ValueError: cannot mmap an empty file on head.log()

[peterbe]

When I do:

log = head.log()

I sometimes get this error:

Traceback (most recent call last):
  File "/usr/local/bin/gg", line 11, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args[1:], **kwargs)
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/gg/builtins/branches/gg_branches.py", line 25, in branches
    print_list(branches_, merged)
  File "/usr/local/lib/python3.6/site-packages/gg/builtins/branches/gg_branches.py", line 96, in print_list
    [wrap(head) for head in heads], key=lambda x: x["info"].get("date")
  File "/usr/local/lib/python3.6/site-packages/gg/builtins/branches/gg_branches.py", line 96, in <listcomp>
    [wrap(head) for head in heads], key=lambda x: x["info"].get("date")
  File "/usr/local/lib/python3.6/site-packages/gg/builtins/branches/gg_branches.py", line 64, in wrap
    log = head.log()
  File "/usr/local/lib/python3.6/site-packages/git/refs/symbolic.py", line 358, in log
    return RefLog.from_file(RefLog.path(self))
  File "/usr/local/lib/python3.6/site-packages/git/refs/log.py", line 177, in from_file
    return cls(filepath)
  File "/usr/local/lib/python3.6/site-packages/git/refs/log.py", line 151, in __init__
    self._read_from_file()
  File "/usr/local/lib/python3.6/site-packages/git/refs/log.py", line 156, in _read_from_file
    fmap = file_contents_ro_filepath(self._path, stream=True, allow_mmap=True)
  File "/usr/local/lib/python3.6/site-packages/gitdb/util.py", line 209, in file_contents_ro_filepath
    return file_contents_ro(fd, stream, allow_mmap)
  File "/usr/local/lib/python3.6/site-packages/gitdb/util.py", line 179, in file_contents_ro
    return mmap.mmap(fd, 0, access=mmap.ACCESS_READ)
ValueError: cannot mmap an empty file

That head is an object like this: <git.Head "refs/heads/bug-1361148-use-locmemcache-in-unit-tests"> when I repr it.

These are old branches usually that it happens to. I think. But I can do:

git checkout bug-1361148-use-locmemcache-in-unit-tests
git log