rebase: use one dirstateguard for when using rebase.singletransaction

This was previously landed as 2519994d25ca but backed out in b63351f6a2 because
it broke hooks mid-rebase and caused conflict resolution data loss in the event
of unexpected exceptions. This new version adds the behavior back but behind a
config flag, since the performance improvement is notable in large repositories.

The old commit message was:

Recently we switched rebases to run the entire rebase inside a single
transaction, which dramatically improved the speed of rebases in repos with
large working copies. Let's also move the dirstate into a single dirstateguard
to get the same benefits. This let's us avoid serializing the dirstate after
each commit.

In a large repo, rebasing 27 commits is sped up by about 20%.

I believe the test changes are because us touching the dirstate gave the
transaction something to actually rollback.
(grafted from 9e3dc3a1638b9754b58a0cb26aaa75d868058109)
(grafted from 7d38b41d2266d9a02a15c64229fae0da5738dcec)

Differential Revision: https://phab.mercurial-scm.org/D135

hgext/rebase.py

@@ -479,12 +479,17 b' class rebaseruntime(object):'
                 editopt = True
             editor = cmdutil.getcommiteditor(edit=editopt, editform=editform)
             revtoreuse = max(self.state)
-            newnode = concludenode(repo, revtoreuse, p1, self.external,
-                                   commitmsg=commitmsg,
-                                   extrafn=_makeextrafn(self.extrafns),
-                                   editor=editor,
-                                   keepbranches=self.keepbranchesf,
-                                   date=self.date)
+
+            dsguard = None
+            if ui.configbool('rebase', 'singletransaction'):
+                dsguard = dirstateguard.dirstateguard(repo, 'rebase')
+            with util.acceptintervention(dsguard):
+                newnode = concludenode(repo, revtoreuse, p1, self.external,
+                                       commitmsg=commitmsg,
+                                       extrafn=_makeextrafn(self.extrafns),
+                                       editor=editor,
+                                       keepbranches=self.keepbranchesf,
+                                       date=self.date)
             if newnode is None:
                 newrev = self.dest
             else:
@@ -711,10 +716,16 b' def rebase(ui, repo, **opts):'
                 return retcode
 
         tr = None
-        if ui.configbool('rebase', 'singletransaction'):
+        dsguard = None
+
+        singletr = ui.configbool('rebase', 'singletransaction')
+        if singletr:
             tr = repo.transaction('rebase')
         with util.acceptintervention(tr):
-            rbsrt._performrebase(tr)
+            if singletr:
+                dsguard = dirstateguard.dirstateguard(repo, 'rebase')
+            with util.acceptintervention(dsguard):
+                rbsrt._performrebase(tr)
 
         rbsrt._finishrebase()
 
@@ -841,8 +852,10 b' def concludenode(repo, rev, p1, p2, comm'
     '''Commit the wd changes with parents p1 and p2. Reuse commit info from rev
     but also store useful information in extra.
     Return node of committed revision.'''
-    dsguard = dirstateguard.dirstateguard(repo, 'rebase')
-    try:
+    dsguard = util.nullcontextmanager()
+    if not repo.ui.configbool('rebase', 'singletransaction'):
+        dsguard = dirstateguard.dirstateguard(repo, 'rebase')
+    with dsguard:
         repo.setparents(repo[p1].node(), repo[p2].node())
         ctx = repo[rev]
         if commitmsg is None:
@@ -864,10 +877,7 b' def concludenode(repo, rev, p1, p2, comm'
                                   date=date, extra=extra, editor=editor)
 
         repo.dirstate.setbranch(repo[newnode].branch())
-        dsguard.close()
         return newnode
-    finally:
-        release(dsguard)
 
 def rebasenode(repo, rev, p1, base, state, collapse, dest):
     'Rebase a single revision rev on top of p1 using base as merge ancestor'

mercurial/dirstateguard.py

@@ -43,6 +43,16 b' class dirstateguard(object):'
             # ``release(tr, ....)``.
             self._abort()
 
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        try:
+            if exc_type is None:
+                self.close()
+        finally:
+            self.release()
+
     def close(self):
         if not self._active: # already inactivated
             msg = (_("can't close already inactivated backup: %s")

mercurial/util.py

@@ -602,6 +602,10 b' def acceptintervention(tr=None):'
     finally:
         tr.release()
 
[e-mail]
+def nullcontextmanager():
+    yield
+
 class _lrucachenode(object):
     """A node in a doubly linked list.
 

tests/test-rebase-base.t

@@ -379,3 +379,40 b' Multiple roots. Two children share two p'
    /
   o  0: A
   
+Rebasing using a single transaction
+
+  $ hg init singletr && cd singletr
+  $ cat >> .hg/hgrc <<EOF
+  > [rebase]
+  > singletransaction=True
+  > EOF
+  $ hg debugdrawdag <<'EOF'
+  >   Z
+  >   |
+  >   | D
+  >   | |
+  >   | C
+  >   | |
+  >   Y B
+  >   |/
+  >   A
+  > EOF
+- We should only see two status stored messages. One from the start, one from
+- the end.
+  $ hg rebase --debug -b D -d Z | grep 'status stored'
+  rebase status stored
+  rebase status stored
+  $ hg tglog
+  o  5: D
+  |
+  o  4: C
+  |
+  o  3: B
+  |
+  o  2: Z
+  |
+  o  1: Y
+  |
+  o  0: A
+  
+  $ cd ..