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.
