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5781871775
According to POSIX and the Linux man page, select(2) is supposed to report read ready if a file is at EOF. In the case of a FIFO, this means that the pipe is empty and there are no writers. But there seems to be an undocumented exception, observed on Linux and other platforms: If no writer has ever been opened, then select(2) does not report read ready. This can happen if a reader is opened with O_NONBLOCK before any writers have opened. This commit makes Cygwin consistent with those other platforms by introducing a special EOF test, fhandler_fifo::select_hit_eof, which returns false if there's never been a writer opened. To implement this we use a new variable '_writer_opened' in the FIFO's shared memory, which is set to 1 the first time a writer opens. New methods writer_opened() and set_writer_opened() are used to test and set this variable. Addresses: https://cygwin.com/pipermail/cygwin/2022-September/252223.html