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Traditionally, libpq's pqPutMsgEnd has rounded down the amount-to-send
to be a multiple of 8K when it is eagerly writing some data. This
still seems like a good idea when sending through a Unix socket, as
pipes typically have a buffer size of 8K or some fraction/multiple of
that. But there's not much argument for it on a TCP connection, since
(a) standard MTU values are not commensurate with that, and (b) the
kernel typically applies its own packet splitting/merging logic.
Worse, our SSL and GSSAPI code paths both have API stipulations that
if they fail to send all the data that was offered in the previous
write attempt, we mustn't offer less data in the next attempt; else
we may get "SSL error: bad length" or "GSSAPI caller failed to
retransmit all data needing to be retried". The previous write
attempt might've been pqFlush attempting to send everything in the
buffer, so pqPutMsgEnd can't safely write less than the full buffer
contents. (Well, we could add some more state to track exactly how
much the previous write attempt was, but there's little value evident
in such extra complication.) Hence, apply the round-down only on
AF_UNIX sockets, where we never use SSL or GSSAPI.
Interestingly, we had a very closely related bug report before,
which I attempted to fix in commit d053a879b. But the test case
we had then seemingly didn't trigger this pqFlush-then-pqPutMsgEnd
scenario, or at least we failed to recognize this variant of the bug.
Bug: #18907
Reported-by: Dorjpalam Batbaatar <htgn.dbat.95@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/18907-d41b9bcf6f29edda@postgresql.org
Backpatch-through: 13
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.