tests/test-transplant.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]>

adding r1
adding r2
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
adding b1
created new head
adding b2
adding b3
4  b3
3  b2
2 0:17ab29e464c6  b1
1  r2
0  r1
updating working directory
4 files updated, 0 files merged, 0 files removed, 0 files unresolved
1 files updated, 0 files merged, 3 files removed, 0 files unresolved
% rebase b onto r1
applying 37a1297eb21b
37a1297eb21b transplanted to e234d668f844
applying 722f4667af76
722f4667af76 transplanted to 539f377d78df
applying a53251cdf717
a53251cdf717 transplanted to ffd6818a3975
7  b3
6  b2
5 1:d11e3596cc1a  b1
4  b3
3  b2
2 0:17ab29e464c6  b1
1  r2
0  r1
updating working directory
4 files updated, 0 files merged, 0 files removed, 0 files unresolved
1 files updated, 0 files merged, 3 files removed, 0 files unresolved
% rebase b onto r1, skipping b2
applying 37a1297eb21b
37a1297eb21b transplanted to e234d668f844
applying a53251cdf717
a53251cdf717 transplanted to 7275fda4d04f
6  b3
5 1:d11e3596cc1a  b1
4  b3
3  b2
2 0:17ab29e464c6  b1
1  r2
0  r1
% remote transplant
requesting all changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 2 changes to 2 files
updating working directory
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
searching for changes
applying 37a1297eb21b
37a1297eb21b transplanted to c19cf0ccb069
applying a53251cdf717
a53251cdf717 transplanted to f7fe5bf98525
3  b3
(transplanted from a53251cdf717679d1907b289f991534be05c997a)
2  b1
(transplanted from 37a1297eb21b3ef5c5d2ffac22121a0988ed9f21)
1  r2
0  r1
% skip previous transplants
searching for changes
applying 722f4667af76
722f4667af76 transplanted to 47156cd86c0b
4  b2
3  b3
(transplanted from a53251cdf717679d1907b289f991534be05c997a)
2  b1
(transplanted from 37a1297eb21b3ef5c5d2ffac22121a0988ed9f21)
1  r2
0  r1
% skip local changes transplanted to the source
adding b4
updating working directory
4 files updated, 0 files merged, 0 files removed, 0 files unresolved
searching for changes
applying 4333daefcb15
4333daefcb15 transplanted to 5f42c04e07cc
% remote transplant with pull
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
updating working directory
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
searching for changes
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
applying a53251cdf717
a53251cdf717 transplanted to 8d9279348abb
2  b3
1  b1
0  r1
% transplant --continue
adding foo
adding toremove
adding added
removing toremove
adding bar
2 files updated, 0 files merged, 2 files removed, 0 files unresolved
created new head
applying a1e30dd1b8e7
patching file foo
Hunk #1 FAILED at 0
1 out of 1 hunks FAILED -- saving rejects to file foo.rej
patch failed to apply
abort: Fix up the merge and run hg transplant --continue
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
applying a1e30dd1b8e7
patching file foo
Hunk #1 FAILED at 0
1 out of 1 hunks FAILED -- saving rejects to file foo.rej
patch failed to apply
abort: Fix up the merge and run hg transplant --continue
a1e30dd1b8e7 transplanted as f1563cf27039
skipping already applied revision 1:a1e30dd1b8e7
applying 1739ac5f6139
1739ac5f6139 transplanted to d649c221319f
applying 0282d5fbbe02
0282d5fbbe02 transplanted to 77418277ccb3
added
bar
foo
% test transplant merge
adding a
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
created new head
% tranplant
applying 42dc4432fd35
1:42dc4432fd35 merged at a9f4acbac129
% test transplant into empty repository
requesting all changes
adding changesets
adding manifests
adding file changes
added 4 changesets with 4 changes to 4 files
% test filter
filtering
applying 17ab29e464c6
17ab29e464c6 transplanted to e9ffc54ea104
filtering
applying 37a1297eb21b
37a1297eb21b transplanted to 348b36d0b6a5
filtering
applying 722f4667af76
722f4667af76 transplanted to 0aa6979afb95
filtering
applying a53251cdf717
a53251cdf717 transplanted to 14f8512272b5
3  b3
2  b2
1  b1
0  r2