Document WAL rules related to PD_ALL_VISIBLE in README.

Also improve comments.

Discussion: https://postgr.es/m/a50005c1c537f89bb359057fd70e66bb83bce969.camel@j-davis.com
Reviewed-by: Peter Geoghegan

src/backend/access/heap/heapam.c

@@ -8193,8 +8193,10 @@ log_heap_freeze(Relation reln, Buffer buffer, TransactionId cutoff_xid,
  * corresponding visibility map block.  Both should have already been modified
  * and dirtied.
  *
- * If checksums are enabled, we also generate a full-page image of
- * heap_buffer, if necessary.
+ * If checksums or wal_log_hints are enabled, we may also generate a full-page
+ * image of heap_buffer. Otherwise, we optimize away the FPI (by specifying
+ * REGBUF_NO_IMAGE for the heap buffer), in which case the caller should *not*
+ * update the heap page's LSN.
  */
 XLogRecPtr
 log_heap_visible(RelFileLocator rlocator, Buffer heap_buffer, Buffer vm_buffer,

src/backend/access/heap/visibilitymap.c

@@ -223,13 +223,13 @@ visibilitymap_pin_ok(BlockNumber heapBlk, Buffer vmbuf)
  *	visibilitymap_set - set bit(s) on a previously pinned page
  *
  * recptr is the LSN of the XLOG record we're replaying, if we're in recovery,
- * or InvalidXLogRecPtr in normal running.  The page LSN is advanced to the
+ * or InvalidXLogRecPtr in normal running.  The VM page LSN is advanced to the
  * one provided; in normal running, we generate a new XLOG record and set the
- * page LSN to that value.  cutoff_xid is the largest xmin on the page being
- * marked all-visible; it is needed for Hot Standby, and can be
- * InvalidTransactionId if the page contains no tuples.  It can also be set
- * to InvalidTransactionId when a page that is already all-visible is being
- * marked all-frozen.
+ * page LSN to that value (though the heap page's LSN may *not* be updated;
+ * see below).  cutoff_xid is the largest xmin on the page being marked
+ * all-visible; it is needed for Hot Standby, and can be InvalidTransactionId
+ * if the page contains no tuples.  It can also be set to InvalidTransactionId
+ * when a page that is already all-visible is being marked all-frozen.
  *
  * Caller is expected to set the heap page's PD_ALL_VISIBLE bit before calling
  * this function. Except in recovery, caller should also pass the heap
@@ -289,6 +289,11 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
 				/*
 				 * If data checksums are enabled (or wal_log_hints=on), we
 				 * need to protect the heap page from being torn.
+				 *
+				 * If not, then we must *not* update the heap page's LSN. In
+				 * this case, the FPI for the heap page was omitted from the
+				 * WAL record inserted above, so it would be incorrect to
+				 * update the heap page's LSN.
 				 */
 				if (XLogHintBitIsNeeded())
 				{

src/backend/access/transam/README

@@ -645,6 +645,23 @@ If you do decide to optimise away a WAL record, then any calls to
 MarkBufferDirty() must be replaced by MarkBufferDirtyHint(),
 otherwise you will expose the risk of partial page writes.
 
+The all-visible hint in a heap page (PD_ALL_VISIBLE) is a special
+case, because it is treated like a durable change in some respects and
+a hint in other respects. It must satisfy the invariant that, if a
+heap page's associated visibilitymap (VM) bit is set, then
+PD_ALL_VISIBLE is set on the heap page itself. Clearing of
+PD_ALL_VISIBLE is always treated like a fully-durable change to
+maintain this invariant. Additionally, if checksums or wal_log_hints
+are enabled, setting PD_ALL_VISIBLE is also treated like a
+fully-durable change to protect against torn pages.
+
+But, if neither checksums nor wal_log_hints are enabled, torn pages
+are of no consequence if the only change is to PD_ALL_VISIBLE; so no
+full heap page image is taken, and the heap page's LSN is not
+updated. NB: it would be incorrect to update the heap page's LSN when
+applying this optimization, even though there is an associated WAL
+record, because subsequent modifiers (e.g. an unrelated UPDATE) of the
+page may falsely believe that a full page image is not required.
 
 Write-Ahead Logging for Filesystem Actions
 ------------------------------------------