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If connectivity to the server has been lost or become flaky, the user might well try to send a query cancel. It's highly annoying if PQcancel hangs up in such a case, but that's exactly what's likely to happen. To ameliorate this problem, apply the PGconn's tcp_user_timeout and keepalives settings to the TCP connection used to send the cancel. This should be safe on Unix machines, since POSIX specifies that setsockopt() is async-signal-safe. We are guessing that WSAIoctl(SIO_KEEPALIVE_VALS) is similarly safe on Windows. (Note that at least in psql and our other frontend programs, there's no safety issue involved anyway, since we run PQcancel in its own thread rather than in a signal handler.) Most of the value here comes from the expectation that tcp_user_timeout will be applied as a connection timeout. That appears to happen on Linux, even though its tcp(7) man page claims differently. The keepalive options probably won't help much, but as long as we can apply them for not much code, we might as well. Jelte Fennema, reviewed by Fujii Masao and myself Discussion: https://postgr.es/m/AM5PR83MB017870DE81FC84D5E21E9D1EF7AA9@AM5PR83MB0178.EURPRD83.prod.outlook.com |
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