Re: Parallel query hangs after a smart shutdown is issued

Thomas Munro <thomas.munro@gmail.com> writes:
> On Thu, Aug 13, 2020 at 6:00 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> One other thing I changed here was to remove PM_WAIT_READONLY from the
>> set of states in which we'll allow promotion to occur or a new walreceiver
>> to start.  I'm not convinced that either of those behaviors aren't
>> bugs; although if someone thinks they're right, we can certainly put
>> back PM_WAIT_CLIENTS in those checks.  (But, for example, it does not
>> appear possible to reach PM_WAIT_READONLY/PM_WAIT_CLIENTS state with
>> Shutdown == NoShutdown, so the test in MaybeStartWalReceiver sure looks
>> like confusingly dead code to me.  If we do want to allow restarting
>> the walreceiver in this state, the Shutdown condition needs fixed.)

> If a walreceiver is allowed to run, why should it not be allowed to
> restart?

I'd come to about the same conclusion after thinking more, so v2
attached undoes that change.  I think putting off promotion is fine
though; it'll get handled at the next postmaster start.  (It looks
like the state machine would just proceed to exit anyway if we allowed
the promotion, but that's a hard-to-test state transition that we could
do without.)

> Yeah, I suppose that other test'd need to be Shutdown <=
> SmartShutdown, just like we do in SIGHUP_handler().  Looking at other
> places where we test Shutdown == NoShutdown, one that jumps out is the
> autovacuum wraparound defence stuff and the nearby
> PMSIGNAL_START_AUTOVAC_WORKER code.

Oh, excellent point!  I'd not thought to look at tests of the Shutdown
variable, but yeah, those should be <= SmartShutdown if we want autovac
to continue to operate in this state.

I also noticed that where reaper() is dealing with startup process
exit(3), it unconditionally sets Shutdown = SmartShutdown which seems
pretty bogus; that variable's value should never be allowed to decrease,
but this could cause it.  In the attached I did

                 StartupStatus = STARTUP_NOT_RUNNING;
-                Shutdown = SmartShutdown;
+                Shutdown = Max(Shutdown, SmartShutdown);
                 TerminateChildren(SIGTERM);

But given that it's forcing immediate termination of all backends,
I wonder if that's not more like a FastShutdown?  (Scary here is
that the coverage report shows we're not testing this path, so who
knows if it works at all.)

            regards, tom lane

diff --git a/doc/src/sgml/ref/pg_ctl-ref.sgml b/doc/src/sgml/ref/pg_ctl-ref.sgml
index e31275a04e..3946fa52ea 100644
--- a/doc/src/sgml/ref/pg_ctl-ref.sgml
+++ b/doc/src/sgml/ref/pg_ctl-ref.sgml
@@ -185,8 +185,8 @@ PostgreSQL documentation
    <option>stop</option> mode shuts down the server that is running in
    the specified data directory.  Three different
    shutdown methods can be selected with the <option>-m</option>
-   option.  <quote>Smart</quote> mode waits for all active
-   clients to disconnect and any online backup to finish.
+   option.  <quote>Smart</quote> mode disallows new connections, then waits
+   for all existing clients to disconnect and any online backup to finish.
    If the server is in hot standby, recovery and streaming replication
    will be terminated once all clients have disconnected.
    <quote>Fast</quote> mode (the default) does not wait for clients to disconnect and
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 38e2c16ac2..e8ad4b67a3 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -148,8 +148,6 @@
 #define BACKEND_TYPE_BGWORKER    0x0008    /* bgworker process */
 #define BACKEND_TYPE_ALL        0x000F    /* OR of all the above */

-#define BACKEND_TYPE_WORKER        (BACKEND_TYPE_AUTOVAC | BACKEND_TYPE_BGWORKER)
-
 /*
  * List of active backends (or child processes anyway; we don't actually
  * know whether a given child has become a backend or is still in the
@@ -319,7 +317,7 @@ static bool FatalError = false; /* T if recovering from backend crash */
  *
  * Notice that this state variable does not distinguish *why* we entered
  * states later than PM_RUN --- Shutdown and FatalError must be consulted
- * to find that out.  FatalError is never true in PM_RECOVERY_* or PM_RUN
+ * to find that out.  FatalError is never true in PM_RECOVERY or PM_RUN
  * states, nor in PM_SHUTDOWN states (because we don't enter those states
  * when trying to recover from a crash).  It can be true in PM_STARTUP state,
  * because we don't clear it until we've successfully started WAL redo.
@@ -332,8 +330,8 @@ typedef enum
     PM_HOT_STANDBY,                /* in hot standby mode */
     PM_RUN,                        /* normal "database is alive" state */
     PM_WAIT_BACKUP,                /* waiting for online backup mode to end */
-    PM_WAIT_READONLY,            /* waiting for read only backends to exit */
-    PM_WAIT_BACKENDS,            /* waiting for live backends to exit */
+    PM_WAIT_CLIENTS,            /* waiting for normal backends to exit */
+    PM_WAIT_BACKENDS,            /* waiting for all backends to exit */
     PM_SHUTDOWN,                /* waiting for checkpointer to do shutdown
                                  * ckpt */
     PM_SHUTDOWN_2,                /* waiting for archiver and walsenders to
@@ -2793,35 +2791,19 @@ pmdie(SIGNAL_ARGS)
             sd_notify(0, "STOPPING=1");
 #endif

-            if (pmState == PM_RUN || pmState == PM_RECOVERY ||
-                pmState == PM_HOT_STANDBY || pmState == PM_STARTUP)
-            {
-                /* autovac workers are told to shut down immediately */
-                /* and bgworkers too; does this need tweaking? */
-                SignalSomeChildren(SIGTERM,
-                                   BACKEND_TYPE_AUTOVAC | BACKEND_TYPE_BGWORKER);
-                /* and the autovac launcher too */
-                if (AutoVacPID != 0)
-                    signal_child(AutoVacPID, SIGTERM);
-                /* and the bgwriter too */
-                if (BgWriterPID != 0)
-                    signal_child(BgWriterPID, SIGTERM);
-                /* and the walwriter too */
-                if (WalWriterPID != 0)
-                    signal_child(WalWriterPID, SIGTERM);
-
-                /*
-                 * If we're in recovery, we can't kill the startup process
-                 * right away, because at present doing so does not release
-                 * its locks.  We might want to change this in a future
-                 * release.  For the time being, the PM_WAIT_READONLY state
-                 * indicates that we're waiting for the regular (read only)
-                 * backends to die off; once they do, we'll kill the startup
-                 * and walreceiver processes.
-                 */
-                pmState = (pmState == PM_RUN) ?
-                    PM_WAIT_BACKUP : PM_WAIT_READONLY;
-            }
+            /*
+             * If we reached normal running, we have to wait for any online
+             * backup mode to end; otherwise go straight to waiting for client
+             * backends to exit.  (The difference is that in the former state,
+             * we'll still let in new superuser clients, so that somebody can
+             * end the online backup mode.)  If already in PM_WAIT_BACKUP or a
+             * later state, do not change it.
+             */
+            if (pmState == PM_RUN)
+                pmState = PM_WAIT_BACKUP;
+            else if (pmState == PM_RECOVERY ||
+                     pmState == PM_HOT_STANDBY || pmState == PM_STARTUP)
+                pmState = PM_WAIT_CLIENTS;

             /*
              * Now wait for online backup mode to end and backends to exit. If
@@ -2871,16 +2853,15 @@ pmdie(SIGNAL_ARGS)
             }
             else if (pmState == PM_RUN ||
                      pmState == PM_WAIT_BACKUP ||
-                     pmState == PM_WAIT_READONLY ||
+                     pmState == PM_WAIT_CLIENTS ||
                      pmState == PM_WAIT_BACKENDS ||
                      pmState == PM_HOT_STANDBY)
             {
                 ereport(LOG,
                         (errmsg("aborting any active transactions")));
-                /* shut down all backends and workers */
+                /* shut down all backends and workers, but not walsenders */
                 SignalSomeChildren(SIGTERM,
-                                   BACKEND_TYPE_NORMAL | BACKEND_TYPE_AUTOVAC |
-                                   BACKEND_TYPE_BGWORKER);
+                                   BACKEND_TYPE_ALL - BACKEND_TYPE_WALSND);
                 /* and the autovac launcher too */
                 if (AutoVacPID != 0)
                     signal_child(AutoVacPID, SIGTERM);
@@ -2987,7 +2968,7 @@ reaper(SIGNAL_ARGS)
                 ereport(LOG,
                         (errmsg("shutdown at recovery target")));
                 StartupStatus = STARTUP_NOT_RUNNING;
-                Shutdown = SmartShutdown;
+                Shutdown = Max(Shutdown, SmartShutdown);
                 TerminateChildren(SIGTERM);
                 pmState = PM_WAIT_BACKENDS;
                 /* PostmasterStateMachine logic does the rest */
@@ -3713,7 +3694,7 @@ HandleChildCrash(int pid, int exitstatus, const char *procname)
         pmState == PM_HOT_STANDBY ||
         pmState == PM_RUN ||
         pmState == PM_WAIT_BACKUP ||
-        pmState == PM_WAIT_READONLY ||
+        pmState == PM_WAIT_CLIENTS ||
         pmState == PM_SHUTDOWN)
         pmState = PM_WAIT_BACKENDS;

@@ -3802,21 +3783,35 @@ PostmasterStateMachine(void)
          * PM_WAIT_BACKUP state ends when online backup mode is not active.
          */
         if (!BackupInProgress())
-            pmState = PM_WAIT_BACKENDS;
+            pmState = PM_WAIT_CLIENTS;
     }

-    if (pmState == PM_WAIT_READONLY)
+    if (pmState == PM_WAIT_CLIENTS)
     {
         /*
-         * PM_WAIT_READONLY state ends when we have no regular backends that
-         * have been started during recovery.  We kill the startup and
-         * walreceiver processes and transition to PM_WAIT_BACKENDS.  Ideally,
-         * we might like to kill these processes first and then wait for
-         * backends to die off, but that doesn't work at present because
-         * killing the startup process doesn't release its locks.
+         * PM_WAIT_CLIENTS state ends when we have no normal client backends
+         * running.  Then signal appropriate support processes, and transition
+         * to PM_WAIT_BACKENDS to wait for them to die.
          */
         if (CountChildren(BACKEND_TYPE_NORMAL) == 0)
         {
+            /*
+             * Signal all backend children except walsenders.  (While there
+             * can't be any normal children left, we might as well include
+             * BACKEND_TYPE_NORMAL in this mask, just to be sure.)
+             */
+            SignalSomeChildren(SIGTERM,
+                               BACKEND_TYPE_ALL - BACKEND_TYPE_WALSND);
+            /* and the autovac launcher too */
+            if (AutoVacPID != 0)
+                signal_child(AutoVacPID, SIGTERM);
+            /* and the bgwriter too */
+            if (BgWriterPID != 0)
+                signal_child(BgWriterPID, SIGTERM);
+            /* and the walwriter too */
+            if (WalWriterPID != 0)
+                signal_child(WalWriterPID, SIGTERM);
+            /* If we're in recovery, also stop startup and walreceiver procs */
             if (StartupPID != 0)
                 signal_child(StartupPID, SIGTERM);
             if (WalReceiverPID != 0)
@@ -3843,7 +3838,7 @@ PostmasterStateMachine(void)
          * later after writing the checkpoint record, like the archiver
          * process.
          */
-        if (CountChildren(BACKEND_TYPE_NORMAL | BACKEND_TYPE_WORKER) == 0 &&
+        if (CountChildren(BACKEND_TYPE_ALL - BACKEND_TYPE_WALSND) == 0 &&
             StartupPID == 0 &&
             WalReceiverPID == 0 &&
             BgWriterPID == 0 &&
@@ -5287,7 +5282,7 @@ sigusr1_handler(SIGNAL_ARGS)
     }

     if (CheckPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER) &&
-        Shutdown == NoShutdown)
+        Shutdown <= SmartShutdown)
     {
         /*
          * Start one iteration of the autovacuum daemon, even if autovacuuming
@@ -5302,7 +5297,7 @@ sigusr1_handler(SIGNAL_ARGS)
     }

     if (CheckPostmasterSignal(PMSIGNAL_START_AUTOVAC_WORKER) &&
-        Shutdown == NoShutdown)
+        Shutdown <= SmartShutdown)
     {
         /* The autovacuum launcher wants us to start a worker process. */
         StartAutovacuumWorker();
@@ -5333,7 +5328,7 @@ sigusr1_handler(SIGNAL_ARGS)

     if (StartupPID != 0 &&
         (pmState == PM_STARTUP || pmState == PM_RECOVERY ||
-         pmState == PM_HOT_STANDBY || pmState == PM_WAIT_READONLY) &&
+         pmState == PM_HOT_STANDBY) &&
         CheckPromoteSignal())
     {
         /*
@@ -5651,8 +5646,8 @@ MaybeStartWalReceiver(void)
 {
     if (WalReceiverPID == 0 &&
         (pmState == PM_STARTUP || pmState == PM_RECOVERY ||
-         pmState == PM_HOT_STANDBY || pmState == PM_WAIT_READONLY) &&
-        Shutdown == NoShutdown)
+         pmState == PM_HOT_STANDBY || pmState == PM_WAIT_CLIENTS) &&
+        Shutdown <= SmartShutdown)
     {
         WalReceiverPID = StartWalReceiver();
         if (WalReceiverPID != 0)
@@ -5905,7 +5900,7 @@ bgworker_should_start_now(BgWorkerStartTime start_time)
         case PM_SHUTDOWN_2:
         case PM_SHUTDOWN:
         case PM_WAIT_BACKENDS:
-        case PM_WAIT_READONLY:
+        case PM_WAIT_CLIENTS:
         case PM_WAIT_BACKUP:
             break;