[PATCH V2] memctx: allow extensions to determine what filectxfn should do

[Siddharth Agarwal]

# HG changeset patch
# User Siddharth Agarwal <sid0 at fb.com>
# Date 1409401778 25200
#      Sat Aug 30 05:29:38 2014 -0700
# Node ID 180c5c9d4a4d9a71ac2798e9671adea813c822ed
# Parent  bdc0e04df243d3995c7266bf7d138fddd0449ba6
memctx: allow extensions to determine what filectxfn should do

Rev 650b5b6e75ed switched the contract for filectxfn from "raise IOError if
file is missing" to "return None if file is missing". Out of tree extensions
need to be updated for that, but for extensions interested in compatibility
with both Mercurial <= 3.1 and default, it is next to impossible to introspect
core Mercurial to figure out what to do.

This patch adds a field to memctx for extensions to use.

diff --git a/mercurial/context.py b/mercurial/context.py
--- a/mercurial/context.py
+++ b/mercurial/context.py
@@ -1593,6 +1593,12 @@
     supported by util.parsedate() and defaults to current date, extra
     is a dictionary of metadata or is left empty.
     """
+
+    # Mercurial <= 3.1 expects the filectxfn to raise IOError for missing files.
+    # Extensions that need to retain compatibility across Mercurial 3.1 can use
+    # this field to determine what to do in filectxfn.
+    _returnnoneformissingfiles = True
+
     def __init__(self, repo, parents, text, files, filectxfn, user=None,
                  date=None, extra=None, editor=False):
         super(memctx, self).__init__(repo, text, user, date, extra)