Merge pull request #4214 from citusdata/degrade-gracefully-when-no-background-workers

Degrade gracefully when no background workers available

src/backend/distributed/metadata/node_metadata.c

@@ -41,6 +41,7 @@
 #include "distributed/resource_lock.h"
 #include "distributed/shardinterval_utils.h"
 #include "distributed/shared_connection_stats.h"
+#include "distributed/string_utils.h"
 #include "distributed/version_compat.h"
 #include "distributed/worker_manager.h"
 #include "distributed/worker_transaction.h"
@@ -101,6 +102,7 @@ static void SetUpDistributedTableDependencies(WorkerNode *workerNode);
 static WorkerNode * TupleToWorkerNode(TupleDesc tupleDescriptor, HeapTuple heapTuple);
 static void PropagateNodeWideObjects(WorkerNode *newWorkerNode);
 static WorkerNode * ModifiableWorkerNode(const char *nodeName, int32 nodePort);
+static void SetLockTimeoutLocally(int32 lock_cooldown);
 static void UpdateNodeLocation(int32 nodeId, char *newNodeName, int32 newNodePort);
 static bool UnsetMetadataSyncedForAll(void);
 static void ErrorIfCoordinatorMetadataSetFalse(WorkerNode *workerNode, Datum value,
@@ -757,6 +759,24 @@ master_update_node(PG_FUNCTION_ARGS)
 		if (force)
 		{
 			handle = StartLockAcquireHelperBackgroundWorker(MyProcPid, lock_cooldown);
+			if (!handle)
+			{
+				/*
+				 * We failed to start a background worker, which probably means that we exceeded
+				 * max_worker_processes, and this is unlikely to be resolved by retrying. We do not want
+				 * to repeatedly throw an error because if master_update_node is called to complete a
+				 * failover then finishing is the only way to bring the cluster back up. Therefore we
+				 * give up on killing other backends and simply wait for the lock. We do set
+				 * lock_timeout to lock_cooldown, because we don't want to wait forever to get a lock.
+				 */
+				SetLockTimeoutLocally(lock_cooldown);
+				ereport(WARNING, (errmsg(
+									  "could not start background worker to kill backends with conflicting"
+									  " locks to force the update. Degrading to acquiring locks "
+									  "with a lock time out."),
+								  errhint(
+									  "Increasing max_worker_processes might help.")));
+			}
 		}
 
 		placementList = AllShardPlacementsOnNodeGroup(workerNode->groupId);
@@ -807,6 +827,19 @@ master_update_node(PG_FUNCTION_ARGS)
 }
 
 
+/*
+ * SetLockTimeoutLocally sets the lock_timeout to the given value.
+ * This setting is local.
+ */
+static void
+SetLockTimeoutLocally(int32 lockCooldown)
+{
+	set_config_option("lock_timeout", ConvertIntToString(lockCooldown),
+					  (superuser() ? PGC_SUSET : PGC_USERSET), PGC_S_SESSION,
+					  GUC_ACTION_LOCAL, true, 0, false);
+}
+
+
 static void
 UpdateNodeLocation(int32 nodeId, char *newNodeName, int32 newNodePort)
 {

src/backend/distributed/utils/acquire_lock.c

@@ -94,9 +94,7 @@ StartLockAcquireHelperBackgroundWorker(int backendToHelp, int32 lock_cooldown)
 
 	if (!RegisterDynamicBackgroundWorker(&worker, &handle))
 	{
-		ereport(ERROR, (errmsg("could not start lock acquiring background worker to "
-							   "force the update"),
-						errhint("Increasing max_worker_processes might help.")));
+		return NULL;
 	}
 
 	MemoryContextCallback *workerCleanup = palloc0(sizeof(MemoryContextCallback));

src/backend/distributed/utils/string_utils.c

@@ -0,0 +1,28 @@
+/*-------------------------------------------------------------------------
+ *
+ * string_utils.c
+ *
+ * This file contains functions to perform useful operations on strings.
+ *
+ * Copyright (c) Citus Data, Inc.
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "distributed/relay_utility.h"
+#include "distributed/string_utils.h"
+
+/*
+ * ConvertIntToString returns the string version of given integer.
+ */
+char *
+ConvertIntToString(int val)
+{
+	StringInfo str = makeStringInfo();
+
+	appendStringInfo(str, "%d", val);
+
+	return str->data;
+}

src/include/distributed/string_utils.h

@@ -0,0 +1,18 @@
+/*-------------------------------------------------------------------------
+ *
+ * string_utils.h
+ *   Utilities related to strings.
+ *
+ * Copyright (c) Citus Data, Inc.
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef CITUS_STRING_UTILS_H
+#define CITUS_STRING_UTILS_H
+
+#include "postgres.h"
+
+extern char * ConvertIntToString(int val);
+
+#endif /* CITUS_STRING_UTILS_H */