commandserver: get around ETIMEDOUT raised by selectors2 stable

commandserver: get around ETIMEDOUT raised by selectors2

selector.select() should exits with an empty event list on timed out, but
selectors2 raises OSError if timeout expires while recovering from EINTR.

Spotted while debugging new chg feature.

--- a/mercurial/commandserver.py	Mon Dec 03 21:31:19 2018 +0900
+++ b/mercurial/commandserver.py	Mon Dec 03 21:45:15 2018 +0900
@@ -472,7 +472,15 @@
                 # waiting for recv() will receive ECONNRESET.
                 self._unlinksocket()
                 exiting = True
-            ready = selector.select(timeout=h.pollinterval)
+            try:
+                ready = selector.select(timeout=h.pollinterval)
+            except OSError as inst:
+                # selectors2 raises ETIMEDOUT if timeout exceeded while
+                # handling signal interrupt. That's probably wrong, but
+                # we can easily get around it.
+                if inst.errno != errno.ETIMEDOUT:
+                    raise
+                ready = []
             if not ready:
                 # only exit if we completed all queued requests
                 if exiting: