localrepo: restore dirstate to one before rollbacking if not parent-gone

'localrepository.rollback()' explicilty restores dirstate, only if at
least one of current parents of the working directory is removed at
rollbacking (a.k.a "parent-gone").

After DirstateTransactionPlan, 'dirstate.write()' will cause marking
'.hg/dirstate' as a file to be restored at rollbacking.

    https://mercurial.selenic.com/wiki/DirstateTransactionPlan

Then, 'transaction.rollback()' restores '.hg/dirstate' regardless of
parents of the working directory at that time, and this causes
unexpected dirstate changes if not "parent-gone" (e.g.  "hg update" to
another branch after "hg commit" or so, then "hg rollback").

To avoid such situation, this patch restores dirstate to one before
rollbacking if not "parent-gone".

  before:
    b1. restore dirstate explicitly, if "parent-gone"

  after:
    a1. save dirstate before actual rollbacking via dirstateguard
    a2. restore dirstate via 'transaction.rollback()'
    a3. if "parent-gone"
        - discard backup (a1)
        - restore dirstate from 'undo.dirstate'
    a4. otherwise, restore dirstate from backup (a1)

Even though restoring dirstate at (a3) after (a2) seems redundant,
this patch keeps this existing code path, because:

  - it isn't ensured that 'dirstate.write()' was invoked at least once
    while transaction running

    If not, '.hg/dirstate' isn't restored at (a2).

    In addition to it, rude 3rd party extension invoking
    'dirstate.write()' without 'repo' while transaction running (see
    subsequent patches for detail) may break consistency of a file
    backup-ed by transaction.

  - this patch mainly focuses on changes for DirstateTransactionPlan

    Restoring dirstate at (a3) itself should be cheaper enough than
    rollbacking itself. Redundancy will be removed in next step.

Newly added test is almost meaningless at this point. It will be used
to detect regression while implementing delayed dirstate write out.

mercurial/localrepo.py

@@ -11,7 +11,7 b' import peer, changegroup, subrepo, pushk'
 import changelog, dirstate, filelog, manifest, context, bookmarks, phases
 import lock as lockmod
 import transaction, store, encoding, exchange, bundle2
-import scmutil, util, extensions, hook, error, revset
+import scmutil, util, extensions, hook, error, revset, cmdutil
 import match as matchmod
 import merge as mergemod
 import tags as tagsmod
@@ -1084,20 +1084,22 b' class localrepository(object):'
             lock.release()
 
     def rollback(self, dryrun=False, force=False):
-        wlock = lock = None
+        wlock = lock = dsguard = None
         try:
             wlock = self.wlock()
             lock = self.lock()
             if self.svfs.exists("undo"):
-                return self._rollback(dryrun, force)
+                dsguard = cmdutil.dirstateguard(self, 'rollback')
+
+                return self._rollback(dryrun, force, dsguard)
             else:
                 self.ui.warn(_("no rollback information available\n"))
                 return 1
         finally:
-            release(lock, wlock)
+            release(dsguard, lock, wlock)
 
     @unfilteredmethod # Until we get smarter cache management
-    def _rollback(self, dryrun, force):
+    def _rollback(self, dryrun, force, dsguard):
         ui = self.ui
         try:
             args = self.vfs.read('undo.desc').splitlines()
@@ -1140,6 +1142,9 b' class localrepository(object):'
         parentgone = (parents[0] not in self.changelog.nodemap or
                       parents[1] not in self.changelog.nodemap)
         if parentgone:
+            # prevent dirstateguard from overwriting already restored one
+            dsguard.close()
+
             self.vfs.rename('undo.dirstate', 'dirstate')
             try:
                 branch = self.vfs.read('undo.branch')

tests/test-import.t

@@ -428,6 +428,25 b' patches: import patch1 patch2; rollback'
   working directory now based on revision 0
   $ hg --cwd b parents --template 'parent: {rev}\n'
   parent: 0
+
+Test that "hg rollback" doesn't restore dirstate to one at the
+beginning of the rollbacked transaction in not-"parent-gone" case.
+
+invoking pretxncommit hook will cause marking '.hg/dirstate' as a file
+to be restored at rollbacking, after DirstateTransactionPlan (see wiki
+page for detail).
+
+  $ hg --cwd b branch -q foobar
+  $ hg --cwd b commit -m foobar
+  $ hg --cwd b update 0 -q
+  $ hg --cwd b import ../patch1 ../patch2 --config hooks.pretxncommit=true
+  applying ../patch1
+  applying ../patch2
+  $ hg --cwd b update -q 1
+  $ hg --cwd b rollback -q
+  $ hg --cwd b parents --template 'parent: {rev}\n'
+  parent: 1
+
   $ rm -r b