Skip WAL for new relfilenodes, under wal_level=minimal.

Until now, only selected bulk operations (e.g. COPY) did this.  If a
given relfilenode received both a WAL-skipping COPY and a WAL-logged
operation (e.g. INSERT), recovery could lose tuples from the COPY.  See
src/backend/access/transam/README section "Skipping WAL for New
RelFileNode" for the new coding rules.  Maintainers of table access
methods should examine that section.

To maintain data durability, just before commit, we choose between an
fsync of the relfilenode and copying its contents to WAL.  A new GUC,
wal_skip_threshold, guides that choice.  If this change slows a workload
that creates small, permanent relfilenodes under wal_level=minimal, try
adjusting wal_skip_threshold.  Users setting a timeout on COMMIT may
need to adjust that timeout, and log_min_duration_statement analysis
will reflect time consumption moving to COMMIT from commands like COPY.

Internally, this requires a reliable determination of whether
RollbackAndReleaseCurrentSubTransaction() would unlink a relation's
current relfilenode.  Introduce rd_firstRelfilenodeSubid.  Amend the
specification of rd_createSubid such that the field is zero when a new
rel has an old rd_node.  Make relcache.c retain entries for certain
dropped relations until end of transaction.

Bump XLOG_PAGE_MAGIC, since this introduces XLOG_GIST_ASSIGN_LSN.
Future servers accept older WAL, so this bump is discretionary.

Kyotaro Horiguchi, reviewed (in earlier, similar versions) by Robert
Haas.  Heikki Linnakangas and Michael Paquier implemented earlier
designs that materially clarified the problem.  Reviewed, in earlier
designs, by Andrew Dunstan, Andres Freund, Alvaro Herrera, Tom Lane,
Fujii Masao, and Simon Riggs.  Reported by Martijn van Oosterhout.

Discussion: https://postgr.es/m/20150702220524.GA9392@svana.org

src/backend/storage/buffer/bufmgr.c

@@ -67,7 +67,7 @@
 #define BUF_WRITTEN				0x01
 #define BUF_REUSABLE			0x02
 
-#define DROP_RELS_BSEARCH_THRESHOLD		20
+#define RELS_BSEARCH_THRESHOLD		20
 
 typedef struct PrivateRefCountEntry
 {
@@ -106,6 +106,19 @@ typedef struct CkptTsStatus
 	int			index;
 } CkptTsStatus;
 
+/*
+ * Type for array used to sort SMgrRelations
+ *
+ * FlushRelationsAllBuffers shares the same comparator function with
+ * DropRelFileNodesAllBuffers. Pointer to this struct and RelFileNode must be
+ * compatible.
+ */
+typedef struct SMgrSortArray
+{
+	RelFileNode rnode;			/* This must be the first member */
+	SMgrRelation srel;
+} SMgrSortArray;
+
 /* GUC variables */
 bool		zero_damaged_pages = false;
 int			bgwriter_lru_maxpages = 100;
@@ -2991,7 +3004,7 @@ DropRelFileNodesAllBuffers(RelFileNodeBackend *rnodes, int nnodes)
 	 * an exactly determined value, as it depends on many factors (CPU and RAM
 	 * speeds, amount of shared buffers etc.).
 	 */
-	use_bsearch = n > DROP_RELS_BSEARCH_THRESHOLD;
+	use_bsearch = n > RELS_BSEARCH_THRESHOLD;
 
 	/* sort the list of rnodes if necessary */
 	if (use_bsearch)
@@ -3241,6 +3254,104 @@ FlushRelationBuffers(Relation rel)
 	}
 }
 
+/* ---------------------------------------------------------------------
+ *		FlushRelationsAllBuffers
+ *
+ *		This function flushes out of the buffer pool all the pages of all
+ *		forks of the specified smgr relations.  It's equivalent to calling
+ *		FlushRelationBuffers once per fork per relation.  The relations are
+ *		assumed not to use local buffers.
+ * --------------------------------------------------------------------
+ */
+void
+FlushRelationsAllBuffers(SMgrRelation *smgrs, int nrels)
+{
+	int			i;
+	SMgrSortArray *srels;
+	bool		use_bsearch;
+
+	if (nrels == 0)
+		return;
+
+	/* fill-in array for qsort */
+	srels = palloc(sizeof(SMgrSortArray) * nrels);
+
+	for (i = 0; i < nrels; i++)
+	{
+		Assert(!RelFileNodeBackendIsTemp(smgrs[i]->smgr_rnode));
+
+		srels[i].rnode = smgrs[i]->smgr_rnode.node;
+		srels[i].srel = smgrs[i];
+	}
+
+	/*
+	 * Save the bsearch overhead for low number of relations to sync. See
+	 * DropRelFileNodesAllBuffers for details.
+	 */
+	use_bsearch = nrels > RELS_BSEARCH_THRESHOLD;
+
+	/* sort the list of SMgrRelations if necessary */
+	if (use_bsearch)
+		pg_qsort(srels, nrels, sizeof(SMgrSortArray), rnode_comparator);
+
+	/* Make sure we can handle the pin inside the loop */
+	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+	for (i = 0; i < NBuffers; i++)
+	{
+		SMgrSortArray *srelent = NULL;
+		BufferDesc *bufHdr = GetBufferDescriptor(i);
+		uint32		buf_state;
+
+		/*
+		 * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
+		 * and saves some cycles.
+		 */
+
+		if (!use_bsearch)
+		{
+			int			j;
+
+			for (j = 0; j < nrels; j++)
+			{
+				if (RelFileNodeEquals(bufHdr->tag.rnode, srels[j].rnode))
+				{
+					srelent = &srels[j];
+					break;
+				}
+			}
+
+		}
+		else
+		{
+			srelent = bsearch((const void *) &(bufHdr->tag.rnode),
+							  srels, nrels, sizeof(SMgrSortArray),
+							  rnode_comparator);
+		}
+
+		/* buffer doesn't belong to any of the given relfilenodes; skip it */
+		if (srelent == NULL)
+			continue;
+
+		ReservePrivateRefCountEntry();
+
+		buf_state = LockBufHdr(bufHdr);
+		if (RelFileNodeEquals(bufHdr->tag.rnode, srelent->rnode) &&
+			(buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
+		{
+			PinBuffer_Locked(bufHdr);
+			LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
+			FlushBuffer(bufHdr, srelent->srel);
+			LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
+			UnpinBuffer(bufHdr, true);
+		}
+		else
+			UnlockBufHdr(bufHdr, buf_state);
+	}
+
+	pfree(srels);
+}
+
 /* ---------------------------------------------------------------------
  *		FlushDatabaseBuffers
  *
@@ -3442,13 +3553,15 @@ MarkBufferDirtyHint(Buffer buffer, bool buffer_std)
 			(pg_atomic_read_u32(&bufHdr->state) & BM_PERMANENT))
 		{
 			/*
-			 * If we're in recovery we cannot dirty a page because of a hint.
-			 * We can set the hint, just not dirty the page as a result so the
-			 * hint is lost when we evict the page or shutdown.
+			 * If we must not write WAL, due to a relfilenode-specific
+			 * condition or being in recovery, don't dirty the page.  We can
+			 * set the hint, just not dirty the page as a result so the hint
+			 * is lost when we evict the page or shutdown.
 			 *
 			 * See src/backend/storage/page/README for longer discussion.
 			 */
-			if (RecoveryInProgress())
+			if (RecoveryInProgress() ||
+				RelFileNodeSkippingWAL(bufHdr->tag.rnode))
 				return;
 
 			/*

src/backend/storage/lmgr/lock.c

@@ -614,6 +614,18 @@ LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
 	return (locallock && locallock->nLocks > 0);
 }
 
+#ifdef USE_ASSERT_CHECKING
+/*
+ * GetLockMethodLocalHash -- return the hash of local locks, for modules that
+ *		evaluate assertions based on all locks held.
+ */
+HTAB *
+GetLockMethodLocalHash(void)
+{
+	return LockMethodLocalHash;
+}
+#endif
+
 /*
  * LockHasWaiters -- look up 'locktag' and check if releasing this
  *		lock would wake up other processes waiting for it.

src/backend/storage/smgr/md.c

@@ -248,11 +248,10 @@ mdcreate(SMgrRelation reln, ForkNumber forkNum, bool isRedo)
  * During replay, we would delete the file and then recreate it, which is fine
  * if the contents of the file were repopulated by subsequent WAL entries.
  * But if we didn't WAL-log insertions, but instead relied on fsyncing the
- * file after populating it (as for instance CLUSTER and CREATE INDEX do),
- * the contents of the file would be lost forever.  By leaving the empty file
- * until after the next checkpoint, we prevent reassignment of the relfilenode
- * number until it's safe, because relfilenode assignment skips over any
- * existing file.
+ * file after populating it (as we do at wal_level=minimal), the contents of
+ * the file would be lost forever.  By leaving the empty file until after the
+ * next checkpoint, we prevent reassignment of the relfilenode number until
+ * it's safe, because relfilenode assignment skips over any existing file.
  *
  * We do not need to go through this dance for temp relations, though, because
  * we never make WAL entries for temp rels, and so a temp rel poses no threat
@@ -877,12 +876,18 @@ mdtruncate(SMgrRelation reln, ForkNumber forknum, BlockNumber nblocks)
  *	mdimmedsync() -- Immediately sync a relation to stable storage.
  *
  * Note that only writes already issued are synced; this routine knows
- * nothing of dirty buffers that may exist inside the buffer manager.
+ * nothing of dirty buffers that may exist inside the buffer manager.  We
+ * sync active and inactive segments; smgrDoPendingSyncs() relies on this.
+ * Consider a relation skipping WAL.  Suppose a checkpoint syncs blocks of
+ * some segment, then mdtruncate() renders that segment inactive.  If we
+ * crash before the next checkpoint syncs the newly-inactive segment, that
+ * segment may survive recovery, reintroducing unwanted data into the table.
  */
 void
 mdimmedsync(SMgrRelation reln, ForkNumber forknum)
 {
 	int			segno;
+	int			min_inactive_seg;
 
 	/*
 	 * NOTE: mdnblocks makes sure we have opened all active segments, so that
@@ -890,7 +895,16 @@ mdimmedsync(SMgrRelation reln, ForkNumber forknum)
 	 */
 	mdnblocks(reln, forknum);
 
-	segno = reln->md_num_open_segs[forknum];
+	min_inactive_seg = segno = reln->md_num_open_segs[forknum];
+
+	/*
+	 * Temporarily open inactive segments, then close them after sync.  There
+	 * may be some inactive segments left opened after fsync() error, but that
+	 * is harmless.  We don't bother to clean them up and take a risk of
+	 * further trouble.  The next mdclose() will soon close them.
+	 */
+	while (_mdfd_openseg(reln, forknum, segno, 0) != NULL)
+		segno++;
 
 	while (segno > 0)
 	{
@@ -901,6 +915,14 @@ mdimmedsync(SMgrRelation reln, ForkNumber forknum)
 					(errcode_for_file_access(),
 					 errmsg("could not fsync file \"%s\": %m",
 							FilePathName(v->mdfd_vfd))));
+
+		/* Close inactive segments immediately */
+		if (segno > min_inactive_seg)
+		{
+			FileClose(v->mdfd_vfd);
+			_fdvec_resize(reln, forknum, segno - 1);
+		}
+
 		segno--;
 	}
 }

src/backend/storage/smgr/smgr.c

@@ -388,6 +388,41 @@ smgrdounlink(SMgrRelation reln, bool isRedo)
 	smgrsw[which].smgr_unlink(rnode, InvalidForkNumber, isRedo);
 }
 
+/*
+ *	smgrdosyncall() -- Immediately sync all forks of all given relations
+ *
+ *		All forks of all given relations are synced out to the store.
+ *
+ *		This is equivalent to FlushRelationBuffers() for each smgr relation,
+ *		then calling smgrimmedsync() for all forks of each relation, but it's
+ *		significantly quicker so should be preferred when possible.
+ */
+void
+smgrdosyncall(SMgrRelation *rels, int nrels)
+{
+	int			i = 0;
+	ForkNumber	forknum;
+
+	if (nrels == 0)
+		return;
+
+	FlushRelationsAllBuffers(rels, nrels);
+
+	/*
+	 * Sync the physical file(s).
+	 */
+	for (i = 0; i < nrels; i++)
+	{
+		int			which = rels[i]->smgr_which;
+
+		for (forknum = 0; forknum <= MAX_FORKNUM; forknum++)
+		{
+			if (smgrsw[which].smgr_exists(rels[i], forknum))
+				smgrsw[which].smgr_immedsync(rels[i], forknum);
+		}
+	}
+}
+
 /*
  *	smgrdounlinkall() -- Immediately unlink all forks of all given relations
  *