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0214a61e06
If libpq output buffer is full, pqSendSome() function tries to drain any incoming data. This avoids deadlock, if the server e.g. sends a lot of NOTICE messages, and blocks until we read them. However, pqSendSome() only did that in blocking mode. In non-blocking mode, the deadlock could still happen. To fix, take a two-pronged approach: 1. Change the documentation to instruct that when PQflush() returns 1, you should wait for both read- and write-ready, and call PQconsumeInput() if it becomes read-ready. That fixes the deadlock, but applications are not going to change overnight. 2. In pqSendSome(), drain the input buffer before returning 1. This alleviates the problem for applications that only wait for write-ready. In particular, a slow but steady stream of NOTICE messages during COPY FROM STDIN will no longer cause a deadlock. The risk remains that the server attempts to send a large burst of data and fills its output buffer, and at the same time the client also sends enough data to fill its output buffer. The application will deadlock if it goes to sleep, waiting for the socket to become write-ready, before the server's data arrives. In practice, NOTICE messages and such that the server might be sending are usually short, so it's highly unlikely that the server would fill its output buffer so quickly. Backpatch to all supported versions. |
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