localrepo: stop doing special dirstate backup at transaction open

Since the dirstate writes are already managed by the transaction, we already do
a backup of the dirstate when necessary (and even trigger one to keep `hg
rollback` happy).

We needs some special code to deal with the initial empty checkout, but it is
not too complicated.

Managing variable filename (as dirstate-v2 uses) at the "journalfile" level, is
complex and fragile (which is consistent with the fact these files are not
journal…). If we no longer do it, our life is significantly simpler.

In some sense, we apply the xkcd-1134¹ solution to our savebackup/restorebackup
problem.

[1] https://xkcd.com/1134/

(the change to test-hardlink are expect as decreasing the number of duplicated
backup drive the hardlink count down)

mercurial/localrepo.py

@@ -2548,8 +2548,6 b' class localrepository:'
                 # out) in this transaction
                 narrowspec.restorebackup(self, b'journal.narrowspec')
                 narrowspec.restorewcbackup(self, b'journal.narrowspec.dirstate')
-                if repo.currentwlock() is not None:
-                    repo.dirstate.restorebackup(None, b'journal.dirstate')
 
                 repo.invalidate(clearfilecache=True)
 
@@ -2676,31 +2674,21 b' class localrepository:'
         return tr
 
     def _journalfiles(self):
-        first = (
+        return (
             (self.svfs, b'journal'),
             (self.svfs, b'journal.narrowspec'),
             (self.vfs, b'journal.narrowspec.dirstate'),
-            (self.vfs, b'journal.dirstate'),
-        )
-        middle = []
-        dirstate_data = self.dirstate.data_backup_filename(b'journal.dirstate')
-        if dirstate_data is not None:
-            middle.append((self.vfs, dirstate_data))
-        end = (
             (self.vfs, b'journal.branch'),
             (self.vfs, b'journal.desc'),
             (bookmarks.bookmarksvfs(self), b'journal.bookmarks'),
             (self.svfs, b'journal.phaseroots'),
         )
-        return first + tuple(middle) + end
 
     def undofiles(self):
         return [(vfs, undoname(x)) for vfs, x in self._journalfiles()]
 
     @unfilteredmethod
     def _writejournal(self, desc):
-        if self.currentwlock() is not None:
-            self.dirstate.savebackup(None, b'journal.dirstate')
         narrowspec.savewcbackup(self, b'journal.narrowspec.dirstate')
         narrowspec.savebackup(self, b'journal.narrowspec')
         self.vfs.write(
@@ -2808,11 +2796,20 b' class localrepository:'
         if self.svfs.exists(b'undo.phaseroots'):
             self.svfs.rename(b'undo.phaseroots', b'phaseroots', checkambig=True)
         self.invalidate()
+        self.dirstate.invalidate()
 
         if parentgone:
+            # replace this with some explicit parent update in the future.
+            has_node = self.changelog.index.has_node
+            if not all(has_node(p) for p in self.dirstate._pl):
+                # There was no dirstate to backup initially, we need to drop
+                # the existing one.
+                with self.dirstate.changing_parents(self):
+                    self.dirstate.setparents(self.nullid)
+                    self.dirstate.clear()
+
             narrowspec.restorebackup(self, b'undo.narrowspec')
             narrowspec.restorewcbackup(self, b'undo.narrowspec.dirstate')
-            self.dirstate.restorebackup(None, b'undo.dirstate')
             try:
                 branch = self.vfs.read(b'undo.branch')
                 self.dirstate.setbranch(encoding.tolocal(branch))

tests/test-hardlinks.t

@@ -261,11 +261,10 b' r4 has hardlinks in the working dir (not'
   2 r4/.hg/store/undo.backup.phaseroots
   2 r4/.hg/store/undo.backupfiles
   2 r4/.hg/store/undo.phaseroots
-  [24] r4/\.hg/undo\.backup\.dirstate (re)
+  2 r4/\.hg/undo\.backup\.dirstate (re)
   2 r4/.hg/undo.bookmarks
   2 r4/.hg/undo.branch
   2 r4/.hg/undo.desc
-  [24] r4/\.hg/undo\.dirstate (re)
   2 r4/.hg/wcache/checkisexec (execbit !)
   2 r4/.hg/wcache/checklink-target (symlink !)
   2 r4/.hg/wcache/checknoexec (execbit !)
@@ -279,7 +278,7 b' Update back to revision 12 in r4 should '
 #if hardlink-whitelisted
   $ nlinksdir r4/.hg/undo.backup.dirstate r4/.hg/dirstate
   2 r4/.hg/dirstate
-  4 r4/.hg/undo.backup.dirstate
+  2 r4/.hg/undo.backup.dirstate
 #endif
 
 
@@ -319,11 +318,10 b' Update back to revision 12 in r4 should '
   2 r4/.hg/store/undo.backup.phaseroots
   2 r4/.hg/store/undo.backupfiles
   2 r4/.hg/store/undo.phaseroots
-  [24] r4/\.hg/undo\.backup\.dirstate (re)
+  2 r4/\.hg/undo\.backup\.dirstate (re)
   2 r4/.hg/undo.bookmarks
   2 r4/.hg/undo.branch
   2 r4/.hg/undo.desc
-  [24] r4/\.hg/undo\.dirstate (re)
   2 r4/.hg/wcache/checkisexec (execbit !)
   2 r4/.hg/wcache/checklink-target (symlink !)
   2 r4/.hg/wcache/checknoexec (execbit !)
@@ -337,7 +335,7 b' Update back to revision 12 in r4 should '
 #if hardlink-whitelisted
   $ nlinksdir r4/.hg/undo.backup.dirstate r4/.hg/dirstate
   1 r4/.hg/dirstate
-  4 r4/.hg/undo.backup.dirstate
+  2 r4/.hg/undo.backup.dirstate
 #endif
 
 Test hardlinking outside hg:

tests/test-rebase-abort.t

@@ -393,7 +393,6 b' New operations are blocked with the corr'
   .hg/merge/state
   .hg/rebasestate
   .hg/undo.backup.dirstate
-  .hg/undo.dirstate
   .hg/updatestate
 
   $ hg rebase -s 3 -d tip