Subversion Merge

Why merge?

Unlike a more simplistic approach of committing the same patch to both the trunk and the branch, using svn merge helps preserve file history information (like renaming files, or adding new files).

svn:mergeinfo

Subversion tracks merges using a property called svn:mergeinfo.

The problem: On branch_3x/, paths that were merged from trunk/ below the top level directory began to accumulate revisions for every single commit that was the result of a merge, even when the commits did not include any direct changes to these paths. This is because a path with an svn:mergeinfo value that differs from its parent's has to keep its own private copy of everything, including a full copy of its parent's merge information. As a result, every time any of such a path's ancestors is a merge target, the private svn:mergeinfo value has to be updated and included in the branch_3x/ commit.

The fix: Always run svn merge from the top level directory (i.e. the directory containing solr/ and lucene/). There are two merge scenarios: a) fully merging all changes from a trunk/ commit into branch_3x; and b) performing a partial merge, where only some of the changes from the trunk/ commit will be merged into branch_3x.

Full Merge

To merge all of the changes from a trunk/ commit into branch_3x/, go to the top level directory in your branch_3x/ working copy, and issue this command:

svn merge -c XXXXXX https://svn.apache.org/repos/asf/lucene/dev/trunk

where XXXXXX is the revision of the source trunk/ commit.

Subversion 1.8 Note

it seems like svn 1.8 resolves all these complications. After, you committed to trunk just change to branch working copy and run

svn merge ^/lucene/dev/trunk -cXXXXXX

Partial Merge

To merge a subset of changes from a trunk/ commit into branch_3x/, go to the highest level directory possible containing the changes you want to include, but not containing any changes you want to exclude, and run {{ svn merge -c XXXXXX }} with the corresponding repository URL. Repeat until you have merged all of the desired changes.

Finally, go to the top level directory and issue the following command:

svn merge --record-only -c XXXXXX https://svn.apache.org/repos/asf/lucene/dev/trunk

The --record-only parameter instructs Subversion to skip performing any actual merging, but to instead modify the svn:mergeinfo property values to include the XXXXXX revision. Crucially, in addition to modifying the svn:mergeinfo property value for the top level directory itself, all paths' svn:mergeinfo property values are also adjusted, so that merged paths with the same merge information as their parents don't carry their own private svn:mergeinfo copies.

Merge Conflicts

If your trunk/ commit contained changes to paths that are not in exactly the same place on both trunk/ and branch_3x/ (e.g. moved paths), Subversion will report a merge conflict.

Once you have merged the desired changes from the trunk/ commit – see the instructions at Partial Merge – tell Subversion that you have resolved the conflict(s):

svn resolve --accept=working  your/conflicted/path

Handling Moved Paths

The Subversion book says:

For long-lived release branches (as described in the section called "Common Branching Patterns"), perform merges only on the root of the branch, not on subdirectories.

However, when one or more of the changed paths in the trunk/ commit has been moved from its location under branch_3x/, e.g. modules/analysis/* and modules/benchmark, merging below the top level directory is necessary; follow the instructions under Partial Merge and Merge Conflicts.

Without taking further action, though, each merged moved path will carry its own private svn:mergeinfo copy of all its ancestors' merge information (see the svn:mergeinfo section for an explanation of why this is a problem).

For the specific case of merged moved paths, to avoid the svn:mergeinfo property value accumulation problem on branch_3x/, we remove the svn:mergeinfo property from all paths except for the top level directory and its immediate children solr/ and lucene/.

svn stat indicates the paths with modified properties via an 

 "M"

in the second status column from the left. For each of these paths (except for the top level directory, solr/, and lucene/), run the following command:

svn propdel svn:mergeinfo  remove/mergeinfo/from/this/path

example merging with moved path from /trunk to branch_3x

you@machine:~/workspace/lucene-branch3x/lucene/contrib/icu$ svn merge -c 1130547 ~/workspace/lucene-clean-trunk/modules/analysis/icu
--- Merging r1130547 into '.':
U build.xml
you@machine:~/workspace/lucene-branch3x$ svn merge -c 1130547 ~/workspace/lucene-clean-trunk
--- Merging r1130547 into '.':
C modules
Summary of conflicts:
Tree conflicts: 1
you@machine:~/workspace/lucene-branch3x$ svn resolved modules
Resolved conflicted state of 'modules'
you@machine:~/workspace/lucene-branch3x$ svn propdel svn:mergeinfo lucene/contrib/icu
property 'svn:mergeinfo' deleted from 'lucene/contrib/icu'.
you@machine:~/workspace/lucene-branch3x$ svn commit -m "merge r1130547"

Example merge from trunk to branch_3x

  1. svn merge https://svn.apache.org/repos/asf/lucene/... at the top level, noting tree conflicts, e.g. from modules/
  2. For each of the conflicts, svn resolve --accept=working conflicted/path/here
  3. For each changed path/file, figure out the highest possible path (root is highest) in common between trunk and branch_3x, change to that directory in branch_3x, then merge from the corresponding trunk dir.
  4. svn stat to discover which directories and files have accumulated svn:mergeinfo properties below the top-level directories that get generated by merging below the top-level dir - the second column from the left for each changed item will have an 
     "M"
    for property changes
  5. For each of the dirs/files below the top-level dir (except solr/ and lucene/, which are allowed to accumulate svn:mergeinfo), svn propdel svn:mergeinfo path/to/item/with/mergeinfo
  6. svn stat again to be sure nothing is left out or unintentionally included
  7. Compile/test, if the build is affected by the changes
  8. svn commit

References

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