Handling Merge Conflicts

About

A merge conflict in Git is a situation where Git is unable to automatically combine changes from two different branches. Git uses a three-way merge algorithm when combining changes. If Git can’t determine which change should take precedence, it stops and reports a conflict. The responsibility then falls to the developer to resolve the conflict manually.

This is Git’s way of saying:

“Both branches have made incompatible changes to the same area of code, and I don’t know which one to keep. Please help.”

Why Do Merge Conflicts Occur?

Merge conflicts typically happen when:

1. Same line changes: Two branches modify the same line in the same file differently.

2. Deletion vs modification: One branch deletes a file or line that the other branch modifies.

3. File mode or rename conflicts: One branch renames or changes file permissions, while the other changes content.

4. Unmerged history: Merging two branches that have drifted significantly or haven’t been regularly rebased or merged with each other.

The Lifecycle of a Merge Conflict

1. Git tries to perform a merge:

   • It uses the current branch (HEAD), the incoming branch (MERGE_HEAD), and the common ancestor.

   • It performs an auto-merge where possible.

2. Conflict detected:

   • If conflicting changes are found, Git marks the file as conflicted and pauses the operation.

3. User intervention:

   • Developer must manually review and decide how to merge the differences.

4. Conflict resolution and staging:

   • After editing the file to resolve the issue, the file must be added (git add) to indicate resolution.

5. Commit or continue operation:

   • Finalize the merge (git commit) or continue the rebase (git rebase --continue) or merge.

What Git Does Internally During Conflict ?

When a conflict happens, Git internally keeps three versions of each conflicted file:

• Base: The common ancestor version.

• Ours: The current branch (your changes).

• Theirs: The incoming branch (other changes).

These are temporarily stored in Git’s index/staging area in different slots (stage 1, 2, and 3) and allow for custom conflict resolution tools or git mergetool to function.

Resolving Conflicts

Git inserts conflict markers directly into the affected file:

<<<<<<< HEAD
Your current changes
=======
Incoming changes from the branch you're merging
>>>>>>> feature-branch

Our task is to:

• Read and understand both versions.

• Decide which to keep, or manually merge parts of each.

• Remove conflict markers.

• Save and stage the resolved file (git add <file>).

What Happens If We Ignore a Conflict?

We cannot complete a merge, rebase, or cherry-pick if conflicts remain unresolved. Git will prevent commits until all conflicts are resolved and marked using git add.

Types of Merge Conflict Scenarios

1. Content Conflict: Most common. Conflicting edits on the same line.

2. File Delete/Modify Conflict: One branch deletes a file while another modifies it.

3. Rename Conflicts: Same file renamed differently in both branches.

4. Binary File Conflicts: Git can't automatically merge binary files like images or compiled objects.

How to Abort a Conflict Operation

If we are overwhelmed or need to start over:

git merge --abort         # Cancels a merge and restores pre-merge state
git rebase --abort        # Cancels a rebase and resets to original HEAD
git cherry-pick --abort   # Aborts a cherry-pick sequence

Tools for Easier Conflict Resolution

Although not required, tools can help:

• git mergetool: Launches a visual merge tool.

• IDEs (e.g., IntelliJ, VS Code) show side-by-side diffs and offer conflict resolution tools.

• git diff helps compare changes before and after resolving.