GitHub network graph representing wrong first parent after merge

Question

I\'ve found a discrepancy between the GitHub network graph and the git log.

Compare the network graph of my GitHub repository called niagara

Answer 1

This occurred for me. Previously the horizontal line in blue on the image below was exhibiting this behavior (like the green quirk in the question). However, doing the back merge (purple line into black, then re-merge into develop) caused the blue line to appear correctly (maybe has something to do with having the same start point as the black line).

EDIT: After further testing and seeing this bug many times in practice, I've noticed that new commits (esp. merge commits that cause the graph to be re-drawn in a new configuration) will often clear the issue and draw the errant branch correctly, with the correct parents. One doesn't necessarily have to back-merge to fix this. Reassuringly it appears to be purely a graph issue rather than any issue with the underlying commit history.

Answer 2

As far as I can tell, you're right: something is off. In particular, that curved green arrow seems to indicate that commit 6d3ca43 is "twice" a parent of commit a731e72, which makes no sense.

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As far as I know, there is no way, in Git, for a commit to have two parents that are actually one and the same. Even if you're in the following situation (and your working directory is clean),

o -- o [master=HEAD,develop]

running

git merge --no-ff develop

is a no-op: the output of that command is simply

Already up-to-date.

Conclusion: my feeling is that it is a bug on GitHub's side...

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Update: I contacted GitHub's support about this and they got back to me. Read below.

Thank you for calling our attention to that network graph -- I agree that looks confusing. I've opened an internal issue to let our team know about this, and we'll get back to you as soon as we have more information.