Branching and Merging in CVS

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Branching and Merging in CVS

• Jamie Nicolson
• May 16, 2005

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What is a branch?

• In the normal course of development, changes are
  checked in serially. The current state of a file
  represents all the changes that have been made up
  to this point.
• Branching creates an alternate timeline of
  changes, starting from some common point in the
  past.

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Why branch?

• In the commercial world, when software is
  released in official versions, you branch to
  separate bug-fix changes from new development.
• For research, it can shield you from other
  people's checkins when you're getting close to a
  deadline.
• Example the Pascal IE challenge deadline was
  only a few weeks after the Java 1.5 switchover.
  We branched to avoid the complications that
  caused.

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How to make a branch

• CVS overloads the tag command to create
  branches
• cvs tag -b ltbranchnamegt ltfilesgt...
• The branch name is actually an alias for a set of
  numeric branch numbers, one per file
• If the branching point is x, the branch will be
  of the form x.0.n, where n is a even number
  starting from 2
• Example branching from 1.1 creates branch 1.1.0.2

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Checking out the branch

• To checkout the branch, specify the branch name
  to cvs co or up
• Example cvs up -r PASCAL_IE_BRANCH
• Checking out on the branch places a sticky tag
  on the file it remembers that it is checked out
  on that branch
• As long as the sticky tag is in place, all
  checkins go onto the branch
• To check out the trunk, you need to clear the
  sticky tag with cvs up -A

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Merging changes

• The CVS merge command is cvs up -j
• To pull all the changes made to the trunk into
  your branch, cvs up -j HEAD
• Be careful now you need to keep track which
  version of the trunk you've synced with
• Solution tag the trunk at the point you've
  synced with
• Then cvs up -j BRANCH_SYNC -j HEAD to pull in
  all changes since then

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Undoing changes with cvs merge

• cvs up -j can generally be used to back out
  changes
• To undo the changes between version 1.x and 1.y,
  cvs up -j 1.y -j 1.x
• Note you are specifying the version numbers in
  reverse chronological order because you are
  moving backwards in time

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Landing your branch

• Eventually you may want to merge your branch back
  into the trunk
• Check out the trunk cvs up -A
• Merge in the branch cvs up -j ltbranchnamegt
• If you've been pulling in code from the trunk,
  you may have conflicts

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Caveats

• Don't add new files to a branch! Add them (in
  some form) to the trunk, then check them out on
  the branch.
• Don't delete branches. There is no way to undo a
  cvs tag -d. CVS maintains no history of
  creating, modifying, or deleting tags.