diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index ce29da90121..f15ae87659c 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/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));
@@ -229,12 +230,13 @@ 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)
 		{
@@ -242,13 +244,13 @@ InitProcGlobal(void)
 			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;
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index e4ac84354ef..7fb9ea733dc 100644
--- a/src/backend/utils/init/postinit.c
+++ b/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)
diff --git a/src/include/miscadmin.h b/src/include/miscadmin.h
index d3ff1d22795..ea7efefaf08 100644
--- a/src/include/miscadmin.h
+++ b/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);
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 9488bf1857c..e1281b44730 100644
--- a/src/include/storage/proc.h
+++ b/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