Don't hold ProcArrayLock longer than needed in rare cases

While cancelling an autovacuum worker, we hold ProcArrayLock while
formatting a debugging log string.  We can make this shorter by saving
the data we need to produce the message and doing the formatting outside
the locked region.

This isn't terribly critical, as it only occurs pretty rarely: when a
backend runs deadlock detection and it happens to be blocked by a
autovacuum running autovacuum.  Still, there's no need to cause a hiccup
in ProcArrayLock processing, which can be very high-traffic in some
cases.

While at it, rework code so that we only print the string when it is
really going to be used, as suggested by Michael Paquier.

Discussion: https://postgr.es/m/20201118214127.GA3179@alvherre.pgsqlReviewed-by: Michael Paquier <michael@paquier.xyz>

src/backend/storage/lmgr/proc.c

@@ -1311,14 +1311,28 @@ ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable)
 		{
 			PGPROC	   *autovac = GetBlockingAutoVacuumPgproc();
 			uint8		statusFlags;
+			uint8		lockmethod_copy;
+			LOCKTAG		locktag_copy;
 
+			/*
+			 * Grab info we need, then release lock immediately.  Note this
+			 * coding means that there is a tiny chance that the process
+			 * terminates its current transaction and starts a different one
+			 * before we have a change to send the signal; the worst possible
+			 * consequence is that a for-wraparound vacuum is cancelled.  But
+			 * that could happen in any case unless we were to do kill() with
+			 * the lock held, which is much more undesirable.
+			 */
 			LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
+			statusFlags = ProcGlobal->statusFlags[autovac->pgxactoff];
+			lockmethod_copy = lock->tag.locktag_lockmethodid;
+			locktag_copy = lock->tag;
+			LWLockRelease(ProcArrayLock);
 
 			/*
 			 * Only do it if the worker is not working to protect against Xid
 			 * wraparound.
 			 */
-			statusFlags = ProcGlobal->statusFlags[autovac->pgxactoff];
 			if ((statusFlags & PROC_IS_AUTOVACUUM) &&
 				!(statusFlags & PROC_VACUUM_FOR_WRAPAROUND))
 			{
@@ -1326,25 +1340,25 @@ ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable)
 				StringInfoData locktagbuf;
 				StringInfoData logbuf;	/* errdetail for server log */
 
+				/* report the case, if configured to do so */
 				initStringInfo(&locktagbuf);
 				initStringInfo(&logbuf);
-				DescribeLockTag(&locktagbuf, &lock->tag);
+				DescribeLockTag(&locktagbuf, &locktag_copy);
 				appendStringInfo(&logbuf,
 								 _("Process %d waits for %s on %s."),
 								 MyProcPid,
-								 GetLockmodeName(lock->tag.locktag_lockmethodid,
-												 lockmode),
+								 GetLockmodeName(lockmethod_copy, lockmode),
 								 locktagbuf.data);
 
-				/* release lock as quickly as possible */
-				LWLockRelease(ProcArrayLock);
-
-				/* send the autovacuum worker Back to Old Kent Road */
 				ereport(DEBUG1,
 						(errmsg("sending cancel to blocking autovacuum PID %d",
 								pid),
 						 errdetail_log("%s", logbuf.data)));
 
+				pfree(logbuf.data);
+				pfree(locktagbuf.data);
+
+				/* send the autovacuum worker Back to Old Kent Road */
 				if (kill(pid, SIGINT) < 0)
 				{
 					/*
@@ -1362,12 +1376,7 @@ ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable)
 								(errmsg("could not send signal to process %d: %m",
 										pid)));
 				}
-
-				pfree(logbuf.data);
-				pfree(locktagbuf.data);
 			}
-			else
-				LWLockRelease(ProcArrayLock);
 
 			/* prevent signal from being sent again more than once */
 			allow_autovacuum_cancel = false;