Clean up manipulations of hash indexes' hasho_flag field.

Standardize on testing a hash index page's type by doing
	(opaque->hasho_flag & LH_PAGE_TYPE) == LH_xxx_PAGE
Various places were taking shortcuts like
	opaque->hasho_flag & LH_BUCKET_PAGE
which while not actually wrong, is still bad practice because
it encourages use of
	opaque->hasho_flag & LH_UNUSED_PAGE
which *is* wrong (LH_UNUSED_PAGE == 0, so the above is constant false).
hash_xlog.c's hash_mask() contained such an incorrect test.

This also ensures that we mask out the additional flag bits that
hasho_flag has accreted since 9.6.  pgstattuple's pgstat_hash_page(),
for one, was failing to do that and was thus actively broken.

Also fix assorted comments that hadn't been updated to reflect the
extended usage of hasho_flag, and fix some macros that were testing
just "(hasho_flag & bit)" to use the less dangerous, project-approved
form "((hasho_flag & bit) != 0)".

Coverity found the bug in hash_mask(); I noted the one in
pgstat_hash_page() through code reading.

contrib/pageinspect/hashfuncs.c

@@ -184,7 +184,8 @@ hash_page_type(PG_FUNCTION_ARGS)
 	bytea	   *raw_page = PG_GETARG_BYTEA_P(0);
 	Page		page;
 	HashPageOpaque opaque;
-	char	   *type;
+	int			pagetype;
+	const char *type;
 
 	if (!superuser())
 		ereport(ERROR,
@@ -200,13 +201,14 @@ hash_page_type(PG_FUNCTION_ARGS)
 		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 
 		/* page type (flags) */
-		if (opaque->hasho_flag & LH_META_PAGE)
+		pagetype = opaque->hasho_flag & LH_PAGE_TYPE;
+		if (pagetype == LH_META_PAGE)
 			type = "metapage";
-		else if (opaque->hasho_flag & LH_OVERFLOW_PAGE)
+		else if (pagetype == LH_OVERFLOW_PAGE)
 			type = "overflow";
-		else if (opaque->hasho_flag & LH_BUCKET_PAGE)
+		else if (pagetype == LH_BUCKET_PAGE)
 			type = "bucket";
-		else if (opaque->hasho_flag & LH_BITMAP_PAGE)
+		else if (pagetype == LH_BITMAP_PAGE)
 			type = "bitmap";
 		else
 			type = "unused";

contrib/pgstattuple/pgstattuple.c

@@ -453,7 +453,7 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 		HashPageOpaque opaque;
 
 		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-		switch (opaque->hasho_flag)
+		switch (opaque->hasho_flag & LH_PAGE_TYPE)
 		{
 			case LH_UNUSED_PAGE:
 				stat->free_space += BLCKSZ;

src/backend/access/hash/hash_xlog.c

@@ -1234,6 +1234,7 @@ hash_mask(char *pagedata, BlockNumber blkno)
 {
 	Page		page = (Page) pagedata;
 	HashPageOpaque opaque;
+	int			pagetype;
 
 	mask_page_lsn(page);
 
@@ -1242,15 +1243,16 @@ hash_mask(char *pagedata, BlockNumber blkno)
 
 	opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 
-	if (opaque->hasho_flag & LH_UNUSED_PAGE)
+	pagetype = opaque->hasho_flag & LH_PAGE_TYPE;
+	if (pagetype == LH_UNUSED_PAGE)
 	{
 		/*
 		 * Mask everything on a UNUSED page.
 		 */
 		mask_page_content(page);
 	}
-	else if ((opaque->hasho_flag & LH_BUCKET_PAGE) ||
-			 (opaque->hasho_flag & LH_OVERFLOW_PAGE))
+	else if (pagetype == LH_BUCKET_PAGE ||
+			 pagetype == LH_OVERFLOW_PAGE)
 	{
 		/*
 		 * In hash bucket and overflow pages, it is possible to modify the

src/backend/access/hash/hashovfl.c

@@ -168,7 +168,7 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin)
 		if (retain_pin)
 		{
 			/* pin will be retained only for the primary bucket page */
-			Assert(pageopaque->hasho_flag & LH_BUCKET_PAGE);
+			Assert((pageopaque->hasho_flag & LH_PAGE_TYPE) == LH_BUCKET_PAGE);
 			LockBuffer(buf, BUFFER_LOCK_UNLOCK);
 		}
 		else

src/backend/access/hash/hashutil.c

@@ -218,8 +218,8 @@ _hash_get_totalbuckets(uint32 splitpoint_phase)
 /*
  * _hash_checkpage -- sanity checks on the format of all hash pages
  *
- * If flags is not zero, it is a bitwise OR of the acceptable values of
- * hasho_flag.
+ * If flags is not zero, it is a bitwise OR of the acceptable page types
+ * (values of hasho_flag & LH_PAGE_TYPE).
  */
 void
 _hash_checkpage(Relation rel, Buffer buf, int flags)

src/include/access/hash.h

@@ -41,13 +41,13 @@ typedef uint32 Bucket;
 /*
  * Special space for hash index pages.
  *
- * hasho_flag tells us which type of page we're looking at.  For
- * example, knowing overflow pages from bucket pages is necessary
- * information when you're deleting tuples from a page. If all the
- * tuples are deleted from an overflow page, the overflow is made
- * available to other buckets by calling _hash_freeovflpage(). If all
- * the tuples are deleted from a bucket page, no additional action is
- * necessary.
+ * hasho_flag's LH_PAGE_TYPE bits tell us which type of page we're looking at.
+ * Additional bits in the flag word are used for more transient purposes.
+ *
+ * To test a page's type, do (hasho_flag & LH_PAGE_TYPE) == LH_xxx_PAGE.
+ * However, we ensure that each used page type has a distinct bit so that
+ * we can OR together page types for uses such as the allowable-page-types
+ * argument of _hash_checkpage().
  */
 #define LH_UNUSED_PAGE			(0)
 #define LH_OVERFLOW_PAGE		(1 << 0)
@@ -60,7 +60,7 @@ typedef uint32 Bucket;
 #define LH_PAGE_HAS_DEAD_TUPLES	(1 << 7)
 
 #define LH_PAGE_TYPE \
-	(LH_OVERFLOW_PAGE|LH_BUCKET_PAGE|LH_BITMAP_PAGE|LH_META_PAGE)
+	(LH_OVERFLOW_PAGE | LH_BUCKET_PAGE | LH_BITMAP_PAGE | LH_META_PAGE)
 
 /*
  * In an overflow page, hasho_prevblkno stores the block number of the previous
@@ -78,16 +78,16 @@ typedef struct HashPageOpaqueData
 	BlockNumber hasho_prevblkno;	/* see above */
 	BlockNumber hasho_nextblkno;	/* see above */
 	Bucket		hasho_bucket;	/* bucket number this pg belongs to */
-	uint16		hasho_flag;		/* page type code, see above */
+	uint16		hasho_flag;		/* page type code + flag bits, see above */
 	uint16		hasho_page_id;	/* for identification of hash indexes */
 } HashPageOpaqueData;
 
 typedef HashPageOpaqueData *HashPageOpaque;
 
-#define H_NEEDS_SPLIT_CLEANUP(opaque)	((opaque)->hasho_flag & LH_BUCKET_NEEDS_SPLIT_CLEANUP)
-#define H_BUCKET_BEING_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_BEING_SPLIT)
-#define H_BUCKET_BEING_POPULATED(opaque)	((opaque)->hasho_flag & LH_BUCKET_BEING_POPULATED)
-#define H_HAS_DEAD_TUPLES(opaque)		((opaque)->hasho_flag & LH_PAGE_HAS_DEAD_TUPLES)
+#define H_NEEDS_SPLIT_CLEANUP(opaque)	(((opaque)->hasho_flag & LH_BUCKET_NEEDS_SPLIT_CLEANUP) != 0)
+#define H_BUCKET_BEING_SPLIT(opaque)	(((opaque)->hasho_flag & LH_BUCKET_BEING_SPLIT) != 0)
+#define H_BUCKET_BEING_POPULATED(opaque)	(((opaque)->hasho_flag & LH_BUCKET_BEING_POPULATED) != 0)
+#define H_HAS_DEAD_TUPLES(opaque)		(((opaque)->hasho_flag & LH_PAGE_HAS_DEAD_TUPLES) != 0)
 
 /*
  * The page ID is for the convenience of pg_filedump and similar utilities,