tests: add extension to emulate invoking dirstate.write at the specific time

This extension fakes 'now' for 'parsers.pack_dirstate()' to emulate
invoking 'dirstate.write()' at the specific time, only when
'dirstate.write()' is invoked via functions below:

  - 'workingctx._checklookup()' (= 'repo.status()')
  - 'committablectx.markcommitted()'

This is useful to reproduce timing critical issues fixed in subsequent
patches.

tests/fakedirstatewritetime.py

@@ -0,0 +1,58 b''
+# extension to emulate invoking 'dirstate.write()' at the time
+# specified by '[fakedirstatewritetime] fakenow', only when
+# 'dirstate.write()' is invoked via functions below:
+#
+#   - 'workingctx._checklookup()' (= 'repo.status()')
+#   - 'committablectx.markcommitted()'
+
+from mercurial import context, extensions, parsers, util
+
+def pack_dirstate(fakenow, orig, dmap, copymap, pl, now):
+    # execute what original parsers.pack_dirstate should do actually
+    # for consistency
+    actualnow = int(now)
+    for f, e in dmap.iteritems():
+        if e[0] == 'n' and e[3] == actualnow:
+            e = parsers.dirstatetuple(e[0], e[1], e[2], -1)
+            dmap[f] = e
+
+    return orig(dmap, copymap, pl, fakenow)
+
+def fakewrite(ui, func):
+    # fake "now" of 'pack_dirstate' only if it is invoked while 'func'
+
+    fakenow = ui.config('fakedirstatewritetime', 'fakenow')
+    if not fakenow:
+        # Execute original one, if fakenow isn't configured. This is
+        # useful to prevent subrepos from executing replaced one,
+        # because replacing 'parsers.pack_dirstate' is also effective
+        # in subrepos.
+        return func()
+
+    # parsing 'fakenow' in YYYYmmddHHMM format makes comparison between
+    # 'fakenow' value and 'touch -t YYYYmmddHHMM' argument easy
+    timestamp = util.parsedate(fakenow, ['%Y%m%d%H%M'])[0]
+    fakenow = float(timestamp)
+
+    orig_pack_dirstate = parsers.pack_dirstate
+    wrapper = lambda *args: pack_dirstate(fakenow, orig_pack_dirstate, *args)
+
+    parsers.pack_dirstate = wrapper
+    try:
+        return func()
+    finally:
+        parsers.pack_dirstate = orig_pack_dirstate
+
+def _checklookup(orig, workingctx, files):
+    ui = workingctx.repo().ui
+    return fakewrite(ui, lambda : orig(workingctx, files))
+
+def markcommitted(orig, committablectx, node):
+    ui = committablectx.repo().ui
+    return fakewrite(ui, lambda : orig(committablectx, node))
+
+def extsetup(ui):
+    extensions.wrapfunction(context.workingctx, '_checklookup',
+                            _checklookup)
+    extensions.wrapfunction(context.committablectx, 'markcommitted',
+                            markcommitted)