[PATCH 1 of 7] dirstate: introduce new context manager for marking dirstate parent changes

[Gregory Szorc]

On Thu, May 18, 2017 at 3:19 PM, Augie Fackler <raf at durin42.com> wrote:

> # HG changeset patch
> # User Augie Fackler <augie at google.com>
> # Date 1495141830 14400
> #      Thu May 18 17:10:30 2017 -0400
> # Node ID a69ba7abd7c7396205bbf44984fc5bd028a7f815
> # Parent  1f3972ed06833775ada5b23d9c90d1cbb240e4fd
> dirstate: introduce new context manager for marking dirstate parent changes
>
> diff --git a/mercurial/dirstate.py b/mercurial/dirstate.py
> --- a/mercurial/dirstate.py
> +++ b/mercurial/dirstate.py
> @@ -8,6 +8,7 @@
>  from __future__ import absolute_import
>
>  import collections
> +import contextlib
>  import errno
>  import os
>  import stat
> @@ -99,6 +100,18 @@ class dirstate(object):
>          # for consistent view between _pl() and _read() invocations
>          self._pendingmode = None
>
> +    @contextlib.contextmanager
> +    def parentchange(self):
> +        '''Context manager for handling dirstate parents.
> +
> +        If an exception occurs in the scope of the context manager,
> +        the incoherent dirstate won't be written when wlock is
> +        released.
> +        '''
> +        self._parentwriters += 1
> +        yield
> +        self._parentwriters -= 1
>
>
I assume the docstring explains why this isn't a try..finally, as one would
expect in most @contextmanager's. While the context is literally right
there today, I can't help but wonder if it warrants an inline comment
instead in case the docstring no longer shows up in the context of a future
diff. Or maybe it should be written to something like:

self._parentwriters += 1
try:
    yield
except Exception:
    # Don't decrement writer count because it shouldn't be written after
error.
    raise
else:
    self._parentwriters -= 1

(Yes, I'm likely being overly pedantic here. But not seeing try..finally in
@contextmanager really bothers my reviewer brain. IMO we should have a
style rule to enforce the pattern because it is such an easy mistake to
make.)


>      def beginparentchange(self):
>          '''Marks the beginning of a set of changes that involve changing
>          the dirstate parents. If there is an exception during this time,
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