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This isn’t simply because of how Git works. You can configure Git to look in multiple places for repo objects. For whatever reason, the GitHub devs either didn’

by brodie 6y ago

This isn’t simply because of how Git works. You can configure Git to look in multiple places for repo objects. For whatever reason, the GitHub devs either didn’t know this, or they didn’t want to implement their forking and pull request systems this way. As someone else mentioned, this may be an intentional design to make it simpler to implement pulling down remote PRs from the destination repo.

est31 6y ago
> You can configure Git to look in multiple places for repo objects. What do you mean by multiple places for repo objects? Do you mean multiple remotes? The remotes are fully inside your local database if you run commands like git pull or git remote update, they are just not in your checkout. Commands like git show <commit hash> work on commit hashes in those remotes as well, even if it's not in one of your local branches. Or do you mean configuring git to use multiple .git/objects directories? I haven't heard of that feature, can you give a link?
- brodie 6y ago
  The feature’s called alternates. You can use it on-the-fly without modifying any repos by using the GIT_ALTERNATE_OBJECT_DIRECTORIES environment variable. If you want the effect permanently, there’s the .git/objects/info/alternates file. For HTTP remotes, there’s apparently a .git/objects/info/http-alternates file as well (no idea how that works though). I’m assuming these files allow multiple alternates as the environment variable does.
  - est31 6y ago
    Oh indeed, very interesting. Apparently the feature also existed since 2005, before Github (2008) so they could have used it from the start. https://stackoverflow.com/a/36125713 https://stackoverflow.com/a/36125713
  - js2 6y ago
    I'm pretty sure GitHub does use alternates in the same way that that GitLab does: https://docs.gitlab.com/ee/development/git_object_deduplication.html https://docs.gitlab.com/ee/development/git_object_deduplicat... I vaguely recall seeing @peff comment about this on HN years ago but I can't find that comment now. Here's a GitLab employee claiming GitHub uses alternates: https://news.ycombinator.com/item?id=22179208 https://news.ycombinator.com/item?id=22179208 The thing is, that both the dmca repo and its forks must have alternates files to the same underlying common repo, otherwise the PR ref in the dmca repo wouldn't be able to see the merge commit pushed to the fork. Pushing the merge must have duplicated all the youtube-dl commits into the common repo used by both the dmca repo and its forks because youtube-dl and dmca would have different common repos.

by brodie 6y ago

est31 6y ago

> You can configure Git to look in multiple places for repo objects. What do you mean by multiple places for repo objects? Do you mean multiple remotes? The remotes are fully inside your local database if you run commands like git pull or git remote update, they are just not in your checkout. Commands like git show <commit hash> work on commit hashes in those remotes as well, even if it's not in one of your local branches. Or do you mean configuring git to use multiple .git/objects directories? I haven't heard of that feature, can you give a link?

brodie 6y ago
The feature’s called alternates. You can use it on-the-fly without modifying any repos by using the GIT_ALTERNATE_OBJECT_DIRECTORIES environment variable. If you want the effect permanently, there’s the .git/objects/info/alternates file. For HTTP remotes, there’s apparently a .git/objects/info/http-alternates file as well (no idea how that works though). I’m assuming these files allow multiple alternates as the environment variable does.
- est31 6y ago
  Oh indeed, very interesting. Apparently the feature also existed since 2005, before Github (2008) so they could have used it from the start. https://stackoverflow.com/a/36125713 https://stackoverflow.com/a/36125713
- js2 6y ago
  I'm pretty sure GitHub does use alternates in the same way that that GitLab does: https://docs.gitlab.com/ee/development/git_object_deduplication.html https://docs.gitlab.com/ee/development/git_object_deduplicat... I vaguely recall seeing @peff comment about this on HN years ago but I can't find that comment now. Here's a GitLab employee claiming GitHub uses alternates: https://news.ycombinator.com/item?id=22179208 https://news.ycombinator.com/item?id=22179208 The thing is, that both the dmca repo and its forks must have alternates files to the same underlying common repo, otherwise the PR ref in the dmca repo wouldn't be able to see the merge commit pushed to the fork. Pushing the merge must have duplicated all the youtube-dl commits into the common repo used by both the dmca repo and its forks because youtube-dl and dmca would have different common repos.