Add a C API for parallel heap scans.

Using this API, one backend can set up a ParallelHeapScanDesc to
which multiple backends can then attach.  Each tuple in the relation
will be returned to exactly one of the scanning backends.  Only
forward scans are supported, and rescans must be carefully
coordinated.

This is not exposed to the planner or executor yet.

The original version of this code was written by me.  Amit Kapila
reviewed it, tested it, and improved it, including adding support for
synchronized scans, per review comments from Jeff Davis.  Extensive
testing of this and related patches was performed by Haribabu Kommi.
Final cleanup of this patch by me.

src/include/access/heapam.h

@@ -96,8 +96,9 @@ extern Relation heap_openrv_extended(const RangeVar *relation,
 
 #define heap_close(r,l)  relation_close(r,l)
 
-/* struct definition appears in relscan.h */
+/* struct definitions appear in relscan.h */
 typedef struct HeapScanDescData *HeapScanDesc;
+typedef struct ParallelHeapScanDescData *ParallelHeapScanDesc;
 
 /*
  * HeapScanIsValid
@@ -126,6 +127,11 @@ extern void heap_rescan_set_params(HeapScanDesc scan, ScanKey key,
 extern void heap_endscan(HeapScanDesc scan);
 extern HeapTuple heap_getnext(HeapScanDesc scan, ScanDirection direction);
 
+extern Size heap_parallelscan_estimate(Snapshot snapshot);
+extern void heap_parallelscan_initialize(ParallelHeapScanDesc target,
+							 Relation relation, Snapshot snapshot);
+extern HeapScanDesc heap_beginscan_parallel(Relation, ParallelHeapScanDesc);
+
 extern bool heap_fetch(Relation relation, Snapshot snapshot,
 		   HeapTuple tuple, Buffer *userbuf, bool keep_buf,
 		   Relation stats_relation);

src/include/access/relscan.h

@@ -20,6 +20,25 @@
 #include "access/itup.h"
 #include "access/tupdesc.h"
 
+/*
+ * Shared state for parallel heap scan.
+ *
+ * Each backend participating in a parallel heap scan has its own
+ * HeapScanDesc in backend-private memory, and those objects all contain
+ * a pointer to this structure.  The information here must be sufficient
+ * to properly initialize each new HeapScanDesc as workers join the scan,
+ * and it must act as a font of block numbers for those workers.
+ */
+typedef struct ParallelHeapScanDescData
+{
+	Oid			phs_relid;		/* OID of relation to scan */
+	bool		phs_syncscan;	/* report location to syncscan logic? */
+	BlockNumber phs_nblocks;	/* # blocks in relation at start of scan */
+	slock_t		phs_mutex;		/* mutual exclusion for block number fields */
+	BlockNumber phs_startblock; /* starting block number */
+	BlockNumber phs_cblock;		/* current block number */
+	char		phs_snapshot_data[FLEXIBLE_ARRAY_MEMBER];
+}	ParallelHeapScanDescData;
 
 typedef struct HeapScanDescData
 {
@@ -49,6 +68,7 @@ typedef struct HeapScanDescData
 	BlockNumber rs_cblock;		/* current block # in scan, if any */
 	Buffer		rs_cbuf;		/* current buffer in scan, if any */
 	/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
+	ParallelHeapScanDesc rs_parallel;	/* parallel scan information */
 
 	/* these fields only used in page-at-a-time mode and for bitmap scans */
 	int			rs_cindex;		/* current tuple's index in vistuples */