Reland "Port _FileDescriptorStreamsNonBlocking to use poll()"

Now that repo 2 requires Python 3, we can reland this.

This reverts commit 91d9587e45.

Change-Id: Id5b178ebb53bdba04bfa79cbb5c698ae5080c957
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/258672
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Theodore Dubois <tbodt@google.com>

platform_utils.py

@@ -90,6 +90,11 @@ class _FileDescriptorStreamsNonBlocking(FileDescriptorStreams):
   """ Implementation of FileDescriptorStreams for platforms that support
   non blocking I/O.
   """
+  def __init__(self):
+    super(_FileDescriptorStreamsNonBlocking, self).__init__()
+    self._poll = select.poll()
+    self._fd_to_stream = {}
+
   class Stream(object):
     """ Encapsulates a file descriptor """
 
@@ -114,11 +119,18 @@ class _FileDescriptorStreamsNonBlocking(FileDescriptorStreams):
       self.fd.close()
 
   def _create_stream(self, fd, dest, std_name):
-    return self.Stream(fd, dest, std_name)
+    stream = self.Stream(fd, dest, std_name)
+    self._fd_to_stream[stream.fileno()] = stream
+    self._poll.register(stream, select.POLLIN)
+    return stream
+
+  def remove(self, stream):
+    self._poll.unregister(stream)
+    del self._fd_to_stream[stream.fileno()]
+    super(_FileDescriptorStreamsNonBlocking, self).remove(stream)
 
   def select(self):
-    ready_streams, _, _ = select.select(self.streams, [], [])
+    return [self._fd_to_stream[fd] for fd, _ in self._poll.poll()]
-    return ready_streams
 
 
 class _FileDescriptorStreamsThreads(FileDescriptorStreams):