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

The need for this was missed in commit 93db6cbda, 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

1 files changed, 12 insertions, 2 deletions

diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 5a3dd5d2d40..14e34d4e933 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -408,7 +408,7 @@ 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;
@@ -443,8 +443,18 @@ extern PGDLLIMPORT PGPROC *PreparedXactProcs;
 #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