Deduplicate freeze plans in freeze WAL records.

Make heapam WAL records that describe freezing performed by VACUUM more
space efficient by storing each distinct "freeze plan" once, alongside
an array of associated page offset numbers (one per freeze plan).  The
freeze plans required for most heap pages tend to naturally have a great
deal of redundancy, so this technique is very effective in practice.  It
often leads to freeze WAL records that are less than 20% of the size of
equivalent WAL records generated using the previous approach.

The freeze plan concept was introduced by commit 3b97e6823b, which fixed
bugs in VACUUM's handling of MultiXacts.  We retain the concept of
freeze plans, but go back to using page offset number arrays.  There is
no loss of generality here because deduplication is an additive process
that gets applied mechanically when FREEZE_PAGE WAL records are built.

More than anything else, freeze plan deduplication is an optimization
that reduces the marginal cost of freezing additional tuples on pages
that will need to have at least one or two tuples frozen in any case.
Ongoing work that adds page-level freezing to VACUUM will take full
advantage of the improved cost profile through batching.

Also refactor some of the details surrounding recovery conflicts needed
to REDO freeze records in passing: make original execution responsible
for generating a standard latestRemovedXid cutoff, rather than working
backwards to get the same cutoff in the REDO routine.  Bugfix commit
66fbcb0d2e did it the other way around, which is equivalent but obscures
what's going on.

Also rename the cutoff field from the WAL record/struct (rename the
field cutoff_xid to latestRemovedXid to match similar WAL records).
Processing of conflicts by REDO routines is already completely uniform,
so tools like pg_waldump should present the information driving the
process uniformly.  There are two remaining WAL record types that still
don't quite follow this convention (heapam's VISIBLE record type and
SP-GiST's VACUUM_REDIRECT record type).  They can be brought into line
by later work that totally standardizes how the cutoffs are presented.

Bump XLOG_PAGE_MAGIC.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-By: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-By: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/CAH2-Wz=XytErMnb8FAyFd+OQEbiipB0Q2FmFdXrggPL4VBnRYQ@mail.gmail.com

src/include/access/heapam.h

@@ -99,6 +99,19 @@ typedef enum
 	HEAPTUPLE_DELETE_IN_PROGRESS	/* deleting xact is still in progress */
 } HTSV_Result;
 
+/* heap_prepare_freeze_tuple state describing how to freeze a tuple */
+typedef struct HeapTupleFreeze
+{
+	/* Fields describing how to process tuple */
+	TransactionId xmax;
+	uint16		t_infomask2;
+	uint16		t_infomask;
+	uint8		frzflags;
+
+	/* Page offset number for tuple */
+	OffsetNumber offset;
+} HeapTupleFreeze;
+
 /* ----------------
  *		function prototypes for heap access method
  *
@@ -164,6 +177,15 @@ extern TM_Result heap_lock_tuple(Relation relation, HeapTuple tuple,
 								 Buffer *buffer, struct TM_FailureData *tmfd);
 
 extern void heap_inplace_update(Relation relation, HeapTuple tuple);
+extern bool heap_prepare_freeze_tuple(HeapTupleHeader tuple,
+									  TransactionId relfrozenxid, TransactionId relminmxid,
+									  TransactionId cutoff_xid, TransactionId cutoff_multi,
+									  HeapTupleFreeze *frz, bool *totally_frozen,
+									  TransactionId *relfrozenxid_out,
+									  MultiXactId *relminmxid_out);
+extern void heap_freeze_execute_prepared(Relation rel, Buffer buffer,
+										 TransactionId FreezeLimit,
+										 HeapTupleFreeze *tuples, int ntuples);
 extern bool heap_freeze_tuple(HeapTupleHeader tuple,
 							  TransactionId relfrozenxid, TransactionId relminmxid,
 							  TransactionId cutoff_xid, TransactionId cutoff_multi);

src/include/access/heapam_xlog.h

@@ -314,35 +314,42 @@ typedef struct xl_heap_inplace
 #define SizeOfHeapInplace	(offsetof(xl_heap_inplace, offnum) + sizeof(OffsetNumber))
 
 /*
- * This struct represents a 'freeze plan', which is what we need to know about
- * a single tuple being frozen during vacuum.
+ * This struct represents a 'freeze plan', which describes how to freeze a
+ * group of one or more heap tuples (appears in xl_heap_freeze_page record)
  */
 /* 0x01 was XLH_FREEZE_XMIN */
 #define		XLH_FREEZE_XVAC		0x02
 #define		XLH_INVALID_XVAC	0x04
 
-typedef struct xl_heap_freeze_tuple
+typedef struct xl_heap_freeze_plan
 {
 	TransactionId xmax;
-	OffsetNumber offset;
 	uint16		t_infomask2;
 	uint16		t_infomask;
 	uint8		frzflags;
-} xl_heap_freeze_tuple;
+
+	/* Length of individual page offset numbers array for this plan */
+	uint16		ntuples;
+} xl_heap_freeze_plan;
 
 /*
  * This is what we need to know about a block being frozen during vacuum
  *
- * Backup block 0's data contains an array of xl_heap_freeze_tuple structs,
- * one for each tuple.
+ * Backup block 0's data contains an array of xl_heap_freeze_plan structs
+ * (with nplans elements), followed by one or more page offset number arrays.
+ * Each such page offset number array corresponds to a single freeze plan
+ * (REDO routine freezes corresponding heap tuples using freeze plan).
  */
 typedef struct xl_heap_freeze_page
 {
-	TransactionId cutoff_xid;
-	uint16		ntuples;
+	TransactionId latestRemovedXid;
+	uint16		nplans;
+
+	/* FREEZE PLANS FOLLOW */
+	/* OFFSET NUMBER ARRAY FOLLOWS */
 } xl_heap_freeze_page;
 
-#define SizeOfHeapFreezePage (offsetof(xl_heap_freeze_page, ntuples) + sizeof(uint16))
+#define SizeOfHeapFreezePage (offsetof(xl_heap_freeze_page, nplans) + sizeof(uint16))
 
 /*
  * This is what we need to know about setting a visibility map bit
@@ -401,20 +408,6 @@ extern void heap2_desc(StringInfo buf, XLogReaderState *record);
 extern const char *heap2_identify(uint8 info);
 extern void heap_xlog_logical_rewrite(XLogReaderState *r);
 
-extern XLogRecPtr log_heap_freeze(Relation reln, Buffer buffer,
-								  TransactionId cutoff_xid, xl_heap_freeze_tuple *tuples,
-								  int ntuples);
-extern bool heap_prepare_freeze_tuple(HeapTupleHeader tuple,
-									  TransactionId relfrozenxid,
-									  TransactionId relminmxid,
-									  TransactionId cutoff_xid,
-									  TransactionId cutoff_multi,
-									  xl_heap_freeze_tuple *frz,
-									  bool *totally_frozen,
-									  TransactionId *relfrozenxid_out,
-									  MultiXactId *relminmxid_out);
-extern void heap_execute_freeze_tuple(HeapTupleHeader tuple,
-									  xl_heap_freeze_tuple *frz);
 extern XLogRecPtr log_heap_visible(RelFileLocator rlocator, Buffer heap_buffer,
 								   Buffer vm_buffer, TransactionId cutoff_xid, uint8 vmflags);
 

src/include/access/xlog_internal.h

@@ -31,7 +31,7 @@
 /*
  * Each page of XLOG file has a header like this:
  */
-#define XLOG_PAGE_MAGIC 0xD110	/* can be used as WAL version indicator */
+#define XLOG_PAGE_MAGIC 0xD111	/* can be used as WAL version indicator */
 
 typedef struct XLogPageHeaderData
 {