Hi all,
After having a closer look at merge and discussing it
with Julian on IRC, there seems to be no silver bullet.
However, we identified a few things that could be changed
and set of constellations that make merge harder than
it needs to be.
For the first, there will be another post soon. The second
boils down to policy. Luckily, SVN has a mechanism to
enforce policies: server-side hook scripts. My proposal
is to develop a small set of scripts that a user can
combine to prevent situations that her life harder than
necessary. This should give us enough time to improve
the merge logic inside the SVN libs.
The following pre-commit scripts / policies would be useful.
* Common parts [not a policy]
We first check whether the commit contains a changed
svn:merge-info property. This limits the performance
impact on non-merge commits and we need to identify
all changed svn:merge-info anyway.
Also, the merges that happened on the source branch
from a different location than the target branch are
of no interest for the policy checkers. E.g.:
r20: merge r19 from ^/sub-branch to ^/branch
txn: merge r10-20 from ^/branch to ^/trunk
Both merges will show up in the merge-info delta but
we only need to evaluate the second one.
* Strict merge hierarchy
A merge from A->B is only allowed, if the copy-from
of A is B or vice versa and the copy source has not
been replaced since the copy). This prevents circular
merges and others (note 1).
In a more sophisticated implementation, we could identify /
allow for renamed branches as well as A and B having
the same relative path to some parents that form a
direct branch (i.e. allow sub-tree merges).
* No sub-tree merges
Like the above but without the check for parents.
* No aggregate merges
There must only be one source branch, i.e. we can't
merge from branches A and B to C in the same revision.
* No distributive merges
For each path being merged (i.e. having a merge-info
delta), the relative paths in source and target must
correspond (i.e. start as the same and then may get
renamed etc.). This is basically the same as the
"sophisticated" part of the check for strict merges.
* No cherry picking
Check that the source branch does not contain revisions
that lie before the last to-be-merged revision but
have neither been merged before nor are being merged
right now.
* No criss-crossing
Prevent situations like the criss-cross examples here:
http://wiki.apache.org/subversion/SymmetricMerge
For a merge A->B, abort if there has been a merge
B->A after the last revision of A to be merged to B.
This only valid for non-cherry-picking merges and
only if the change sets of both merges overlap.
Except for the last one those checks will simply verify
that the user followed certain policies. They should,
therefore, rarely reject a commit.
Again, the user shall be free to combine (or not use)
these policies although not all combinations are meaningful.
Thoughts?
-- Stefan^2.
Note 1:
One thing that we might want to support is integration
branches where a temporary branch is being used as
an intermediate merge target:
integrate A->B as
rN: copy B->A_integration
rN+1: merge A->A_integration
rN+x: ... various changes on A_integration
rN+y: merge A_integration->B
rN+y+1: delete A_integration
These checks become more complicated, requires
naming conventions for the integration branches etc.
Received on 2012-04-19 12:49:41 CEST