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In earlier chapters, we developed an extended example out of the
hello-world project.
Alice and Bob, the primary programmers on the project, started one archive and created some revisions there.
Candice, a user of the project, created her own archive, started a
branch of the hello-world project, and began maintaining her own
local modifications.
In this chapter, we'll begin to consider a situation that is more
typical of free software projects in the real world. Here, we'll
consider Alice and Bob to be the maintainers of a public project, and
Candice as a major remote contributor to the project. We'll identify
the new revision control needs that arise from that arrangement, and
look at some arch commands that help to satisfy those needs.
So far, if you've been following the examples, Candice has an elementary branch. She made a branch from the mainline, made some local changes, and has kept her branch up-to-date with Alice and Bob's mainline.
We're supposing, at this point, that Alice and Bob want to merge Candice's changes into the mainline.
Well, that merging work has already been done. Candice's latest revision is exactly the tree that Alice and Bob want. They can incorporate that merge into their mainline very simply, by committing Candice's latest revision to their own mainline:
% tla get -A candice@candice.net--2003-candice \
hello-world--candice--0.1 \
hw-C
[...]
% cd hw-C
% tla set-tree-version -A lord@emf.net--2003-example \
hello-world--mainline--0.1
% tla make-log
++log.hello-world--mainline--0.1--lord@emf.net--2003-example
[... edit log file (consider `tla log-for-merge') ... ]
% cat ++log.hello-world--mainline--0.1--lord@emf.net--2003-example
Summary: merge from Candice's Branch
Keywords:
Patches applied:
* candice@candice.net--2003-candice/hello-world--candice--0.1--patch-2
merge from mainline sources
* candice@candice.net--2003-candice/hello-world--candice--0.1--patch-1
Punctuated the output correctly
* candice@candice.net--2003-candice/hello-world--candice--0.1--base-0
tag of
lord@emf.net--2003-example/hello-world--mainline--0.1--patch-1
% tla commit
[....]
Read Carefully Note: Note carefully the trick we just used.
Candice's latest revision was exactly what Alice and Bob wanted –
they combined get with set-tree-version to turn Candice's tree
into one they could easily commit to their own mainline.
Let's consider what happens as development proceeds on both branches. For this purpose, we'll introduce something new: a way of diagraming branches and the merges between them.
After the examples so far, we have this situation:
mainline--0.1 candice--0.1
------------- ------------
base-0 -----------> base-0 (a tag)
patch-1 ---------' patch-1
patch-2 ----------> patch-2
patch-3 ----------' --------'
patch-4 <-----------'
which tells us that the candice branch is a tag
of patch-1 from the mainline; that at patch-2 of the candice
branch, there was a merge of everything up to patch-3 of the
mainline; and finally that patch-4 of the mainline merges
in everything up to patch-2 from the candice branch.
Whenever we have a such a diagram in which none of the merge lines cross, that is a simple development branch.
The significance of a simple development branch is that it's a model for how two development efforts can work asynchronously on one project. Within each effort – on each branch – programmer's use the "update/commit" style of cooperation (see The update/commit Style of Cooperation). However, changes on one branch have no effect on the other until the two branches are merged.
Let's suppose that more work happens on both the mainline and
candice branches, leaving us with:
mainline--0.1 candice--0.1
------------- ------------
base-0 -----------> base-0 (a tag)
patch-1 ---------' patch-1
patch-2 ----------> patch-2
patch-3 ----------' --------' patch-3
patch-4 <-----------' patch-4
patch-5
patch-6
% tla revisions --summary -A candice@candice.net--2003-candice \
hello-world--candice--0.1
base-0
tag of
lord@emf.net--2003-example/hello-world--mainline--0.1--patch-1
patch-1
Punctuated the output correctly
patch-2
merge from mainline sources
patch-3
added a period to output string
patch-4
capitalized the output string
% tla revisions --summary -A lord@emf.net--2003-example \
hello-world--mainline--0.1
base-0
initial import
patch-1
Fix bugs in the "hello world" string
patch-2
commented return from main
patch-3
added copywrong statements
patch-4
merge from Candice's Branch
patch-5
fixed the copyrwrong for hw.c
patch-6
fixed the copyrwrong for main.c
Let's consider a scenario in which our goal is to merge the new work
on the mainline branch into the candice branch. In other words,
we want to wind up with:
mainline--0.1 candice--0.1
------------- ------------
base-0 -----------> base-0 (a tag)
patch-1 ---------' patch-1
patch-2 ----------> patch-2
patch-3 ----------' --------' patch-3
patch-4 <-----------' patch-4
patch-5 --------> patch-5
patch-6 ------------'
How can we perform that merge? Let's start with the latest pre-merge
candice revision (patch-4):
% tla get -A candice@candice.net--2003-candice \
hello-world--candice--0.1--patch-4 \
hw-C-4
[....]
% cd hw-C-4
Here are two techniques that don't work:
replay will try to apply all "missing" changes from the mainline
into the candice tree. The list of changeset it will apply is
given by:
% tla missing --summary \
-A candice@candice.net--2003-example \
hello-world--mainline--0.1
patch-4
merge from Candice's Branch
patch-5
fixed the copyrwrong for hw.c
patch-6
fixed the copyrwrong for main.c
Problematic in that list is patch-4. It's a merge that includes
all of the changes from the candice branch up to its patch-2
level. Yet those changes are already present in the patch-4
revision of the candice branch – so replay will be applying them
redundantly (cause patch conflicts).
Note of Warning: The replay command will not prevent you from running
further replays even though the source tree is not in a consistant state.
TLA in its current incarnation does not merge reject files. This leaves
open the possibility that patch rejects will be lost if a second replay
is performed before the rejects from the first replay are resolved. (Some day
TLA may be able to merge multiple rejects into a combined reject.)
Advanced User Note: The replay command has options that would
allows us to skip the patch-4 revision from the mainline. That
sort of solves the problem, but it has some drawbacks. First, it
means that patch-4 will continue to appear in the missing output
of the candice branch. Second, there is nothing that guarantees us
that the patch-4 changeset contains only merges from the candice
branch. If Alice and Bob made other changes in patch-4, and we skip
that changeset, those other changes will be lost.
Suppose we try to update from the mainline branch. Recall that
update will compute a changeset from the youngest mainline
ancestor of the project tree to the tree itself, then apply that
changeset to the latest mainline revision.
We have a notation for this. A changeset from X to Y is written:
delta(X, Y)
In this case, update will start by computing a changeset from the
mainline patch-3 revision to our project tree:
delta(mainline--0.1--patch-3, hw-C-4)
The tree that results for applying a changeset from X to Y to a
tree Z is written:
delta(X, Y) [ Z ]
In other words, the result of update in our example can be described
as:
delta(mainline--0.1--patch-3, hw-C-4) [mainline--0.1--patch-6]
Here's the problem, though. The patch-3 revision of mainline was
not previously merged with the candice branch. Thus, the changeset
delta(mainline--0.1--patch-3, hw-C-4)
will include, among other changes, the changes from patch-1 and
patch-2 of the candice branch.
Unfortunately, the tree we'll be applying that changeset to,
mainline--0.1--patch-6, has already been merged with
base-0...patch-2 of the candice branch.
As with replay, update will cause merge conflicts by making
zredundant changes.
Using just our delta notation and merge diagrams, let's look at
solving this merge problem cleanly.
Remember that we currently have:
mainline--0.1 candice--0.1
------------- ------------
base-0 -----------> base-0 (a tag)
patch-1 ---------' patch-1
patch-2 ----------> patch-2
patch-3 ----------' --------' patch-3
patch-4 <-----------' patch-4
patch-5
patch-6
and our goal is to create a new merge, for patch-5 of Candice's branch:
--------> patch-5
patch-6 ------------'
We might decide to start with a mainline branch and merge in missing
candice changes, or start with a candice tree and merge in missing
mainline changes. Let's assume the latter (merging into a candice
tree).
In this case, mainline-0.1 revision patch-6 is "up to date" with
candice-0.1 revision patch-2. We want to apply all changes
since then to the latest candice revision:
with:
ancestor := candice--0.1--patch-2
merge_in := mainline--0.1--patch-6
target := canidice--0.1--patch-4
answer := delta(ancestor, merge_in)[target]
The arrows in the merge diagram are critical to figuring out the right
answer. For example, suppose that the arrow from Candice's patch-2
to the mainline revision patch-4 wasn't there. Then the answer
would be:
with:
ancestor := mainline--0.1--patch-3
merge_in := mainline--0.1--patch-6
target := canidice--0.1--patch-4
answer := delta(ancestor, merge_in)[target]
Tracing out the arrows for a given merge is a tedious process.
It's automated by the star-merge command:
It's a bit beyond the scope of this tutorial to explain the complete solution to the development branch merging problem in general. The two solutions shown above illustrate two cases, but slightly different solutions are sometimes necessary.
What you should know is that when you have simple development
branches (see Simple Development Branches in Development Branches – The star-merge Style of Cooperation), the command
star-merge knows how to merge between them without causing spurious
merge conflicts.
In ordinary use, you invoke star-merge in the tree you want to merge
info, providing as an argument the tree you want to merge from:
% tla get -A candice@candice.net--2003-candice \
hello-world--candice--0.1--patch-4 \
merge-temp
% tla star-merge lord@emf.net--2003/hello-world--mainline--0.1