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uncommitted changes in your tree beforehand. This has two purposes:
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if you mess up the merge you won't lose anything important; secondly
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this makes it more likely that the merge will be relatively pure.
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It is a good idea to commit as soon as a merge is complete and
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satisfactorily resolved, so as to protect the work you did in the
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merge and to keep it separate from later development. (Mark suggests
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an option to automatically commit when the merge is complete.)
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bzr records what branches have been merged so far. This is useful as
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historical information and also for later choosing a merge ancestor.
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For each revision we record all the other revisions which have come
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into this tree, either by being completely merged or as cherry-picks.
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(This design is similar to the PatchLogPruning__ draft from baz.)
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__ http://wiki.gnuarch.org/PatchLogPruning
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This list of merged revisions is generally append-only, but can be
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reduced if changes are taken back out. Changes can be
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anti-cherry-picked, which causes any successors to change from being
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fully-merged to being cherry-picked.
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The list of merged patches is stored delta-compressed.
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``tla update`` performs a useful but slightly subtle change: it pulls
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in only changes that have been made on the other branch since you last
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merged. That is to say, it sets the merge basis as the most recent
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merged-from point on the other branch.
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This means that any changes which were taken from your branch into the
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other and then reversed or modified will not be reversed. Those
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changes will always be considered as new in your branch and will have
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The basic idea of a merge that only brings in remote work and doesn't
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revert your own changes is good. It could be handled by a three way
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merge with a specified version but perhaps there is a better way.
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Merge is conducted at two levels: merging the tree shape, and merging
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the file contents. Merging the tree shape means accounting for
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renames, adds, deletes, etc. This is almost the same as merging the
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two inventories, but we need to do a smart merge on them to enforce
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structural invariants.
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Some tools insist that you complete the entire merge while the
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``merge`` command is running; you cannot exit the program or restart
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the computer because state is held in memory. We should avoid that.
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At least when the tool is waiting for user input it should have
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written everything to disk sufficient to pick up and continue the
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merge from that point.
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This suggests that there should be a command to continue a merge;
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perhaps ``bzr resolve`` should look for any unresolved changes and
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start resolving them. ``bzr merge`` can (by default) automatically
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One hard aspect is transformation of the tree state such as renames,
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directory creation, etc. This might include files swapping place,
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etc. We would like to do atomically but cannot.
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If a merge has been begun but not committed then ``bzr revert`` should
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put everything back as it was in the previous revision. This includes
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resetting the tree state and texts, and also clearing the list of
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pending-merged revisions.
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It should be possible to download all the data necessary to do a merge
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from a remote branch, then disconnect and complete the merge. It
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should be possible to interrupt and continue the merge during this
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This implies that all the data is pulled down and stored somewhere
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locally before the actual merge begins. It could be pulled either
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into the revision history on non-trunk revisions, or into temporary
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It seems useful to move all revisions and texts from the other branch
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into the storage of this branch, in concordance with the general idea
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of every branch moving towards complete knowledge. This allows the
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most options for an offline merge, and also for later looking back to
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see what was merged in and what decisions were made during the merge.
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What does cherry-picking mean here? It means we merged the changes
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from a revision relative to its predecessor? But what if we actually
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want to merge the delta relative to something else? Can that be
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ddaa says perhaps we should have three states: with respect to a
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branch any foreign revision can be *merged*, *not-merged*, or *rejected*.
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The difference between *not-merged* and *rejected* is that not-merged
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patches will try to merge in when you next sync from their
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branch, whereas rejected changes will not.
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'rejected' seems technically equivalent to it merged with the text
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changes not present. But perhaps there should be something more?
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Bitkeeper has the very interesting feature of *excluded* changesets,
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where something previously committed to or merged into this branch is
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treated as if it never occurred. Because of their representation of
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changes as a weave they can do this without dependency problems on
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Although the later changes will not mechanically conflict, there is of
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course no guarantee that excluding the patch will generate anything
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Cool though this is, it seems that few people understand it well
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enough to use it confidently. It complicates the merge algorithms and
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seems to have been a source of some bugs. It may be better just to
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apply a reversed patch.
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Each revision has a pointer to two sets, represented as lists of
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* ``merged-revisions``
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* ``picked-revisions``
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When a branch is merged in, its revision history plus merged-revisions
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are added to the merged-revisions file. When changes are
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cherry-picked in they are added to the picked-revisions.
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These lists are stored in the text store and their sha1 and id is
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stored in the revision.
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While a merge is underway, these are stored in
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``.bzr/pending-merged-revisions`` and
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``.bzr/pending-picked-revisions``.
added
removed
doc/merge.txt
