# HG changeset patch
# User Mike Hommey <mh+mozilla@glandium.org>
# Date 1502856976 -32400
# Node ID 8d9ae8c45dd07496f164364f24c5f43edcf2eb6e
# Parent  fa02d334033f0039ef82b3d7dc194312ba250279
Bug 1384062 - Make SystemResourceMonitor.stop more resilient to errors. r=ahal,gps

The poll() call in SystemResourceMonitor.stop might fail even though
there is something to read from the pipe, in some corner cases, and
python won't let us know about it. In that case, an exception is thrown,
leaving the SystemResourceMonitor (and its callers) in a weird state. In
practice, this leads BuildMonitor.__exit__ to recall stop, which then
fails.

So when poll() throws an exception, we pretend there's still something
to read, and we try to read anyways. If there is something to read,
recv() will return it, otherwise, it will throw an exception of its own,
which we catch, pretending we're done.

Furthermore, when there is nothing to read from the pipe, poll() simply
returns False, and our loop never sets `done` to True, and we then hit
an assert, which doesn't have its place here, so we remove it.

Finally, the other end of the pipe might have died at any time, making
sending over the pipe fail, so we also protect against that.

With all these changes, it feels like the reason to backout bug 1239939
in bug 1272782 should have been dealt with, and we can drop the timeout
again.

diff --git a/testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py b/testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py
--- a/testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py
+++ b/testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py
@@ -284,57 +284,71 @@ class SystemResourceMonitor(object):
         """
         if not self._process:
             self._stopped = True
             return
 
         assert self._running
         assert not self._stopped
 
-        self._pipe.send(('terminate',))
+        try:
+            self._pipe.send(('terminate',))
+        except Exception:
+            pass
         self._running = False
         self._stopped = True
 
         self.measurements = []
 
-        done = False
-
         # The child process will send each data sample over the pipe
         # as a separate data structure. When it has finished sending
         # samples, it sends a special "done" message to indicate it
         # is finished.
-        while self._pipe.poll(1.0):
-            start_time, end_time, io_diff, cpu_diff, cpu_percent, virt_mem, \
-                swap_mem = self._pipe.recv()
+
+        # multiprocessing.Pipe is not actually a pipe on at least Linux.  that
+        # has an effect on the expected outcome of reading from it when the
+        # other end of the pipe dies, leading to possibly hanging on revc()
+        # below. So we must poll().
+        def poll():
+            try:
+                return self._pipe.poll(0.1)
+            except Exception:
+                # Poll might throw an exception even though there's still
+                # data to read. That happens when the underlying system call
+                # returns both POLLERR and POLLIN, but python doesn't tell us
+                # about it. So assume there is something to read, and we'll
+                # get an exception when trying to read the data.
+                return True
+        while poll():
+            try:
+                start_time, end_time, io_diff, cpu_diff, cpu_percent, virt_mem, \
+                    swap_mem = self._pipe.recv()
+            except Exception:
+                # Let's assume we're done here
+                break
 
             # There should be nothing after the "done" message so
             # terminate.
             if start_time == 'done':
-                done = True
                 break
 
             io = self._io_type(*io_diff)
             virt = self._virt_type(*virt_mem)
             swap = self._swap_type(*swap_mem)
             cpu_times = [self._cpu_times_type(*v) for v in cpu_diff]
 
             self.measurements.append(SystemResourceUsage(start_time, end_time,
                                                          cpu_times, cpu_percent, io, virt, swap))
 
         # We establish a timeout so we don't hang forever if the child
         # process has crashed.
         self._process.join(10)
         if self._process.is_alive():
             self._process.terminate()
             self._process.join(10)
-        else:
-            # We should have received a "done" message from the
-            # child indicating it shut down properly. This only
-            # happens if the child shuts down cleanly.
-            assert done
 
         if len(self.measurements):
             self.start_time = self.measurements[0].start
             self.end_time = self.measurements[-1].end
 
     # Methods to record events alongside the monitored data.
 
     def record_event(self, name):