src/backend/storage/ipc/shm_toc.c

@@ -208,6 +208,9 @@ shm_toc_insert(shm_toc *toc, uint64 key, void *address)
 /*
  * Look up a TOC entry.
  *
+ * If the key is not found, returns NULL if noError is true, otherwise
+ * throws elog(ERROR).
+ *
  * Unlike the other functions in this file, this operation acquires no lock;
  * it uses only barriers.  It probably wouldn't hurt concurrency very much even
  * if it did get a lock, but since it's reasonably likely that a group of
@@ -215,7 +218,7 @@ shm_toc_insert(shm_toc *toc, uint64 key, void *address)
  * right around the same time, there seems to be some value in avoiding it.
  */
 void *
-shm_toc_lookup(shm_toc *toc, uint64 key)
+shm_toc_lookup(shm_toc *toc, uint64 key, bool noError)
 {
 	uint64		nentry;
 	uint64		i;
@@ -226,10 +229,15 @@ shm_toc_lookup(shm_toc *toc, uint64 key)
 
 	/* Now search for a matching entry. */
 	for (i = 0; i < nentry; ++i)
+	{
 		if (toc->toc_entry[i].key == key)
 			return ((char *) toc) + toc->toc_entry[i].offset;
+	}
 
 	/* No matching entry was found. */
+	if (!noError)
+		elog(ERROR, "could not find key " UINT64_FORMAT " in shm TOC at %p",
+			 key, toc);
 	return NULL;
 }
 

src/include/storage/shm_toc.h

@@ -32,7 +32,7 @@ extern shm_toc *shm_toc_attach(uint64 magic, void *address);
 extern void *shm_toc_allocate(shm_toc *toc, Size nbytes);
 extern Size shm_toc_freespace(shm_toc *toc);
 extern void shm_toc_insert(shm_toc *toc, uint64 key, void *address);
-extern void *shm_toc_lookup(shm_toc *toc, uint64 key);
+extern void *shm_toc_lookup(shm_toc *toc, uint64 key, bool noError);
 
 /*
  * Tools for estimating how large a chunk of shared memory will be needed

src/test/modules/test_shm_mq/worker.c

@@ -95,7 +95,7 @@ test_shm_mq_main(Datum main_arg)
 	 * find it.  Our worker number gives our identity: there may be just one
 	 * worker involved in this parallel operation, or there may be many.
 	 */
-	hdr = shm_toc_lookup(toc, 0);
+	hdr = shm_toc_lookup(toc, 0, false);
 	SpinLockAcquire(&hdr->mutex);
 	myworkernumber = ++hdr->workers_attached;
 	SpinLockRelease(&hdr->mutex);
@@ -158,10 +158,10 @@ attach_to_queues(dsm_segment *seg, shm_toc *toc, int myworkernumber,
 	shm_mq	   *inq;
 	shm_mq	   *outq;
 
-	inq = shm_toc_lookup(toc, myworkernumber);
+	inq = shm_toc_lookup(toc, myworkernumber, false);
 	shm_mq_set_receiver(inq, MyProc);
 	*inqhp = shm_mq_attach(inq, seg, NULL);
-	outq = shm_toc_lookup(toc, myworkernumber + 1);
+	outq = shm_toc_lookup(toc, myworkernumber + 1, false);
 	shm_mq_set_sender(outq, MyProc);
 	*outqhp = shm_mq_attach(outq, seg, NULL);
 }