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