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This patch adds support for Microsoft Windows in the Process subsystem. Libevent does not support mixing different types of handles (sockets, named pipes, etc.) on Windows in its core event loop code. This have historically meant that Tor have avoided attaching any non-networking handles to the event loop. This patch uses a slightly different approach to roughly support the same features for the Process subsystem as we do with the Unix backend. In this patch we use Windows Extended I/O functions (ReadFileEx() and WriteFileEx()) which executes asynchronously in the background and executes a completion routine when the scheduled read or write operation have completed. This is much different from the Unix backend where the operating system signals to us whenever a file descriptor is "ready" to either being read from or written to. To make the Windows operating system execute the completion routines of ReadFileEx() and WriteFileEx() we must get the Tor process into what Microsoft calls an "alertable" state. To do this we execute SleepEx() with a zero millisecond sleep time from a main loop timer that ticks once a second. This moves the process into the "alertable" state and when we return from the zero millisecond timeout all the outstanding I/O completion routines will be called and we can schedule the next reads and writes. The timer loop is also responsible for detecting whether our child processes have terminated since the last timer tick. See: https://bugs.torproject.org/28179 |
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