tests/test-tag.out

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]>

changeset:   0:0acdaf898367
tag:         tip
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     test

changeset:   1:3ecf002a1c57
tag:         tip
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     Added tag bleah for changeset 0acdaf898367

changeset:   0:0acdaf898367
tag:         bleah
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     test

abort: working copy of .hgtags is changed (please commit .hgtags manually)
failed
abort: tag names must be unique
failed
abort: the name 'tip' is reserved
failed
abort: tag 'bleah' already exists (use -f to force)
failed
abort: tag 'bleah' already exists (use -f to force)
failed
abort: tag 'blecch' does not exist
failed
abort: tag 'blecch' does not exist
failed
0acdaf8983679e0aac16e811534eb49d7ee1f2b4 bleah
0acdaf8983679e0aac16e811534eb49d7ee1f2b4 bleah0
868cc8fbb43b754ad09fa109885d243fc49adae7 gack
868cc8fbb43b754ad09fa109885d243fc49adae7 gawk
868cc8fbb43b754ad09fa109885d243fc49adae7 gorp
868cc8fbb43b754ad09fa109885d243fc49adae7 gack
3807bcf62c5614cb6c16436b514d7764ca5f1631 gack
3807bcf62c5614cb6c16436b514d7764ca5f1631 gack
0000000000000000000000000000000000000000 gack
868cc8fbb43b754ad09fa109885d243fc49adae7 gorp
0000000000000000000000000000000000000000 gorp
3ecf002a1c572a2f3bb4e665417e60fca65bbd42 bleah1
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
0acdaf8983679e0aac16e811534eb49d7ee1f2b4 foobar
3ecf002a1c572a2f3bb4e665417e60fca65bbd42 bleah1
abort: '\n' cannot be used in a tag name
abort: ':' cannot be used in a tag name
% cloning local tags
changeset:   0:0acdaf898367
tag:         foobar
tag:         bleah
tag:         bleah0
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     test

changeset:   1:3ecf002a1c57
tag:         bleah1
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     Added tag bleah for changeset 0acdaf898367

changeset:   2:868cc8fbb43b
tag:         gawk
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     Added tag bleah0 for changeset 0acdaf898367

changeset:   3:3807bcf62c56
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     Added tag gack, gawk, gorp for changeset 868cc8fbb43b

changeset:   4:140c6e8597b4
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     Added tag gack for changeset 3807bcf62c56

changeset:   5:470a65fa7cc9
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     Removed tag gack, gorp

1[tip]   3ecf002a1c57   1970-01-12 13:46 +0000   test
  Added tag bleah for changeset 0acdaf898367

5[tip]   470a65fa7cc9   1970-01-12 13:46 +0000   test
  Removed tag gack, gorp

% issue 601
3ecf002a1c572a2f3bb4e665417e60fca65bbd42 bleah13ecf002a1c572a2f3bb4e665417e60fca65bbd42 bleah1
f68b039e72eacbb2e68b0543e1f6e50990aa2bb5 localnewline
0acdaf8983679e0aac16e811534eb49d7ee1f2b4 foobar0acdaf8983679e0aac16e811534eb49d7ee1f2b4 foobar
6ae703d793c8b1f097116869275ecd97b2977a2b newline