named branches: server branchmap wire protocol support (issue736)

The repository command, 'branchmap', returns a dictionary, branchname
-> [branchheads], and will be implemented for localrepo, httprepo and
sshrepo.

The following wire format is used for returning data:

branchname1 branch1head2 branch1head2 ...
branchname2 ...
...

Branch names are URL encoded to escape white space, and branch heads
are sent as hex encoded node ids. All branches and all their heads are
sent.

The background and motivation for this command is the desire for a
richer named branch semantics when pushing changesets. The details are
explained in the original proposal which is included below.


1. BACKGROUND

The algorithm currently implemented in Mercurial only considers the
graph theoretical heads when determining whether new heads are
created, rather than using the branch heads as a count (the algorithm
considers a branch head effectively closed when it is merged into
another branch or a new named branch is started from that point
onward).

Our particular problem with the algorithm is that we'd like to see the
following case working without forcing a push:

Upsteam has:

(0:dev) ---- (1:dev)
\
 `--- (2:stable)

Someone merges stable into dev:

(0:dev) ---- (1:dev) ------(3:dev)
\                         /
 `--- (2:stable) --------´

This can be pushed without --force (as it should).
Now someone else does some coding on stable (a bug fix, say):

(0:dev) ---- (1:dev) ------(3:dev)
\                          /
 `--- (2:stable) ---------´---------(4:stable)

This time we need --force to push.

We allow this to be pushed without using --force by getting all the
remote branch heads (by extending the wire protocol with a new
function).

We would, furthermore, also prefer if it is impossible to push a new
branch without --force (or a later --newbranch option so --force isn't
shoe-horned into too many disparate functions, if need be), except of
course in the case where the remote repository is empty.

This is what our patches accomplish.


2. ALTERNATIVES

We have, of course, considered some alternatives to reconstructing
enough information to decide whether we are creating new remote branch
heads, before we added the new wire protocol command.

2.1. LOOKUP ON REMOTE

The main alternative is to use the information from remote.heads() and
remote.lookup() to try to reconstruct enough graph information to
decide whether we are creating new heads. This is not adequate as
illustrated below.

Remember that each lookup is typically a request-response pair over
SSH or HTTP(S).

If we have a simple repository at the remote end like this:

(0:dev) ---- (1:dev) ---- (3:stable)
\
 `--- (2:dev)

then remote.heads() will yield [2, 3]. Assume we have nodes [0, 1, 2]
locally and want to create a new node, 4:dev, as a descendant from
(1:dev), which should be OK as 1:dev is a branch head.

If we do remote.lookup('dev') we will get [2]. Thus, we can get
information about whether a branch exists on the remote server or not,
but this does not solve our problem of figuring out whether we are
creating new heads or not.

Pushing 4:dev ought to be OK, since after the push, we still only have
two heads on branch a.

Using remote.lookup() and remote.heads() is thus not adequate to
consistently decide whether we are creating new remote heads (e.g. in
this situation the latter would never return 1:dev).

2.2. USING INCOMING TO RECONSTRUCT THE GRAPH

An alternative would be to use information equivalent to hg incoming
to get the full remote graph in addition to the local graph.

To do this, we would have to get a changegroup(subset) bundle
representing the remote end (which may be a substantial amount of
data), getting the branch heads from an instantiated bundlerepository,
deleting the bundle, and finally, we can compute the prepush logic.

While this is backwards compatible, it will cause a possibly
substantial slowdown of the push command as it first needs to pull in
all changes.


3. FURTHER ARGUMENTS IN FAVOUR OF THE BRANCHMAP WIRE-PROTOCOL EXTENSION

Currently, the commands incoming and pull, work based on the tip of a
given branch if used with "-r branchname", making it hard to get all
revisions of a certain branch only (if it has multiple heads). This
can be solved by requesting the remote's branchheads and letting the
revisions to be used with the command be these heads. This can be done
by extending the commands with a new option, e.g.:

hg pull -b branchname

which will be turned into the equivalent of:

hg pull -r branchhead1 -r branchhead2 -r branchhead3

We have a simple follow-up patch that can do this ready as well
(although not submitted yet as it is pending the acceptance of the
branch patch).


4. WRAP-UP

We generally find that the branchmap wire protocol extension can
provide better named branch support to Mercurial. Currently, some
things, like the initial push scenario in this mail, are fairly
counter-intuitive, and the more often you have to force push, the more
it is likely you will get a lot of spurious and unnecessary merge
nodes. Also, restricting incoming and pull to all changes on a branch
rather than changes on the tip-most head would be a sensible extension
to making named branches a first class citizen in Mercurial.
Currently, named branches sometimes feel like a late-coming unwanted
step-child.

We have run it in a production environment for a while, with fewer
multiple heads occurring in our repositories and fewer confused users
as a result.

Also, it fixes the long-standing issue 736.

Co-contributor: Sune Foldager <[e-mail]>

mercurial/hgweb/protocol.py

@@ -5,7 +5,7
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2, incorporated herein by reference.
 
-import cStringIO, zlib, tempfile, errno, os, sys
+import cStringIO, zlib, tempfile, errno, os, sys, urllib
 from mercurial import util, streamclone
 from mercurial.node import bin, hex
 from mercurial import changegroup as changegroupmod
@@ -17,6 +17,7 from common import ErrorResponse, HTTP_O
 __all__ = [
    'lookup', 'heads', 'branches', 'between', 'changegroup',
    'changegroupsubset', 'capabilities', 'unbundle', 'stream_out',
+   'branchmap',
 ]
 
 HGTYPE = 'application/mercurial-0.1'
@@ -37,6 +38,17 def heads(repo, req):
     req.respond(HTTP_OK, HGTYPE, length=len(resp))
     yield resp
 
+def branchmap(repo, req):
+    branches = repo.branchmap()
+    heads = []
+    for branch, nodes in branches.iteritems():
+        branchname = urllib.quote(branch)
+        branchnodes = [hex(node) for node in nodes]
+        heads.append('%s %s' % (branchname, ' '.join(branchnodes)))
+    resp = '\n'.join(heads)
+    req.respond(HTTP_OK, HGTYPE, length=len(resp))
+    yield resp
+
 def branches(repo, req):
     nodes = []
     if 'nodes' in req.form:
@@ -97,7 +109,7 def changegroupsubset(repo, req):
     yield z.flush()
 
 def capabilities(repo, req):
-    caps = ['lookup', 'changegroupsubset']
+    caps = ['lookup', 'changegroupsubset', 'branchmap']
     if repo.ui.configbool('server', 'uncompressed', untrusted=True):
         caps.append('stream=%d' % repo.changelog.version)
     if changegroupmod.bundlepriority:

mercurial/localrepo.py

@@ -18,7 +18,7 import weakref, stat, errno, os, time, i
 propertycache = util.propertycache
 
 class localrepository(repo.repository):
-    capabilities = set(('lookup', 'changegroupsubset'))
+    capabilities = set(('lookup', 'changegroupsubset', 'branchmap'))
     supported = set('revlogv1 store fncache'.split())
 
     def __init__(self, baseui, path=None, create=0):
@@ -360,7 +360,7 class localrepository(repo.repository):
 
         return partial
 
-    def _branchheads(self):
+    def branchmap(self):
         tip = self.changelog.tip()
         if self.branchcache is not None and self._branchcachetip == tip:
             return self.branchcache
@@ -392,7 +392,7 class localrepository(repo.repository):
         '''return a dict where branch names map to the tipmost head of
         the branch, open heads come before closed'''
         bt = {}
-        for bn, heads in self._branchheads().iteritems():
+        for bn, heads in self.branchmap().iteritems():
             head = None
             for i in range(len(heads)-1, -1, -1):
                 h = heads[i]
@@ -1125,7 +1125,7 class localrepository(repo.repository):
     def branchheads(self, branch=None, start=None, closed=True):
         if branch is None:
             branch = self[None].branch()
-        branches = self._branchheads()
+        branches = self.branchmap()
         if branch not in branches:
             return []
         bheads = branches[branch]

mercurial/sshserver.py

@@ -9,7 +9,7
 from i18n import _
 from node import bin, hex
 import streamclone, util, hook
-import os, sys, tempfile
+import os, sys, tempfile, urllib
 
 class sshserver(object):
     def __init__(self, ui, repo):
@@ -64,6 +64,15 class sshserver(object):
             success = 0
         self.respond("%s %s\n" % (success, r))
 
+    def do_branchmap(self):
+        branchmap = self.repo.branchmap()
+        heads = []
+        for branch, nodes in branchmap.iteritems():
+            branchname = urllib.quote(branch)
+            branchnodes = [hex(node) for node in nodes]
+            heads.append('%s %s' % (branchname, ' '.join(branchnodes)))
+        self.respond('\n'.join(heads))
+
     def do_heads(self):
         h = self.repo.heads()
         self.respond(" ".join(map(hex, h)) + "\n")
@@ -77,7 +86,7 class sshserver(object):
         capabilities: space separated list of tokens
         '''
 
-        caps = ['unbundle', 'lookup', 'changegroupsubset']
+        caps = ['unbundle', 'lookup', 'changegroupsubset', 'branchmap']
         if self.ui.configbool('server', 'uncompressed'):
             caps.append('stream=%d' % self.repo.changelog.version)
         self.respond("capabilities: %s\n" % (' '.join(caps),))

tests/test-hgweb-commands.out

@@ -848,7 +848,7 graph.render(data);
 % capabilities
 200 Script output follows
 
-lookup changegroupsubset unbundle=HG10GZ,HG10BZ,HG10UN% heads
+lookup changegroupsubset branchmap unbundle=HG10GZ,HG10BZ,HG10UN% heads
 200 Script output follows
 
 1d22e65f027e5a0609357e7d8e7508cd2ba5d2fe