tests/test-convert-cvs-detectmerge.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 <cryo@cyanite.org>
% create cvs repository with one project
cvs -f -q -d *REPO* init
% populate cvs repository
cvs -f -Q co proj
cvs -f -Q add file1
cvs -f ci -madd file1 on trunk
% create two release branches
cvs -f -q tag -b v1_0
T file1
cvs -f -q tag -b v1_1
T file1
% modify file1 on branch v1_0
cvs -f -Q update -rv1_0
cvs -f ci -madd text
% make unrelated change on v1_1
cvs -f -Q update -rv1_1
cvs -f -Q add unrelated
cvs -f ci -munrelated change
% merge file1 to v1_1
cvs -f -Q update -jv1_0
RCS file: *REPO*/proj/file1,v
retrieving revision 1.1
retrieving revision 1.1.2.1
Merging differences between 1.1 and 1.1.2.1 into file1
cvs -f ci -madd text [MERGE from v1_0]
% merge change to trunk
cvs -f -Q update -A
cvs -f -Q update -jv1_1
RCS file: *REPO*/proj/file1,v
retrieving revision 1.1
retrieving revision 1.1.4.1
Merging differences between 1.1 and 1.1.4.1 into file1
cvs -f ci -madd text [MERGE from v1_1]
% non-merged change on trunk
cvs -f -Q add file2
cvs -f ci -madd file2 on trunk file2
% change on trunk to backport
cvs -f ci -madd other text file1
revision 1.3
add other text
----------------------------
revision 1.2
add text [MERGE from v1_1]
----------------------------
revision 1.1
branches: 1.1.2; 1.1.4;
add file1 on trunk
----------------------------
revision 1.1.4.1
add text [MERGE from v1_0]
----------------------------
revision 1.1.2.1
add text
=============================================================================
% backport trunk change to v1_1
cvs -f -Q update -rv1_1
cvs -f -Q update -j1.2 -j1.3 file1
RCS file: *REPO*/proj/file1,v
retrieving revision 1.2
retrieving revision 1.3
Merging differences between 1.2 and 1.3 into file1
cvs -f ci -madd other text [MERGE from HEAD] file1
% fix bug on v1_1, merge to trunk with error
cvs -f -Q update -rv1_1
cvs -f -Q tag unmerged
cvs -f ci -mfix file1
cvs -f -Q update -A
cvs -f -Q update -junmerged -jv1_1
RCS file: *REPO*/proj/file1,v
retrieving revision 1.1.4.2
retrieving revision 1.1.4.3
Merging differences between 1.1.4.2 and 1.1.4.3 into file1
cvs -f ci -mfix file1 [MERGE from v1-1]
% convert to hg
warning: CVS commit message references non-existent branch 'v1-1':
fix file1 [MERGE from v1-1]
initializing destination proj.hg repository
connecting to *REPO*
scanning source...
using builtin cvsps
collecting CVS rlog
12 log entries
creating changesets
10 changeset entries
sorting...
converting...
9 add file1 on trunk
8 add text
7 unrelated change
6 add text [MERGE from v1_0]
5 add text [MERGE from v1_1]
4 add file2 on trunk
3 add other text
2 add other text [MERGE from HEAD]
1 fix file1
0 fix file1 [MERGE from v1-1]
% complete log
9: '' fix file1 [MERGE from v1-1]
8: 'v1_1' fix file1
7: 'v1_1' add other text [MERGE from HEAD]
6: '' add other text
5: '' add file2 on trunk
4: '' add text [MERGE from v1_1]
3: 'v1_1' add text [MERGE from v1_0]
2: 'v1_1' unrelated change
1: 'v1_0' add text
0: '' add file1 on trunk
% graphical log
o 9: '' fix file1 [MERGE from v1-1]
|
| o 8: 'v1_1' fix file1
| |
| o 7: 'v1_1' add other text [MERGE from HEAD]
|/|
o | 6: '' add other text
| |
o | 5: '' add file2 on trunk
| |
o | 4: '' add text [MERGE from v1_1]
|\|
| o 3: 'v1_1' add text [MERGE from v1_0]
| |\
+---o 2: 'v1_1' unrelated change
| |
| o 1: 'v1_0' add text
|/
o 0: '' add file1 on trunk