Git merge fails unexpected

Question

I've implemented automated merging script in Python. It should merge branches automatically and create pull requests if there are some merging conflicts.

Part of my script looks like:

from subprocess import check_call, CalledProcessError

# E.g. destination_branch = "master", source_branch = "release"
try:
    check_call(['git', 'checkout', '%s' % destination_branch])
    check_call(['git', 'merge', '--no-ff', '%s' % source_branch])
except CalledProcessError:
    # Creating pull request.

It looks like everything is good, but there are some issues here. After some automated merges I've got following errors: error: you need to resolve your current index first Dockerfile: needs merge. Also I'm printing status code of these two step. Status code is 1, which is not good.

As a result, I can see too many pull requests, most of them do not have any merging conflicts.

What is wrong here?

UPDATE:

Before merging something I also have a stuff for branch updating (to keep up-to-date version). It looks like:

try:
    check_call(['git', 'checkout', str(source_branch)])
    # branch successfully checked out
    check_call(['git', 'pull', 'origin', str(source_branch)])
except CalledProcessError:
    # Logging an errors.

One important thing to add:

As guys: @torek, @MarkAdelsberger mentioned in their comments, I've tried also their solution with adding git merge --abort command after failed merging for some reason. So, it doesn't help. It fails with another error: check_call([GIT_CMD, 'merge', '--abort']) File "/usr/lib/python2.7/subprocess.py", line 540, in check_call raise CalledProcessError(retcode, cmd) subprocess.CalledProcessError: Command '['git', 'merge', '--abort']' returned non-zero exit status 128

So, I'm looking for some solution again....

Ideas?

Answer 1

An error message of the form you need to resolve your current index first means you, earlier, started a regular (non-fast-forward) merge and it failed with a merge conflict.

When git merge fails with a merge conflict, Git produces a status 1 exit. The check_call code will raise CalledProcessError if that particular failed merge occurred as part of this code, but all we know for sure is that there was such a failed merge earlier, not that it happened right here:

subprocess.CalledProcessError: Command '...' returned non-zero exit status 1

Once this happens—whether as part of your Python code, or by some other means—the repository's work-tree and index are left in this "merge failed, hand treatment required" state. If you don't know the repository's state in advance, you should find it out before you start, e.g., using git status --porcelain or git status --porcelain=v2 (see the documentation for git status; note that this web version does not show the new v2 format, which was first release in Git 2.11).

If you have put the work-tree and index into this partially merged, intermediate, requires-hand-treatment state and wish to restore it to the pre-merge state, simply run git merge --abort. Do not attempt a commit or a pull request from this half-vast¹ state: the merge must be either finished, or aborted entirely.

Aside: '%s' % expr is silly, just use str(expr), or expr itself if it is already known to be a string.

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¹Say "half-vast" aloud, three times, fast. :-)