Avoid having two copies of the HOT-chain search logic.

It's been like this since HOT was originally introduced, but the logic
is complex enough that this is a recipe for bugs, as we've already
found out with SSI.  So refactor heap_hot_search_buffer() so that it
can satisfy the needs of index_getnext(), and make index_getnext() use
that rather than duplicating the logic.

This change was originally proposed by Heikki Linnakangas as part of a
larger refactoring oriented towards allowing index-only scans.  I
extracted and adjusted this part, since it seems to have independent
merit.  Review by Jeff Davis.

src/include/access/heapam.h

@@ -83,7 +83,8 @@ extern bool heap_fetch(Relation relation, Snapshot snapshot,
 		   HeapTuple tuple, Buffer *userbuf, bool keep_buf,
 		   Relation stats_relation);
 extern bool heap_hot_search_buffer(ItemPointer tid, Relation relation,
-					   Buffer buffer, Snapshot snapshot, bool *all_dead);
+					   Buffer buffer, Snapshot snapshot, HeapTuple heapTuple,
+					   bool *all_dead, bool first_call);
 extern bool heap_hot_search(ItemPointer tid, Relation relation,
 				Snapshot snapshot, bool *all_dead);
 

src/include/access/relscan.h

@@ -84,9 +84,7 @@ typedef struct IndexScanDescData
 	bool		xs_recheck;		/* T means scan keys must be rechecked */
 
 	/* state data for traversing HOT chains in index_getnext */
-	bool		xs_hot_dead;	/* T if all members of HOT chain are dead */
-	OffsetNumber xs_next_hot;	/* next member of HOT chain, if any */
-	TransactionId xs_prev_xmax; /* previous HOT chain member's XMAX, if any */
+	bool		xs_continue_hot;	/* T if must keep walking HOT chain */
 }	IndexScanDescData;
 
 /* Struct for heap-or-index scans of system tables */