Reserve a PGPROC slot and semaphore for the slotsync worker process.

The need for this was missed in commit 93db6cb, with the result
being that if we launch a slotsync worker it would consume one of
the PGPROCs in the max_connections pool.  That could lead to inability
to launch the worker, or to subsequent failures of connection requests
that should have succeeded according to the configured settings.

Rather than create some one-off infrastructure to support this,
let's group the slotsync worker with the existing autovac launcher
in a new category of "special worker" processes.  These are kind of
like auxiliary processes, but they cannot use that infrastructure
because they need to be able to run transactions.

For the moment, make these processes share the PGPROC freelist
used for autovac workers (which previously supplied the autovac
launcher too).  This is partly to avoid an ABI change in v17,
and partly because it seems silly to have a freelist with
at most two members.  This might be worth revisiting if we grow
enough workers in this category.

Tom Lane and Hou Zhijie.  Back-patch to v17.

Discussion: https://postgr.es/m/1808397.1735156190@sss.pgh.pa.us

Author: tglsfdc

src/backend/storage/lmgr/proc.c

@@ -183,11 +183,12 @@ InitProcGlobal(void)
 
 	/*
 	 * Create and initialize all the PGPROC structures we'll need.  There are
-	 * five separate consumers: (1) normal backends, (2) autovacuum workers
-	 * and the autovacuum launcher, (3) background workers, (4) auxiliary
-	 * processes, and (5) prepared transactions.  Each PGPROC structure is
-	 * dedicated to exactly one of these purposes, and they do not move
-	 * between groups.
+	 * six separate consumers: (1) normal backends, (2) autovacuum workers and
+	 * special workers, (3) background workers, (4) walsenders, (5) auxiliary
+	 * processes, and (6) prepared transactions.  (For largely-historical
+	 * reasons, we combine autovacuum and special workers into one category
+	 * with a single freelist.)  Each PGPROC structure is dedicated to exactly
+	 * one of these purposes, and they do not move between groups.
 	 */
 	procs = (PGPROC *) ShmemAlloc(TotalProcs * sizeof(PGPROC));
 	MemSet(procs, 0, TotalProcs * sizeof(PGPROC));
@@ -229,26 +230,27 @@ InitProcGlobal(void)
 		}
 
 		/*
-		 * Newly created PGPROCs for normal backends, autovacuum and bgworkers
-		 * must be queued up on the appropriate free list.  Because there can
-		 * only ever be a small, fixed number of auxiliary processes, no free
-		 * list is used in that case; InitAuxiliaryProcess() instead uses a
-		 * linear search.   PGPROCs for prepared transactions are added to a
-		 * free list by TwoPhaseShmemInit().
+		 * Newly created PGPROCs for normal backends, autovacuum workers,
+		 * special workers, bgworkers, and walsenders must be queued up on the
+		 * appropriate free list.  Because there can only ever be a small,
+		 * fixed number of auxiliary processes, no free list is used in that
+		 * case; InitAuxiliaryProcess() instead uses a linear search.  PGPROCs
+		 * for prepared transactions are added to a free list by
+		 * TwoPhaseShmemInit().
 		 */
 		if (i < MaxConnections)
 		{
 			/* PGPROC for normal backend, add to freeProcs list */
 			dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->freeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1)
+		else if (i < MaxConnections + autovacuum_max_workers + NUM_SPECIAL_WORKER_PROCS)
 		{
-			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
+			/* PGPROC for AV or special worker, add to autovacFreeProcs list */
 			dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
 			proc->procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
+		else if (i < MaxConnections + autovacuum_max_workers + NUM_SPECIAL_WORKER_PROCS + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
@@ -307,8 +309,11 @@ InitProcess(void)
 	if (MyProc != NULL)
 		elog(ERROR, "you already exist");
 
-	/* Decide which list should supply our PGPROC. */
-	if (AmAutoVacuumLauncherProcess() || AmAutoVacuumWorkerProcess())
+	/*
+	 * Decide which list should supply our PGPROC.  This logic must match the
+	 * way the freelists were constructed in InitProcGlobal().
+	 */
+	if (AmAutoVacuumWorkerProcess() || AmSpecialWorkerProcess())
 		procgloballist = &ProcGlobal->autovacFreeProcs;
 	else if (AmBackgroundWorkerProcess())
 		procgloballist = &ProcGlobal->bgworkerFreeProcs;

src/backend/utils/init/postinit.c

@@ -576,9 +576,9 @@ InitializeMaxBackends(void)
 {
 	Assert(MaxBackends == 0);
 
-	/* the extra unit accounts for the autovacuum launcher */
-	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
-		max_worker_processes + max_wal_senders;
+	/* Note that this does not include "auxiliary" processes */
+	MaxBackends = MaxConnections + autovacuum_max_workers +
+		max_worker_processes + max_wal_senders + NUM_SPECIAL_WORKER_PROCS;
 
 	/* internal error because the values were all checked previously */
 	if (MaxBackends > MAX_BACKENDS)

src/include/miscadmin.h

@@ -346,8 +346,9 @@ typedef enum BackendType
 
 	/*
 	 * Auxiliary processes. These have PGPROC entries, but they are not
-	 * attached to any particular database. There can be only one of each of
-	 * these running at a time.
+	 * attached to any particular database, and cannot run transactions or
+	 * even take heavyweight locks. There can be only one of each of these
+	 * running at a time.
 	 *
 	 * If you modify these, make sure to update NUM_AUXILIARY_PROCS and the
 	 * glossary in the docs.
@@ -384,6 +385,10 @@ extern PGDLLIMPORT BackendType MyBackendType;
 #define AmWalSummarizerProcess()	(MyBackendType == B_WAL_SUMMARIZER)
 #define AmWalWriterProcess()		(MyBackendType == B_WAL_WRITER)
 
+#define AmSpecialWorkerProcess() \
+	(AmAutoVacuumLauncherProcess() || \
+	 AmLogicalSlotSyncWorkerProcess())
+
 extern const char *GetBackendTypeDesc(BackendType backendType);
 
 extern void SetDatabasePath(const char *path);

src/include/storage/proc.h

@@ -404,7 +404,7 @@ typedef struct PROC_HDR
 	uint32		allProcCount;
 	/* Head of list of free PGPROC structures */
 	dlist_head	freeProcs;
-	/* Head of list of autovacuum's free PGPROC structures */
+	/* Head of list of autovacuum & special worker free PGPROC structures */
 	dlist_head	autovacFreeProcs;
 	/* Head of list of bgworker free PGPROC structures */
 	dlist_head	bgworkerFreeProcs;
@@ -434,9 +434,19 @@ extern PGDLLIMPORT PGPROC *PreparedXactProcs;
 #define GetPGProcByNumber(n) (&ProcGlobal->allProcs[(n)])
 #define GetNumberFromPGProc(proc) ((proc) - &ProcGlobal->allProcs[0])
 
+/*
+ * We set aside some extra PGPROC structures for "special worker" processes,
+ * which are full-fledged backends (they can run transactions)
+ * but are unique animals that there's never more than one of.
+ * Currently there are two such processes: the autovacuum launcher
+ * and the slotsync worker.
+ */
+#define NUM_SPECIAL_WORKER_PROCS	2
+
 /*
  * We set aside some extra PGPROC structures for auxiliary processes,
- * ie things that aren't full-fledged backends but need shmem access.
+ * ie things that aren't full-fledged backends (they cannot run transactions
+ * or take heavyweight locks) but need shmem access.
  *
  * Background writer, checkpointer, WAL writer, WAL summarizer, and archiver
  * run during normal operation.  Startup process and WAL receiver also consume