Pass extra data to bgworkers, and use this to fix parallel contexts.

Up until now, the total amount of data that could be passed to a
background worker at startup was one datum, which can be a small as
4 bytes on some systems.  That's enough to pass a dsm_handle or an
array index, but not much else.  Add a bgw_extra flag to the
BackgroundWorker struct, allowing up to 128 bytes to be passed to
a new worker on any platform.

Use this to fix a problem I recently discovered with the parallel
context machinery added in 9.5: the master assigns each worker an
array index, and each worker subsequently assigns itself an array
index, and there's nothing to guarantee that the two sets of indexes
match, leading to chaos.

Normally, I would not back-patch the change to add bgw_extra, since it
is basically a feature addition.  However, since 9.5 is still in beta
and there seems to be no other sensible way to repair the broken
parallel context machinery, back-patch to 9.5.  Existing background
worker code can ignore the bgw_extra field without a problem, but
might need to be recompiled since the structure size has changed.

Report and patch by me.  Review by Amit Kapila.

doc/src/sgml/bgworker.sgml

@@ -58,6 +58,7 @@ typedef struct BackgroundWorker
     char        bgw_library_name[BGW_MAXLEN];   /* only if bgw_main is NULL */
     char        bgw_function_name[BGW_MAXLEN];  /* only if bgw_main is NULL */
     Datum       bgw_main_arg;
+    char        bgw_extra[BGW_EXTRALEN];
     int         bgw_notify_pid;
 } BackgroundWorker;
 </programlisting>
@@ -136,6 +137,13 @@ typedef struct BackgroundWorker
    <structfield>bgw_main</structfield> is NULL.
   </para>
 
+  <para>
+   <structfield>bgw_extra</structfield> can contain extra data to be passed
+   to the background worker.  Unlike <structfield>bgw_main_arg</>, this data
+   is not passed as an argument to the worker's main function, but it can be
+   accessed via <literal>MyBgworkerEntry</literal>, as discussed above.
+  </para>
+
   <para>
    <structfield>bgw_notify_pid</structfield> is the PID of a PostgreSQL
    backend process to which the postmaster should send <literal>SIGUSR1</>

src/backend/access/transam/parallel.c

@@ -77,10 +77,6 @@ typedef struct FixedParallelState
 	/* Mutex protects remaining fields. */
 	slock_t		mutex;
 
-	/* Track whether workers have attached. */
-	int			workers_expected;
-	int			workers_attached;
-
 	/* Maximum XactLastRecEnd of any worker. */
 	XLogRecPtr	last_xlog_end;
 } FixedParallelState;
@@ -286,8 +282,6 @@ InitializeParallelDSM(ParallelContext *pcxt)
 	fps->parallel_master_backend_id = MyBackendId;
 	fps->entrypoint = pcxt->entrypoint;
 	SpinLockInit(&fps->mutex);
-	fps->workers_expected = pcxt->nworkers;
-	fps->workers_attached = 0;
 	fps->last_xlog_end = 0;
 	shm_toc_insert(pcxt->toc, PARALLEL_KEY_FIXED, fps);
 
@@ -406,6 +400,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 	worker.bgw_main = ParallelWorkerMain;
 	worker.bgw_main_arg = UInt32GetDatum(dsm_segment_handle(pcxt->seg));
 	worker.bgw_notify_pid = MyProcPid;
+	memset(&worker.bgw_extra, 0, BGW_EXTRALEN);
 
 	/*
 	 * Start workers.
@@ -417,6 +412,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 	 */
 	for (i = 0; i < pcxt->nworkers; ++i)
 	{
+		memcpy(worker.bgw_extra, &i, sizeof(int));
 		if (!any_registrations_failed &&
 			RegisterDynamicBackgroundWorker(&worker,
 											&pcxt->worker[i].bgwhandle))
@@ -825,6 +821,10 @@ ParallelWorkerMain(Datum main_arg)
 	pqsignal(SIGTERM, die);
 	BackgroundWorkerUnblockSignals();
 
+	/* Determine and set our parallel worker number. */
+	Assert(ParallelWorkerNumber == -1);
+	memcpy(&ParallelWorkerNumber, MyBgworkerEntry->bgw_extra, sizeof(int));
+
 	/* Set up a memory context and resource owner. */
 	Assert(CurrentResourceOwner == NULL);
 	CurrentResourceOwner = ResourceOwnerCreate(NULL, "parallel toplevel");
@@ -849,18 +849,9 @@ ParallelWorkerMain(Datum main_arg)
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 			   errmsg("bad magic number in dynamic shared memory segment")));
 
-	/* Determine and set our worker number. */
+	/* Look up fixed parallel state. */
 	fps = shm_toc_lookup(toc, PARALLEL_KEY_FIXED);
 	Assert(fps != NULL);
-	Assert(ParallelWorkerNumber == -1);
-	SpinLockAcquire(&fps->mutex);
-	if (fps->workers_attached < fps->workers_expected)
-		ParallelWorkerNumber = fps->workers_attached++;
-	SpinLockRelease(&fps->mutex);
-	if (ParallelWorkerNumber < 0)
-		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("too many parallel workers already attached")));
 	MyFixedParallelState = fps;
 
 	/*

src/backend/postmaster/bgworker.c

@@ -314,6 +314,7 @@ BackgroundWorkerStateChange(void)
 		rw->rw_worker.bgw_restart_time = slot->worker.bgw_restart_time;
 		rw->rw_worker.bgw_main = slot->worker.bgw_main;
 		rw->rw_worker.bgw_main_arg = slot->worker.bgw_main_arg;
+		memcpy(rw->rw_worker.bgw_extra, slot->worker.bgw_extra, BGW_EXTRALEN);
 
 		/*
 		 * Copy the PID to be notified about state changes, but only if the

src/include/postmaster/bgworker.h

@@ -74,6 +74,7 @@ typedef enum
 #define BGW_DEFAULT_RESTART_INTERVAL	60
 #define BGW_NEVER_RESTART				-1
 #define BGW_MAXLEN						64
+#define BGW_EXTRALEN					128
 
 typedef struct BackgroundWorker
 {
@@ -85,6 +86,7 @@ typedef struct BackgroundWorker
 	char		bgw_library_name[BGW_MAXLEN];	/* only if bgw_main is NULL */
 	char		bgw_function_name[BGW_MAXLEN];	/* only if bgw_main is NULL */
 	Datum		bgw_main_arg;
+	char		bgw_extra[BGW_EXTRALEN];
 	pid_t		bgw_notify_pid; /* SIGUSR1 this backend on start/stop */
 } BackgroundWorker;