Log a better message when canceling autovacuum.

The old message was at DEBUG2, so typically it didn't show up in the
log at all.  As a result, in most cases where autovacuum was canceled,
the only information that was logged was the table being vacuumed,
with no indication as to what problem caused the cancel.  Crank up
the level to LOG and add some more details to assist with debugging.

Back-patch all the way, per discussion on pgsql-hackers.

src/backend/storage/lmgr/proc.c

@@ -1100,13 +1100,30 @@ ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable)
 				!(autovac_pgxact->vacuumFlags & PROC_VACUUM_FOR_WRAPAROUND))
 			{
 				int			pid = autovac->pid;
+				StringInfoData locktagbuf;
+				StringInfoData logbuf;		/* errdetail for server log */
 
-				elog(DEBUG2, "sending cancel to blocking autovacuum PID %d",
-					 pid);
+				initStringInfo(&locktagbuf);
+				initStringInfo(&logbuf);
+				DescribeLockTag(&locktagbuf, &lock->tag);
+				appendStringInfo(&logbuf,
+					  _("Process %d waits for %s on %s"),
+						 MyProcPid,
+						 GetLockmodeName(lock->tag.locktag_lockmethodid,
+										 lockmode),
+						 locktagbuf.data);
 
-				/* don't hold the lock across the kill() syscall */
+				/* release lock as quickly as possible */
 				LWLockRelease(ProcArrayLock);
 
+				ereport(LOG,
+						(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)
 				{