[PATCH] Transaction traceability - txid_status(bigint)

Hi all

Following on from

bigint txids vs 'xid' type, new txid_recent(bigint) => xid

https://www.postgresql.org/message-id/CAMsr+YFDZMN_iZ7KrRoe+j0KVLQvFVgvZxbcVxR-MLjgtoZugA@mail.gmail.com

here's a patch that implements a txid_status(bigint) function to report the
commit status of a function.

If an application is disconnected while a COMMIT request is in flight,
the backend crashes mid-commit, etc, then an application may not be
sure whether or not a commit completed successfully or was rolled
back. While two-phase commit solves this it does so at a considerable
overhead, so introduce a lighter alternative.

txid_status(bigint) lets an application determine the status of a
a commit based on an xid-with-epoch as returned by txid_current()
or similar. Status may be committed, aborted, in-progress (including
prepared xacts) or null if the xact is too old for its commit status
to still be retained because it has passed the wrap-around epoch
boundary.

Applications must call txid_current() in their transactions to make
much use of this since PostgreSQL does not automatically report an xid
to the client when one is assigned.

A future protocol enhancement to report txid assignment would be very
useful, but quite separate to this.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services