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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
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