Unit-testing – If two individual branches pass unit tests, once they’re merged, is the result also guaranteed to pass unit tests

continuous integrationtestingunit testing

Suppose we have two branches A and B which have been forked from master.

Both branches A and B make some changes and implement some unit tests. They pass all current and new tests, then are merged back into master. For simplicity, there are no merge conflicts.

Is it guaranteed that the resulting code on master will also pass the unit tests?

The reason I ask the question, is I often see GitHub unit tests run automatically once a pull request is made. If they pass, then the code may be merged into master. However, I think master could still end up failing tests if two pull requests break each other? I would have thought
a better solution would be:

  1. When a pull request is made, run the unit tests to catch anything egregious.
  2. Have conversations, code reviews etc…
  3. Once the pull request is ready to be merged, do a test merge into master, run the unit tests, if all succeeds, commit the merge.

So you never actually commit broken code into master.

Best Answer

No.

The simplest example I've seen is: branch A cleans unused imports in a file. Branch B adds code that actually uses some of the unused imports. Git merges automatically since the lines that were changed were not the same. Code can no longer compile and unit tests can not run.

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