Fix contrib/bloom to work for unlogged indexes.

blbuildempty did not do even approximately the right thing: it tried
to add a metapage to the relation's regular data fork, which already
has one at that point.  It should look like the ambuildempty methods
for all the standard index types, ie, initialize a metapage image in
some transient storage and then write it directly to the init fork.
To support that, refactor BloomInitMetapage into two functions.

In passing, fix BloomInitMetapage so it doesn't leave the rd_options
field of the index's relcache entry pointing at transient storage.
I'm not sure this had any visible consequence, since nothing much
else is likely to look at a bloom index's rd_options, but it's
certainly poor practice.

Per bug #14155 from Zhou Digoal.

Report: <20160524144146.22598.42558@wrigleys.postgresql.org>

contrib/bloom/blinsert.c

@@ -18,6 +18,7 @@
 #include "miscadmin.h"
 #include "storage/bufmgr.h"
 #include "storage/indexfsm.h"
+#include "storage/smgr.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
 
@@ -159,12 +160,26 @@ blbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 void
 blbuildempty(Relation index)
 {
-	if (RelationGetNumberOfBlocks(index) != 0)
-		elog(ERROR, "index \"%s\" already contains data",
-			 RelationGetRelationName(index));
+	Page		metapage;
 
-	/* Initialize the meta page */
-	BloomInitMetapage(index);
+	/* Construct metapage. */
+	metapage = (Page) palloc(BLCKSZ);
+	BloomFillMetapage(index, metapage);
+
+	/* Write the page.  If archiving/streaming, XLOG it. */
+	PageSetChecksumInplace(metapage, BLOOM_METAPAGE_BLKNO);
+	smgrwrite(index->rd_smgr, INIT_FORKNUM, BLOOM_METAPAGE_BLKNO,
+			  (char *) metapage, true);
+	if (XLogIsNeeded())
+		log_newpage(&index->rd_smgr->smgr_rnode.node, INIT_FORKNUM,
+					BLOOM_METAPAGE_BLKNO, metapage, false);
+
+	/*
+	 * An immediate sync is required even if we xlog'd the page, because the
+	 * write did not go through shared_buffers and therefore a concurrent
+	 * checkpoint may have moved the redo pointer past our xlog record.
+	 */
+	smgrimmedsync(index->rd_smgr, INIT_FORKNUM);
 }
 
 /*