Rethink handling of [Prevent|Is]InTransactionBlock in pipeline mode.

Commits f92944137 et al. made IsInTransactionBlock() set the
XACT_FLAGS_NEEDIMMEDIATECOMMIT flag before returning "false",
on the grounds that that kept its API promises equivalent to those of
PreventInTransactionBlock().  This turns out to be a bad idea though,
because it allows an ANALYZE in a pipelined series of commands to
cause an immediate commit, which is unexpected.

Furthermore, if we return "false" then we have another issue,
which is that ANALYZE will decide it's allowed to do internal
commit-and-start-transaction sequences, thus possibly unexpectedly
committing the effects of previous commands in the pipeline.

To fix the latter situation, invent another transaction state flag
XACT_FLAGS_PIPELINING, which explicitly records the fact that we
have executed some extended-protocol command and not yet seen a
commit for it.  Then, require that flag to not be set before allowing
InTransactionBlock() to return "false".

Having done that, we can remove its setting of NEEDIMMEDIATECOMMIT
without fear of causing problems.  This means that the API guarantees
of IsInTransactionBlock now diverge from PreventInTransactionBlock,
which is mildly annoying, but it seems OK given the very limited usage
of IsInTransactionBlock.  (In any case, a caller preferring the old
behavior could always set NEEDIMMEDIATECOMMIT for itself.)

For consistency also require XACT_FLAGS_PIPELINING to not be set
in PreventInTransactionBlock.  This too is meant to prevent commands
such as CREATE DATABASE from silently committing previous commands
in a pipeline.

Per report from Peter Eisentraut.  As before, back-patch to all
supported branches (which sadly no longer includes v10).

Discussion: https://postgr.es/m/65a899dd-aebc-f667-1d0a-abb89ff3abf8@enterprisedb.com

1 files changed, 7 insertions, 0 deletions

diff --git a/src/include/access/xact.h b/src/include/access/xact.h
index c604ee11f85..898b065b4f3 100644
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.h
@@ -114,6 +114,13 @@ extern PGDLLIMPORT int MyXactFlags;
 #define XACT_FLAGS_NEEDIMMEDIATECOMMIT			(1U << 2)
 
 /*
+ * XACT_FLAGS_PIPELINING - set when we complete an extended-query-protocol
+ * Execute message.  This is useful for detecting that an implicit transaction
+ * block has been created via pipelining.
+ */
+#define XACT_FLAGS_PIPELINING					(1U << 3)
+
+/*
  *	start- and end-of-transaction callbacks for dynamically loaded modules
  */
 typedef enum