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Git merge

Merge policies

Fast-forward merge

  • It is only possible when there are no further commits in the receiving branch after the branch creation
  • There is no merge conflict in FF
  • First checkout to the master and then merge feature branch
  • Git simply takes the master branch pointer and moves it to the last commit in feature branch!
  • After merging the branch can be safely removed git branch -d branch-name

HEAD

3-way merge

  • A new merge commit in master is created. The new merge commit incorporates all the changes of the secondary branch
  • Conflicts may appear!
  • This new commit has 2 parents: one from master, one from feature branch
  • The feature branch is incorporated to the master branch, all its structure now lives in the master branch
  • All the commits in feature branch is incorporated as a single commit in master

HEAD

Merge conflicts

  • Merge conflicts occur when same files were edited in both branches
  • Conflicts must be resolved on the merge commit
  • When a merge has failed due to conflict, the following message is shown
Auto-merging file7.txt
CONFLICT (content): Merge conflict in file7.txt
Automatic merge failed; fix conflicts and then commit the result.
  • That means the merge commit has not yet been created and it's WIP until you manually fix and commit it
  • Conflict resolution pattern
<<<<<<< HEAD
Line replaced in master branch
=======
Line replaced in BR-3 branch+
>>>>>>> BR-3

  • Merge conflicts are identified between the conflicting files appear 3 times in git ls-files -s

  • 0 (no conflict): file is the same in the repository

  • 1: file version that is common for both branches - ancestor commit
  • 2: last file version from master branch <<<<<<< HEAD
  • 3: last file version from feature branch >>>>>>> feature

  • As soon as the conflict is solved the file changes to 0

  • After fixing the conflicts, git commit (a edit for message will open)

  • .git/MERGE_HEAD is the pointer for last commit of feature branch.
  • .git/HEAD is the pointer of last commit in master branch (that is merging)
  • Both of them will be parents in the new merge commit