Port _FileDescriptorStreamsNonBlocking to use poll()

select() has a limit of FD_SETSIZE file descriptors. If you run repo
sync -j500 you'll pretty quickly hit this limit and get "file descriptor
out of range for select" errors. poll() has no such limit.

Change-Id: I21f350e472bda1db03dcbcc437645c23dbc7a901
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/248852
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 """
     def __init__(self, fd, dest, std_name):
@@ -113,11 +118,18 @@ 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 ready_streams
+    return [self._fd_to_stream[fd] for fd, _ in self._poll.poll()]
 
 
 class _FileDescriptorStreamsThreads(FileDescriptorStreams):