How do you get git bisect to ignore merged branches?

Question

I'm aware that git bisect is branch-aware by design, so that if between good commit, G, and bad commit, B, you merged in a branch, it needs to take those changes into consideration as well, as the bug may be contained in the branch.

In my case I have a dependency as a side branch and I merge in changes to my main project from time to time. The dependency can be considered a library that has a different way of running, different build-system etc. from my main project, but I still want recent changes from it via merges to the main branch.

The problem is then that while bisecting in this scenario, you end up on non-compilable commits in the commits from the dependency.

I would really just want to consider each branch merge as a single commit while doing the bisection.

A workaround I've found so far is making a list of valid commits G..B with git log --first-parent, and then while bisecting, do git bisect skip if the current commit isn't in that list. That takes a lot of time though (lots of files to checkout/change for each skip).

So the question is: Is there any way of doing --first-parent with git bisect or providing a list of commits i feel are valid to be able to avoid checking out branches I know already are not compilable? How do we only check the commits marked o in the diagram?

G---o---o---o---o---o---o---B  main project branch
   /       /       / 
  x---x---x---x---x            dependency
           \ /
            x'                 dependency project taskbranch

Edit: added diagram for clarity

Answer 1

I've been looking for something like this too. As far as I've got is that git rev-list --bisect --first-parent seems to do what you want to, and the docs for rev-list implies that the --bisect option is what bisect uses internally - but getting git bisect to add that flag to its call(s) to rev-list seems less trivial:

The bisect command is implemented by a shell script git-bisect, which in turn uses a builtin command bisect--helper to actually do the interesting part ("computation, display and checkout" says the comment...), apparently based on a bunch of magic state files in .git/. And it seems to be the rev-list command that is reusing code from bisect--helper rather than the other way around as you might expect.

So, you'd have to extend the bisect--helper code's commit filtering to do it, I think.

As a workaround, something like this might work: after bisect checks something out for you, reset to a different one using git rev-list --bisect --first-parent, test that and mark it good/bad/skip and continue from there.

Answer 2

If the history looks like:

A - B - C - H - I - J - K - L
         \              /
          D - E - F - G

where L is bad, B is good, and you want to ignore the DEFG branch, then running

$ git bisect start
$ git bisect skip $( git rev-list G ^C )
$ git bisect bad L
$ git bisect good B

where B,C,G,and L are the respective shas seems to do what you want.

Answer 3

I thought of one possible solution, but I'm still hoping to find something more elegant:

Mark all second-parents of all merges into the main branch as good

Marking all remote parents of each merge as good will consider all the commits preceding them as good (and as such skipped by bisect). This solution should also be generic enough to handle multiple merges from multiple branches, leaving only the commits on the main branch.

git rev-list --first-parent --merges --parents GOOD..BAD \
| sed 's/^[^ ][^ ]* [^ ][^ ]* //' \
| xargs git bisect good

(replace GOOD and BAD with the relevant commits)

The regex in sed removes the first two commits of each line; the merge commit itself, and the first parent, leaving the rest of the parents (usually just the second one).

Given the history stated in the question, running the one-liner would give you:

G---o---o---o---o---o---o---B  main project branch
   /       /       / 
  G---x---G---x---G            dependency
           \ /
            x'                 dependency project taskbranch

This would make bisect traverse only the commits on the main branch:

    o---o---o---o---o---o

If any of the merged branches are indirectly the cause of the problem, it will be discovered when you test the merge commit via bisect, which could be reason to investigate further on that branch.

Answer 4

I don't see a one-step method, however, based on your current solution: git bisect skip can take a list of commits to skip. git log branchname will list commits on branch branchname. So this should let you specify the list of commits.

If your dependency and your main code live in different filesystem spaces, you can specify the paths to include with git bisect start. Depending on your code layout, that may be the best option. (Almost certainly is if you have a list of files that may contain the bug!)

The man page has details; the see also there is interesting reading, too.

Answer 5

You can instruct git-bisect to only go through the first-parents of the merge commits by running the following command:

git bisect skip $(comm -23 <(git rev-list G | sort) <(git rev-list --first-parent G | sort))

Answer 6

Björn Steinbrink's answer works great, but recently started printing this:

hint: Support for /info/grafts is deprecated
hint: and will be removed in a future Git version.
hint: 
hint: Please use "git replace --convert-graft-file"
hint: to convert the grafts into replace refs.
hint: 
hint: Turn this message off by running
hint: "git config advice.graftFileDeprecated false"

Here's a more modern version of his solution using "git replace" instead of grafts:

git rev-list --first-parent --merges --parents HEAD | \
  while read COMMIT PARENT1 PARENT2; do 
    git replace --graft $COMMIT $PARENT1; 
  done

Unfortunately, it is much slower for large repos (around 3 minutes for 150k commits); git replace doesn't seem to have a bulk-mode yet. You might want to restrict the rev-list to only the commits in scope of your bisect.

To remove the replacements when you are done, you can rm .git/refs/replace/* .

Answer 7

You might be able to use git bisect start --no-checkout to avoid having to actually checkout the commit into the working tree. Then I suspect that you can do git checkout BISECT_HEAD for commits that you actually want to test (i.e. only first parent commits on the main branch). I have not tried this but I hope it would work.

Answer 8

You can make git treat you history as linear using grafts. To linearize the whole first parent history you can use:

git rev-list --first-parent --merges --parents HEAD | cut -d' ' -f1,2 > .git/info/grafts

Just drop the grafts file when you're done with the bisection.

Answer 9

So, the assumption that the first parent of a merge commit is always the same branch isn't always correct. For instance, if you are off on a topic branch and merge master to it to get up to date (so for this merge commit, the first parent is the topic branch) and then checkout master and merge topic back to it, you get a fast forward merge, which just moves master to the merge commit that had first parent as your topic branch. This may seem contrived, but its actually a pretty normal workflow - I always merge master into my branch so that my merge back to master will be a trivial merge (i.e., fast forward-able) (Sorry James, always forget to rebase it).

There is one way that I've found to help figure out which parent is your branch - the merge commit comment itself. By default, git composes a merge commit comment that says which branch was merged and you can use this to deduce which parent is the branch you are interested in, so long as the person doing the merge commit didn't overwrite this merge commit comment.

So I tried this out and it seems to work for me. I wrote a Python script to help do this on github. If you run this script, it will try to trace backwards and follow your branch and emit a list of commit ids that are the tips of the branches that are merged into your branch. With this list, you can give these to "git bisect good" and bisect will then omit all of the commits on the merged branches from your bisection, achieving the desired result.

Answer 10

Is there any way of doing --first-parent with git bisect

Yes: with Git 2.29 (Q4 2020), "git bisect"^(man) learns the "--first-parent" option to find the first breakage along the first-parent chain.

See commit ad464a4, commit e8861ff, commit be5fe20, commit 0fe305a, commit 15a4802 (07 Aug 2020) by Aaron Lipman (alipman88).
^{(Merged by Junio C Hamano -- gitster -- in commit 47f0f94, 17 Aug 2020)}

bisect: introduce first-parent flag

^{Signed-off-by: Aaron Lipman}

Upon seeing a merge commit when bisecting, this option may be used to follow only the first parent.

In detecting regressions introduced through the merging of a branch, the merge commit will be identified as introduction of the bug and its ancestors will be ignored.

This option is particularly useful in avoiding false positives when a merged branch contained broken or non-buildable commits, but the merge itself was OK.

git bisect [--no-checkout] [--first-parent] [<bad> [<good>...]] [--] [<paths>...]

git bisect now includes in its man page:

--first-parent

Follow only the first parent commit upon seeing a merge commit.

In detecting regressions introduced through the merging of a branch, the merge commit will be identified as introduction of the bug and its ancestors will be ignored.

This option is particularly useful in avoiding false positives when a merged branch contained broken or non-buildable commits, but the merge itself was OK.