dirstate-v2: actually fix the dirstate-v2 upgrade race

It looks like the previous fix for the dirstate-v2 upgrade race didn't work.
The problem is that it only recovers in case the size of the v1 `dirstate` file
is smaller than the `v2` one, whereas in real life it's always larger.

This commit changes the test to be more realistic, which reveals the crash,
and changes the code to fix the crash.

mercurial/dirstatemap.py

@@ -4,7 +4,6 b''
 # GNU General Public License version 2 or any later version.
 
 
-import struct
 from .i18n import _
 
 from . import (
@@ -152,15 +151,13 b' class _dirstatemapcommon:'
                     b'dirstate only has a docket in v2 format'
                 )
             self._set_identity()
-            try:
+            data = self._readdirstatefile()
+            if data == b'' or data.startswith(docketmod.V2_FORMAT_MARKER):
                 self._docket = docketmod.DirstateDocket.parse(
-                    self._readdirstatefile(), self._nodeconstants
+                    data, self._nodeconstants
                 )
-            except struct.error:
-                self._ui.debug(b"failed to read dirstate-v2 data")
-                raise error.CorruptedDirstate(
-                    b"failed to read dirstate-v2 data"
-                )
+            else:
+                raise error.CorruptedDirstate(b"dirstate is not in v2 format")
         return self._docket
 
     def _read_v2_data(self):

tests/test-dirstate-version-fallback.t

@@ -10,8 +10,8 b' Set up a v1 repo'
   $ hg init repo
   $ cd repo
   $ echo a > a
-  $ hg add a
-  $ hg commit -m a
+  $ touch file-with-somewhat-long-name-to-make-dirstate-v1-bigger-than-v2
+  $ hg commit -Aqm a
   $ hg debugrequires | grep dirstate
   [1]
   $ ls -1 .hg/dirstate*