How does merging a branch actually work (under the hood)?

Question

Hi, this may be a naive question, but, as asked in the object, what is the actual way used by versioning softwares to merge a branch back into the main trunk without generating broken code?

Here's a quick example: I make a branch out of the main trunk for the program "Hello World Power edition". I add support for Klingon. This is a radical change that changes the way the function printHelloWorld() works.

Meanwhile, because of bug #749 that cause "Hello World" to be written "Helo World", the function printHelloWorld() in the main trunk has been changed.

Now, the problem that I see here is: when i merge by branch back to the main trunk i experiment a clash in the function printHelloWorld() within the file sayHello.py

How does a VCS program know how to add the Klingon support from my branch and keep the bug fix in the main trunk? What are the human-driven or software-driven strategies to avoid this?

Thanks in advance.

Answer 1

In such a case the version control system can't merge automatically, you have to do the merge by hand. Good unit tests will help you to ensure that no functionality is lost.

Answer 2

Before you can merge your branch back to trunk, the version control system will update your working copy with the changes in trunk since you branched out. It will just not allow you to merge without this update. This ensures you get all the bug fixes in the trunk in your next commit.

A good strategy for working on a branch is to port changes in trunk into you branch on very regular basis. This ensures that you don't drift too apart from trunk, leading you to have problems when you eventually merge back into trunk.

Answer 3

How does a VCS program know how to add the Klingon support from my branch and keep the bug fix in the main trunk?

VCS knows nothing about semantic of your source code it sees it as a bunch of text/binary files. VCS system uses diff / merge algorithms to detect conflicts between yours and current file version. It is your responsibility to resolve such conflicts because only you know semantic of these changes. Some VCSs like SVN would require you to update your working copy with latest changes from trunk before allowing you to commit changes to make sure that nothing is lost.

To make sure that you didn't break anything and all previous bug fixes were not broken you should use code reviews, unit tests and other practices. Continuous integration is a good way to keep software healthy.