SIGQUIT debugging breaks networking because httplib (and others?) don't handle EINTR properly

If you're wondering why your branch over http: is going terribly slowly, you can type C-\ to break into pdb, but when you type 'c' to continue the http networking layer will die with an interrupted system call:

Traceback (most recent call last):
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/commands.py", line 802, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/commands.py", line 758, in run_bzr
    ret = run(*run_argv)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/commands.py", line 492, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/builtins.py", line 893, in run
    possible_transports=[to_transport])
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/bzrdir.py", line 829, in sprout
    revision_id=revision_id)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/decorators.py", line 127, in read_locked
    return unbound(self, *args, **kwargs)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/repository.py", line 960, in sprout
    dest_repo.fetch(self, revision_id=revision_id)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/repository.py", line 873, in fetch
    return inter.fetch(revision_id=revision_id, pb=pb, find_ghosts=find_ghosts)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/decorators.py", line 165, in write_locked
    return unbound(self, *args, **kwargs)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/repository.py", line 2468, in fetch
    pb=pb)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/fetch.py", line 103, in __init__
    self.__fetch()
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/fetch.py", line 132, in __fetch
    self._fetch_everything_for_revisions(revs, pp)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/fetch.py", line 150, in _fetch_everything_for_revisions
    for knit_kind, file_id, revisions in data_to_fetch:
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/repository.py", line 1368, in item_keys_introduced_by
    file_ids = self.fileids_altered_by_revision_ids(revision_ids)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/repository.py", line 1214, in fileids_altered_by_revision_ids
    selected_revision_ids)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/repository.py", line 1192, in _find_file_ids_from_xml_inventory_lines
    line_iterator).iterkeys():
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/repository.py", line 1132, in _find_text_key_references_from_xml_inventory_lines
    for line, version_id in line_iterator:
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/knit.py", line 1140, in iter_lines_added_or_present_in_versions
    enumerate(self._data.read_records_iter(version_id_records)):
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/knit.py", line 2226, in read_records_iter
    izip(iter(needed_records), raw_data):
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/knit.py", line 1934, in get_raw_records
    for pos, data in self._transport.readv(self._filename, read_vector):
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/transport/http/__init__.py", line 270, in _readv
    f = self._get_ranges_hinted(relpath, coalesced)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/transport/http/__init__.py", line 232, in _get_ranges_hinted
    code, f = self._get(relpath, ranges)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/transport/http/_urllib.py", line 139, in _get
    data = handle_response(abspath, code, response.headers, response)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/transport/http/response.py", line 264, in handle_response
    return HttpMultipartRangeResponse(url, content_type, data)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/transport/http/response.py", line 198, in __init__
    RangeFile.__init__(self, path, input_file)
  File "/home/mwh/src/bzr/bzr.dev/bzrlib/transport/http/response.py", line 76, in __init__
    self._data = input_file.read()
  File "socket.py", line 291, in read
    data = self._sock.recv(recv_size)
  File "httplib.py", line 529, in read
    s = self.fp.read(amt)
  File "socket.py", line 309, in read
    data = self._sock.recv(recv_size)
error: (4, 'Interrupted system call')