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Commit ff8cd116 authored by Stan Hu's avatar Stan Hu

Disable --follow in `git log` to avoid loading duplicate commit data in infinite scroll

`git` doesn't work properly when `--follow` and `--skip` are specified together. We could even be **omitting commits in the Web log** as a result.

Here are the gory details. Let's say you ran:

```
git log -n=5 --skip=2 README
```

This is the working case since it omits `--follow`. This is what happens:

1. `git` starts at `HEAD` and traverses down the tree until it finds the top-most commit relevant to README.
2. Once this is found, this commit is returned via `get_revision_1()`.
3. If the `skip_count` is positive, decrement and repeat step 2. Otherwise go onto step 4.
4. `show_log()` gets called with that commit.
5. Repeat step 1 until we have all five entries.

That's exactly what we want. What happens when you use `--follow`? You have to understand how step 1 is performed:

* When you specify a pathspec on the command-line (e.g. README), a flag `prune` [gets set here](https://github.com/git/git/blob/master/revision.c#L2351).
* If the `prune` flag is active, `get_commit_action()` determines whether the commit should be [scanned for matching paths](https://github.com/git/git/blob/master/revision.c#L2989).
* In the case of `--follow`, however, `prune` is [disabled here](https://github.com/git/git/blob/master/revision.c#L2350).
* As a result, a commit is never scanned for matching paths and therefore never pruned. `HEAD` will always get returned as the first commit, even if it's not relevant to the README.
* Making matters worse, the `--skip` in the example above would actually skip every other after `HEAD` N times. If README were changed in these skipped commits, we would actually miss information!

Since git uses a matching algorithm to determine whether a file was renamed, I
believe `git` needs to generate a diff of each commit to do this and traverse
each commit one-by-one to do this. I think that's the rationale for disabling
the `prune` functionality since you can't just do a simple string comparison.

Closes #4181, #4229, #3574, #2410
parent 350d6550
......@@ -10,6 +10,7 @@ v 8.4.0 (unreleased)
- Add link to merge request on build detail page.
v 8.3.2 (unreleased)
- Disable --follow in `git log` to avoid loading duplicate commit data in infinite scroll (Stan Hu)
- Enable "Add key" button when user fills in a proper key
v 8.3.1
......
......@@ -76,7 +76,9 @@ def commits(ref, path = nil, limit = nil, offset = nil, skip_merges = false)
path: path,
limit: limit,
offset: offset,
follow: path.present?
# --follow doesn't play well with --skip. See:
# https://gitlab.com/gitlab-org/gitlab-ce/issues/3574#note_3040520
follow: false
}
commits = Gitlab::Git::Commit.where(options)
......
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