Delay checking process for termination until both stdout and stderr are closed.

This patch makes us delay checking for whether we have an exit code
value (via GetExitCodeProcess()) until both stdout and stderr have been
closed by the operating system either by the process itself or by
process cleanup after termination.

See: https://bugs.torproject.org/28179

src/lib/process/process_win32.c

@@ -525,6 +525,28 @@ process_win32_timer_test_process(process_t *process)
   BOOL ret = FALSE;
   DWORD exit_code = 0;
 
+  /* Sometimes the Windows kernel wont give us the EOF/Broken Pipe error
+   * message until some time after the process have actually terminated. We
+   * make sure that our ReadFileEx() calls for the process have *all* returned
+   * and both standard out and error have been marked as EOF before we try to
+   * see if the process terminated.
+   *
+   * This ensures that we *never* call the exit callback of the `process_t`,
+   * which potentially ends up calling `process_free()` on our `process_t`,
+   * before all data have been received from the process.
+   *
+   * We do NOT have a check here for whether standard in reached EOF since
+   * standard in's WriteFileEx() function is only called on-demand when we have
+   * something to write and is thus usually not awaiting to finish any
+   * operations. If we WriteFileEx() to a file that has terminated we'll simply
+   * get an error from ReadFileEx() or its completion routine and move on with
+   * life.  */
+  if (! win32_process->stdout_handle.reached_eof)
+    return false;
+
+  if (! win32_process->stderr_handle.reached_eof)
+    return false;
+
   /* We start by testing whether our process is still running. */
   ret = GetExitCodeProcess(win32_process->process_information.hProcess,
                            &exit_code);