pull: avoid race condition with 'hg pull --rev name --update' (issue4706)

The previous scheme was:

1) lookup node for all pulled revision,
2) pull said node
3) lookup the node of the checkout target
4) update the repository there.

If the remote repo changes between (1) and (3), the resolved name will be
different and (3) crash. There is actually no need for a remote lookup during
(3), we could just set the value in (1). This prevent the race condition and
save a possible network roundtrip.

mercurial/commands.py

@@ -5112,7 +5112,13 b' def pull(ui, repo, source="default", **o'
 
         if revs:
             try:
-                revs = [other.lookup(rev) for rev in revs]
+                oldrevs = revs
+                revs = [] # actually, nodes
+                for r in oldrevs:
+                    node = other.lookup(r)
+                    revs.append(node)
+                    if r == checkout:
+                        checkout = node
             except error.CapabilityError:
                 err = _("other repository doesn't support revision lookup, "
                         "so a rev cannot be specified.")
@@ -5122,7 +5128,7 b' def pull(ui, repo, source="default", **o'
                                  force=opts.get('force'),
                                  bookmarks=opts.get('bookmark', ())).cgresult
         if checkout:
-            checkout = str(repo.changelog.rev(other.lookup(checkout)))
+            checkout = str(repo.changelog.rev(checkout))
         repo._subtoppath = source
         try:
             ret = postincoming(ui, repo, modheads, opts.get('update'), checkout)

tests/test-pull-r.t

@@ -101,4 +101,44 b' This used to abort: received changelog g'
 
   $ hg pull -qr 1 ../repo
 
+Test race condition with -r and -U (issue4707)
+
+We pull '-U -r <name>' and the name change right after/during the changegroup emission.
+We use http because http is better is our racy-est option.
+
+
+  $ echo babar > ../repo/jungle
+  $ cat <<EOF > ../repo/.hg/hgrc
+  > [hooks]
+  > outgoing.makecommit = hg ci -Am 'racy commit'; echo committed in pull-race
+  > EOF
+  $ hg -R ../repo serve -p $HGPORT2 -d --pid-file=../repo.pid
+  $ cat ../repo.pid >> $DAEMON_PIDS
+  $ hg pull --rev default --update http://localhost:$HGPORT2/
+  pulling from http://localhost:$HGPORT2/
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files (+1 heads)
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg log -G
+  @  changeset:   2:effea6de0384
+  |  tag:         tip
+  |  parent:      0:bbd179dfa0a7
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     add bar
+  |
+  | o  changeset:   1:ed1b79f46b9a
+  |/   user:        test
+  |    date:        Thu Jan 01 00:00:00 1970 +0000
+  |    summary:     change foo
+  |
+  o  changeset:   0:bbd179dfa0a7
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     add foo
+  
+
   $ cd ..