Revisit cosmetics of "For inplace update, send nontransactional invalidations."

This removes a never-used CacheInvalidateHeapTupleInplace() parameter.
It adds README content about inplace update visibility in logical
decoding.  It rewrites other comments.

Back-patch to v18, where commit 243e9b40f1b2dd09d6e5bf91ebf6e822a2cd3704
first appeared.  Since this removes a CacheInvalidateHeapTupleInplace()
parameter, expect a v18 ".abi-compliance-history" edit to follow.  PGXN
contains no calls to that function.

Reported-by: Paul A Jungwirth <pj@illuminatedcomputing.com>
Reported-by: Ilyasov Ian <ianilyasov@outlook.com>
Reviewed-by: Paul A Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Surya Poondla <s_poondla@apple.com>
Discussion: https://postgr.es/m/CA+renyU+LGLvCqS0=fHit-N1J-2=2_mPK97AQxvcfKm+F-DxJA@mail.gmail.com
Backpatch-through: 18

2 files changed, 46 insertions, 16 deletions

diff --git a/src/backend/access/heap/README.tuplock b/src/backend/access/heap/README.tuplock
index 843c2e58f92..16f7d78b7d2 100644
--- a/src/backend/access/heap/README.tuplock
+++ b/src/backend/access/heap/README.tuplock
@@ -199,3 +199,35 @@ under a reader holding a pin.  A reader of a heap_fetch() result tuple may
 witness a torn read.  Current inplace-updated fields are aligned and are no
 wider than four bytes, and current readers don't need consistency across
 fields.  Hence, they get by with just fetching each field once.
+
+During logical decoding, caches reflect an inplace update no later than the
+next XLOG_XACT_INVALIDATIONS.  That record witnesses the end of a command.
+Tuples of its cmin are then visible to decoding, as are inplace updates of any
+lower LSN.  Inplace updates of a higher LSN may also be visible, even if those
+updates would have been invisible to a non-historic snapshot matching
+decoding's historic snapshot.  (In other words, decoding may see inplace
+updates that were not visible to a similar snapshot taken during original
+transaction processing.)  That's a consequence of inplace update violating
+MVCC: there are no snapshot-specific versions of inplace-updated values.  This
+all makes it hard to reason about inplace-updated column reads during logical
+decoding, but the behavior does suffice for relhasindex.  A relhasindex=t in
+CREATE INDEX becomes visible no later than the new pg_index row.  While it may
+be visible earlier, that's harmless.  Finding zero indexes despite
+relhasindex=t is normal in more cases than this, e.g. after DROP INDEX.
+Example of a case that meaningfully reacts to the inplace inval:
+
+CREATE TABLE cat (c int) WITH (user_catalog_table = true);
+CREATE TABLE normal (d int);
+...
+CREATE INDEX ON cat (c)\; INSERT INTO normal VALUES (1);
+
+If the output plugin reads "cat" during decoding of the INSERT, it's fair to
+want that read to see relhasindex=t and use the new index.
+
+An alternative would be to have decoding of XLOG_HEAP_INPLACE immediately
+execute its invals.  That would behave more like invals during original
+transaction processing.  It would remove the decoding-specific delay in e.g. a
+decoding plugin witnessing a relfrozenxid change.  However, a good use case
+for that is unlikely, since the plugin would still witness relfrozenxid
+changes prematurely.  Hence, inplace update takes the trivial approach of
+delegating to XLOG_XACT_INVALIDATIONS.
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 3be8fabd7fb..6daf4a87dec 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -6396,15 +6396,17 @@ heap_inplace_lock(Relation relation,
 	Assert(BufferIsValid(buffer));
 
 	/*
-	 * Construct shared cache inval if necessary.  Because we pass a tuple
-	 * version without our own inplace changes or inplace changes other
-	 * sessions complete while we wait for locks, inplace update mustn't
-	 * change catcache lookup keys.  But we aren't bothering with index
-	 * updates either, so that's true a fortiori.  After LockBuffer(), it
-	 * would be too late, because this might reach a
-	 * CatalogCacheInitializeCache() that locks "buffer".
+	 * Register shared cache invals if necessary.  Other sessions may finish
+	 * inplace updates of this tuple between this step and LockTuple().  Since
+	 * inplace updates don't change cache keys, that's harmless.
+	 *
+	 * While it's tempting to register invals only after confirming we can
+	 * return true, the following obstacle precludes reordering steps that
+	 * way.  Registering invals might reach a CatalogCacheInitializeCache()
+	 * that locks "buffer".  That would hang indefinitely if running after our
+	 * own LockBuffer().  Hence, we must register invals before LockBuffer().
 	 */
-	CacheInvalidateHeapTupleInplace(relation, oldtup_ptr, NULL);
+	CacheInvalidateHeapTupleInplace(relation, oldtup_ptr);
 
 	LockTuple(relation, &oldtup.t_self, InplaceUpdateTupleLock);
 	LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
@@ -6642,10 +6644,6 @@ heap_inplace_update_and_unlock(Relation relation,
 	/*
 	 * Send invalidations to shared queue.  SearchSysCacheLocked1() assumes we
 	 * do this before UnlockTuple().
-	 *
-	 * If we're mutating a tuple visible only to this transaction, there's an
-	 * equivalent transactional inval from the action that created the tuple,
-	 * and this inval is superfluous.
 	 */
 	AtInplace_Inval();
 
@@ -6656,10 +6654,10 @@ heap_inplace_update_and_unlock(Relation relation,
 	AcceptInvalidationMessages();	/* local processing of just-sent inval */
 
 	/*
-	 * Queue a transactional inval.  The immediate invalidation we just sent
-	 * is the only one known to be necessary.  To reduce risk from the
-	 * transition to immediate invalidation, continue sending a transactional
-	 * invalidation like we've long done.  Third-party code might rely on it.
+	 * Queue a transactional inval, for logical decoding and for third-party
+	 * code that might have been relying on it since long before inplace
+	 * update adopted immediate invalidation.  See README.tuplock section
+	 * "Reading inplace-updated columns" for logical decoding details.
 	 */
 	if (!IsBootstrapProcessingMode())
 		CacheInvalidateHeapTuple(relation, tuple, NULL);