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

diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index ce29da901213e7cc40b1f43c80b63ed00198f39e..f15ae87659c3fa158fd47708d3b0ffaa86a05267 100644 (file)
--- 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 e4ac84354ef7bea674cf925ef6e5eed32bcce777..7fb9ea733dcbffa465e0e3e71bbc17d1e623996d 100644 (file)
--- 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 d3ff1d22795216596b2b246dcbbb120a9040f2d9..ea7efefaf0824e7b81606cfe3a32d63b42c1cc2d 100644 (file)
--- 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 9488bf1857c8e5b9b3675cac5786bea0989903ec..e1281b44730ff0bd31824e999132c22f2e1a5101 100644 (file)
--- 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