Bisection is a mightily powerful tool for debugging issues that were quietly introduced into a codebase and not noticed for an extended period of time. With a fine-grained linear history I have found it often to be possible to quickly bisect to the single-line change that causes an issue, even amongst tens of thousands of commits spanning many years from a shifting team of developers, armed only with a reproducible test case.
Bisection is particularly powerful when it identifies a small commit as the culprit. To increase the chances of this happening, keep all commits as small as possible. Wide-reaching but low-risk changes like renaming classes or auto-reformatting code should be kept separate from semantically important changes. It’s not a good feeling when you isolate the offending commit and it comprises thousands of whitespace changes plus one or two non-whitespace bits that are hidden in the noise.
It’s also useful for bisection if most commits contain code that compiles and passes (a reasonable proportion of) all of the tests. This obviously isn’t always possible.
Prefer rebase to merge
Merging is where two independent sequences of commits (green and blue) are combined at a merge commit (red):
Rebasing is where one of the sequences (blue) is moved on top of the other one (green):
Both of these process can require manual intervention if the underlying changes are in conflict with each other. There are two related advantages of rebasing that come about particularly if there’s an issue that isn’t immediately noticed:
When merging, the issue may be present in neither the green nor the blue commits, but may be present at the merge commit. When using bisection to isolate where a issue was introduced, it often ends up pointing at a merge commit. The problem with this is that merge commits may be quite a bit larger than non-merge commits, which makes bisection that bit less useful as a debugging tool. When rebasing, the resulting commits are all non-merge commits so can be made much more supportive of bisection.
When rebasing, there is a clear division of responsibility between the two developers. The green developer pushed first, so the onus is on the blue developer to rebase their changes correctly. When merging, the green and blue commits may all be correct and yet the merged code is not, but neither contributor really owns the merge commit in the same way. Again, bisection does not work well as a tool to find a problem in the merge commit.
Additionally, bisection only really works if there is always a well-defined commit half-way between the known-good and known-bad states. In the merging diagram above there is no particularly sensible half-way point between the red and black commits.
Prefer merge to fast-forward
The trouble with naively rebasing is that you end up with a sequence of commits, some of which contain work-in-progress and others contain finished features:
When looking back through history, it’s useful to be able to distinguish the finished commits from the WIP. My preferred way to do this is to perform a trivial merge at the end of each feature, rather than simply to fast-forward to the tip of the feature branch:
Since each merge is trivial, the problems with merging discussed above do not apply.
If a bisection hits an untestably bad WIP commit it’s normally a good idea to jump to a nearby merge and continue from there.
Prefer not to squash
Squashing a whole feature branch into a single commit (often done at rebase) destroys the usefulness of bisection because the resulting commit is not fine-grained, so not particularly helpful for later debugging. The argument in favour of squashing seems to be that it yields a “tidier” history, which I charitably interpret to mean “contains no WIP commits”. I much prefer trivial merges for distinguishing the WIP from finished features without losing the valuable detail of the WIP commits either.