Move BRIN page type to page's last two bytes

... which is the usual convention among AMs, so that pg_filedump and
similar utilities can tell apart pages of different AMs.  It was also
the intent of the original code, but I failed to realize that alignment
considerations would move the whole thing to the previous-to-last word
in the page.

The new definition of the associated macro makes surrounding code a bit
leaner, too.

Per note from Heikki at
http://www.postgresql.org/message-id/546A16EF.9070005@vmware.com

contrib/pageinspect/brinfuncs.c

@@ -58,12 +58,9 @@ brin_page_type(PG_FUNCTION_ARGS)
 {
 	bytea	   *raw_page = PG_GETARG_BYTEA_P(0);
 	Page		page = VARDATA(raw_page);
-	BrinSpecialSpace *special;
 	char *type;
 
-	special = (BrinSpecialSpace *) PageGetSpecialPointer(page);
-
-	switch (special->type)
+	switch (BrinPageType(page))
 	{
 		case BRIN_PAGETYPE_META:
 			type = "meta";
@@ -75,7 +72,7 @@ brin_page_type(PG_FUNCTION_ARGS)
 			type = "regular";
 			break;
 		default:
-			type = psprintf("unknown (%02x)", special->type);
+			type = psprintf("unknown (%02x)", BrinPageType(page));
 			break;
 	}
 
@@ -91,7 +88,6 @@ verify_brin_page(bytea *raw_page, uint16 type, const char *strtype)
 {
 	Page	page;
 	int		raw_page_size;
-	BrinSpecialSpace *special;
 
 	raw_page_size = VARSIZE(raw_page) - VARHDRSZ;
 
@@ -104,13 +100,12 @@ verify_brin_page(bytea *raw_page, uint16 type, const char *strtype)
 	page = VARDATA(raw_page);
 
 	/* verify the special space says this page is what we want */
-	special = (BrinSpecialSpace *) PageGetSpecialPointer(page);
-	if (special->type != type)
+	if (BrinPageType(page) != type)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("page is not a BRIN page of type \"%s\"", strtype),
 				 errdetail("Expected special type %08x, got %08x.",
-						   type, special->type)));
+						   type, BrinPageType(page))));
 
 	return page;
 }

src/backend/access/brin/brin_pageops.c

@@ -53,7 +53,6 @@ brin_doupdate(Relation idxrel, BlockNumber pagesPerRange,
 	BrinTuple  *oldtup;
 	Size		oldsz;
 	Buffer		newbuf;
-	BrinSpecialSpace *special;
 	bool		extended = false;
 
 	newsz = MAXALIGN(newsz);
@@ -113,8 +112,6 @@ brin_doupdate(Relation idxrel, BlockNumber pagesPerRange,
 		return false;
 	}
 
-	special = (BrinSpecialSpace *) PageGetSpecialPointer(oldpage);
-
 	/*
 	 * Great, the old tuple is intact.  We can proceed with the update.
 	 *
@@ -124,7 +121,7 @@ brin_doupdate(Relation idxrel, BlockNumber pagesPerRange,
 	 * caller told us there isn't, if a concurrent update moved another tuple
 	 * elsewhere or replaced a tuple with a smaller one.
 	 */
-	if (((special->flags & BRIN_EVACUATE_PAGE) == 0) &&
+	if (((BrinPageFlags(oldpage) & BRIN_EVACUATE_PAGE) == 0) &&
 		brin_can_do_samepage_update(oldbuf, origsz, newsz))
 	{
 		if (BufferIsValid(newbuf))
@@ -374,12 +371,9 @@ brin_doinsert(Relation idxrel, BlockNumber pagesPerRange,
 void
 brin_page_init(Page page, uint16 type)
 {
-	BrinSpecialSpace *special;
-
 	PageInit(page, BLCKSZ, sizeof(BrinSpecialSpace));
 
-	special = (BrinSpecialSpace *) PageGetSpecialPointer(page);
-	special->type = type;
+	BrinPageType(page) = type;
 }
 
 /*
@@ -420,7 +414,6 @@ brin_start_evacuating_page(Relation idxRel, Buffer buf)
 {
 	OffsetNumber off;
 	OffsetNumber maxoff;
-	BrinSpecialSpace *special;
 	Page		page;
 
 	page = BufferGetPage(buf);
@@ -428,8 +421,6 @@ brin_start_evacuating_page(Relation idxRel, Buffer buf)
 	if (PageIsNew(page))
 		return false;
 
-	special = (BrinSpecialSpace *) PageGetSpecialPointer(page);
-
 	maxoff = PageGetMaxOffsetNumber(page);
 	for (off = FirstOffsetNumber; off <= maxoff; off++)
 	{
@@ -439,7 +430,7 @@ brin_start_evacuating_page(Relation idxRel, Buffer buf)
 		if (ItemIdIsUsed(lp))
 		{
 			/* prevent other backends from adding more stuff to this page */
-			special->flags |= BRIN_EVACUATE_PAGE;
+			BrinPageFlags(page) |= BRIN_EVACUATE_PAGE;
 			MarkBufferDirtyHint(buf, true);
 
 			return true;
@@ -463,8 +454,7 @@ brin_evacuate_page(Relation idxRel, BlockNumber pagesPerRange,
 
 	page = BufferGetPage(buf);
 
-	Assert(((BrinSpecialSpace *)
-			PageGetSpecialPointer(page))->flags & BRIN_EVACUATE_PAGE);
+	Assert(BrinPageFlags(page) & BRIN_EVACUATE_PAGE);
 
 	maxoff = PageGetMaxOffsetNumber(page);
 	for (off = FirstOffsetNumber; off <= maxoff; off++)
@@ -677,11 +667,8 @@ brin_getinsertbuffer(Relation irel, Buffer oldbuf, Size itemsz,
 static Size
 br_page_get_freespace(Page page)
 {
-	BrinSpecialSpace *special;
-
-	special = (BrinSpecialSpace *) PageGetSpecialPointer(page);
 	if (!BRIN_IS_REGULAR_PAGE(page) ||
-		(special->flags & BRIN_EVACUATE_PAGE) != 0)
+		(BrinPageFlags(page) & BRIN_EVACUATE_PAGE) != 0)
 		return 0;
 	else
 		return PageGetFreeSpace(page);

src/backend/access/brin/brin_revmap.c

@@ -446,7 +446,7 @@ revmap_physical_extend(BrinRevmap *revmap)
 		ereport(ERROR,
 				(errcode(ERRCODE_INDEX_CORRUPTED),
 				 errmsg("unexpected page type 0x%04X in BRIN index \"%s\" block %u",
-						BRIN_PAGE_TYPE(page),
+						BrinPageType(page),
 						RelationGetRelationName(irel),
 						BufferGetBlockNumber(buf))));
 

src/include/access/brin_page.h

@@ -20,24 +20,44 @@
 #include "storage/block.h"
 #include "storage/itemptr.h"
 
+/*
+ * Special area of BRIN pages.
+ *
+ * We define it in this odd way so that it always occupies the last
+ * MAXALIGN-sized element of each page.
+ */
+typedef struct BrinSpecialSpace
+{
+	uint16		vector[MAXALIGN(1) / sizeof(uint16)];
+} BrinSpecialSpace;
+
+/*
+ * Make the page type be the last half-word in the page, for consumption by
+ * pg_filedump and similar utilities.  We don't really care much about the
+ * position of the "flags" half-word, but it's simpler to apply a consistent
+ * rule to both.
+ *
+ * See comments above GinPageOpaqueData.
+ */
+#define BrinPageType(page)		\
+	(((BrinSpecialSpace *)  	\
+	  PageGetSpecialPointer(page))->vector[MAXALIGN(1) / sizeof(uint16) - 1])
+
+#define BrinPageFlags(page)		\
+	(((BrinSpecialSpace *)  	\
+	  PageGetSpecialPointer(page))->vector[MAXALIGN(1) / sizeof(uint16) - 2])
+
 /* special space on all BRIN pages stores a "type" identifier */
 #define		BRIN_PAGETYPE_META			0xF091
 #define		BRIN_PAGETYPE_REVMAP		0xF092
 #define		BRIN_PAGETYPE_REGULAR		0xF093
 
-#define BRIN_PAGE_TYPE(page)	\
-	(((BrinSpecialSpace *) PageGetSpecialPointer(page))->type)
-#define BRIN_IS_REVMAP_PAGE(page) (BRIN_PAGE_TYPE(page) == BRIN_PAGETYPE_REVMAP)
-#define BRIN_IS_REGULAR_PAGE(page) (BRIN_PAGE_TYPE(page) == BRIN_PAGETYPE_REGULAR)
+#define BRIN_IS_REVMAP_PAGE(page) (BrinPageType(page) == BRIN_PAGETYPE_REVMAP)
+#define BRIN_IS_REGULAR_PAGE(page) (BrinPageType(page) == BRIN_PAGETYPE_REGULAR)
 
 /* flags for BrinSpecialSpace */
 #define		BRIN_EVACUATE_PAGE			(1 << 0)
 
-typedef struct BrinSpecialSpace
-{
-	uint16		flags;
-	uint16		type;
-} BrinSpecialSpace;
 
 /* Metapage definitions */
 typedef struct BrinMetaPageData

src/include/access/gin_private.h

@@ -24,10 +24,10 @@
  * Note: GIN does not include a page ID word as do the other index types.
  * This is OK because the opaque data is only 8 bytes and so can be reliably
  * distinguished by size.  Revisit this if the size ever increases.
- * Further note: as of 9.2, SP-GiST also uses 8-byte special space.  This is
- * still OK, as long as GIN isn't using all of the high-order bits in its
- * flags word, because that way the flags word cannot match the page ID used
- * by SP-GiST.
+ * Further note: as of 9.2, SP-GiST also uses 8-byte special space, as does
+ * BRIN as of 9.5.  This is still OK, as long as GIN isn't using all of the
+ * high-order bits in its flags word, because that way the flags word cannot
+ * match the page IDs used by SP-GiST and BRIN.
  */
 typedef struct GinPageOpaqueData
 {

src/include/access/spgist_private.h

@@ -64,6 +64,8 @@ typedef SpGistPageOpaqueData *SpGistPageOpaque;
  * which otherwise would have a hard time telling pages of different index
  * types apart.  It should be the last 2 bytes on the page.  This is more or
  * less "free" due to alignment considerations.
+ *
+ * See comments above GinPageOpaqueData.
  */
 #define SPGIST_PAGE_ID		0xFF82