如何在没有更改的情况下将分支合并到git中的master分支中？

Question

Having a basic doubt in git branching and merge . Can a branch be merged without any changes into master . If so , What happens to the delta changes that exists between master and branch . Will that be overridden with master branch changes . Refer the below scenario :

I have two branches master and BugBranch.

STEP 1:

C--B--A   (master)
      |
      X   (branch)

Took a branch from master say BugBranch . Now I have a file TestMerge.java.

public static void main(String[] args){
...
...
callSomeMethod();
  switch(decision)
  {
    case A:
        do_something
        break;
    case B:
        do_something
        break;
  }
}

So at STEP 1 , both master and BugBranch looks same.

STEP 2: The file(TestMerge) is changed in master as below . ( just removed the switch )

public static void main(String[] args){
...
...
callSomeMethod();
}

C--B--A--1  (master)
      |
      X     (branch)

STEP 3: The problem is when I try to merge the branch with the master . Expecting a merge conflict in file , since there exists a delta for sure and want to preserve the changes in BugBranch . But the strange thing is that , its getting overridden with the master changes .

Tried out the below commands

git checkout master

git merge BugBranch

This is really confusing !

Answer 1

A Git branch is just a pointer to a commit. The X branch in your question points to commit A and for many Git operations, X can be used instead of A and vice-versa and you get the same result.

Only by its creation, a Git branch does not produce any change in the repository. Until you do some changes in the files and commit them, the branch does not diverge from its source branch ( master in your case).

You change some files and committed on master (commit 1 ). If you didn't change and commit anything on branch X , it still points to commit A which is in the history of branch 1 (actually, it's its parent).

When you want to commit branch X into branch master , Git detects that the branch X is in the past of branch master ; starting from master , if Git repeatedly jumps from one commit to one of its parents it can reach the commit pointed by branch X (which is the A commit).

This means the branch X doesn't contain anything that is not already in branch master . In other words, there is nothing to merge and Git informs you about this situation with the message "Already up-to-date.".

On the other hand, if you want to merge master into X , because master is in the future of X , Git transforms the merge operation into a "fast-forward" operation. This means, it doesn't create an unnecessary new merge commit and just pushes the X branch (the branch you merge into) forward until it reaches the master commit (the branch you merge from).

You can force Git to create a merge commit by using the --no-ff flag in the git merge command line but, except for really exceptional situations, you don't have any reason to do it. It produces a commit that doesn't introduce any change in the files.