Hi folks, As hinted by a couple recent patches, I'm planning on some pretty big submodule work over the next 6 months or so - and Ævar pointed out to me in https://lore.kernel.org/git/87v98p17im.fsf@xxxxxxxxxxxxxxxxxxx that I probably should share some of those plans ahead of time. :) So attached is a lightly modified version of the doc that we've been working on internally at Google, focusing on what we think would be an ideal submodule workflow. I'm hoping that folks will get a chance to read some or all of it and let us know what sounds cool (or sounds extremely broken). The best spot to start is probably the "Overview" section, which describes what the "main path" would look like for a user working on a project with submodules. Most of the work that we're planning on doing is under the "What doesn't already work" headings. Thanks in advance for any time you spend reading/discussing :) - Emily Background ========== It's worth mentioning that the main goal that's funding this work is to provide an alternative for users whose projects use repo (https://source.android.com/setup/develop#repo) today. That means that the main focus is to try and reach feature parity to repo for an easier transition for those who want to switch. As a result, some of the direction below is aimed towards learning from what has worked well with repo (but hopefully more flexible for users who want to do more, or differently). There are also a few things mentioned that are specifically targeted to ease use with Gerrit, which is in wide use here at Google (and therefore also a consideration we need to make to keep getting paid ;) ). Overview ======= When the work is completed, users should be able to have a clean, obvious workflow when using best practices: To download the code, they should be able to run simply git clone https://example.com/superproject to download the project and all its submodules; if partial clone is configured, they should receive only the objects allowed by the filter in their superproject as well as in each submodule. To begin working on a feature, from the superproject they can 'git switch -c feature', and since the new branch is being created, a new branch 'feature' will be created for each submodule, pointing to the submodule's current 'HEAD'. They can move to a submodule directory and begin to make changes, and when they commit these changes normally with 'git commit' from the submodule directory, running git status in the superproject will reflect that a submodule has changed. Next, they can switch to a second submodule, making and committing more changes. When they are ready to send these changes which are ready for review but need to be linked together, they can switch back to the superproject, where 'git status' indicates that there are changes in both submodules. They can commit these changes to the superproject and use 'git push' to send a review; Git will recurse into affected submodules and push those submodule commits appropriately as well. While the user is waiting for feedback on their review, to work on their next task, they can 'git switch other-feature', which will checkout the branches specified in the superproject commit at the tip of 'other-feature'; now the user can continue working as before. When it's time to update their local repo, the user can do so as with a single-repo project. First they can 'git checkout main && git pull' (or 'git pull -r'); Git will first checkout the branches associated with main in each submodule, then fetch and merge/rebase in each submodule appropriately. Finally, they can 'git switch feature && git rebase', at which time Git will recursively checkout the branches associated with 'feature' in each submodule and rebase each submodule appropriately. Detailed Design =============== The Well-Tread Path: Basic Contribution Workflow ------------------------------------------------ - git clone 1. git clone initializes the directory indicated by the user 2. git clone fetches the superproject 3. git clone checks out the superproject at server's HEAD (or at another commit as specified by the user, e.g. with --branch) 4. git clone warns the user that a recommended hook/config setup exists and provides a tip on how to install it 5. For each submodule encountered in step 3, git clone is invoked for the submodule, and steps 1-4 are repeated (but in directories indicated by the superproject commit, not by the user). Note that this means options like '--branch' *don't* propagate directly to the submodules. If superproject branch "foo" points its submodule to branch "main", then 'git clone --branch foo https://superproject.git' will clone superproject/submodule to branch 'main' instead. (It *may* be OK to take '--branch' to mean "the branch specified by the parent *and* the branch named in --branch if applicable, but no other branches".) What doesn't already work: * --recurse-submodules should turn on submodule.recurse=true * superproject gets top-level config inherited by submodules * New --recurse-submodules --single-branch semantics * Progress bar for clone (see work estimates) * Recommended config from project owner -- Partial clone 1. git clone initializes the directory indicated by the user 2. git clone applies the appropriate configs for the partial clone filter requested by the user a) These configs go to the config file shared by superproject and submodules. 3. git clone fetches the superproject 4. git clone checks out the superproject at server's HEAD 5. git clone warns the user that a recommended hook/config setup exists and provides a tip on how to install it 6. For each submodule encountered in step 4, git clone is invoked for the submodule, and steps 1-4 are repeated (but in directories indicated by the superproject commit, not by the user). The same filter supplied to the superproject applies to the submodules. What doesn't already work: * --filter=blob:none with submodules (it's using global variables) * propagating --filter=blob:none to submodules (via submodules.config) * Recommended config from project owner - git fetch By default, git fetch looks for (1) the remote name(s) supplied at the command line, (2) the remote which the currently checked out branch is tracking, or (3) the remote named origin, in that order. For submodules, there is no guarantee that (1) has anything to do with the state of the submodule referenced by the superproject commit, so just start from (2). This operation can be extremely long-running if the project contains many large submodules, so progress indicators should be displayed. Caveat: this will mean that we should be more careful about ensuring that submodule branches have tracking info set up correctly; that may be an issue for users who want to branch within their submodule. This may be OK because users will probably still have 'origin' as their submodule's remote, and if they want more complicated behavior, they will be able to configure it. What doesn't already work: * Make sure not to propagate (1) to submodules while recursing * Fetching new submodules. * Not having 0.95 success probability ** 100 = low success probability (that is, we need more retries during submodule fetch) * Progress indicators - git switch / git checkout Submodules should continue to perform these operations the same way that they have before, that is, the way that single-repo Git works. But superprojects should behave as follows: -- Create mode (git switch -c / git checkout -b) 1. The current worktree is checked for uncommitted changes to tracked files. The current worktree of each submodule is also checked. 2. A new branch is created on the superproject; that branch's ref is pointed to the current HEAD. 3. The new branch is checked out on the superproject. 4. A new branch with the same name is created on each submodule. a. If there is a naming conflict, we could prompt the user to resolve it, or we could just check out the branch by that name and print a warning to the user with advice on how to solve it (cd submodule && git switch -c different-branch-name HEAD@{1}). Maybe we could skip the warning/advice if the tree is identical to the tree we would have used as the start point (that is, the user switched branches in the submodule, then said "oh crap" and went back and switched branches in the superproject). b. Tracking info is set appropriately on each new branch to the upstream of the branch referenced by the parent of the new superproject commit, OR to the default branch's upstream. 5. The new branch is checked out on each of the submodules. What doesn't already work: * Safety check when leaving uncommitted submodule changes * Propagating branch names to submodules currently requires a custom hacky repolike patch * Error handling + graceful non-error handling if the branch already exists * "Knowing what branch to push to": copying over which-branch-is-upstream info ** Needs some UX help, push.default is a mess * Tracking info setups -- Switching to an existing branch (git switch / git checkout) 1. The current worktree is checked for uncommitted changes to tracked files. The current worktree of each submodule is also checked. 2. The requested branch is checked out on the superproject. 3. The submodule commit or branch referenced by the newly-checked-out superproject commit is checked out on each submodule. What doesn't already work: * Same as in create mode - git status -- From superproject The superproject is clean if: * No tracked files in the superproject have been modified and not committed * No tracked files in any submodules have been modified and not committed * No commits in any submodules differ from the commits referenced by the tip commit of the superproject Advices should describe: * How to commit or drop changes to files in the superproject * How to commit or drop changes to files in the submodules * How to commit changes to submodule references * Which commit/branch to switch the submodule back to if the current work should be dropped: "Submodule "foo" no longer points to "main", 'git -C foo switch main' to discard changes" What doesn't already work: * "git status" being super fast and actually possible to use. ** (That is, we've seen it move very slowly on projects with many submodules.) * Advice updates to use the appropriate submodule-y commands. -- From submodule git status's behavior for submodules does not change compared to single-repository Git, except that a red warning line will also display if the superproject commit does not point to the HEAD of the submodule. (This could look similar to the detached-HEAD warning and tracking branch lines in git status today, e.g. "HEAD is ahead of parent project by 2 commits".) What doesn't already work: * "git status" from a submodule being aware of the superproject. - git push -- From superproject Ideally, a push of the superproject commit results in a push of each submodule which changed, to the appropriate Gerrit upstream. Commits pushed this way across submodules should somehow be associated in the Gerrit UI, similar to the "submitted together" display. This will need some work to make happen. What doesn't already work: * Automatically setting Gerrit topic (with a hook) * "push --recurse-submodules" knowing where to push to in submodules to initiate a Gerrit review ** From `branch` field in .gitmodules? ** Gerrit accepting 'git push -o review origin main' pushes? ** Review URL with a remote helper that rewrites refs/heads/main to refs/for/main? ** Need UX help >From submodule No change to client behavior is needed. With Gerrit submodule subscriptions, the server knows how to generate superproject commits when merging submodule commits. - git pull / git rebase Note: We're still thinking about this one :) 1. Performs a fetch as described above 2. For each superproject commit, replay the submodule commits against the newly updated submodule base; then, make a new superproject commit containing those changes What doesn't already work: * Rewriting gitlinks in a superproject commit when 'rebase --recurse-submodules'-ing * Resuming after resolving a conflict during rebase - git merge The story for merges is a little bit muddled... and for our goals we don't need it for quite a while, so we haven't thought much about it :) Any suggestions folks have about reasonable ways to 'git merge --recurse-submodules' are totally welcome. For now, though, we'll probably just stick in some error message saying that merges with submodules isn't currently supported (maybe we will even add that downstream). What doesn't already work: * Erroring out for "not supported" Aligning Teams -------------- There's two pieces of work that we are relying on a lot, and both have been mentioned upstream by now, so I'll just link out: 1. Recommended Hook Configurations (https://lore.kernel.org/git/pull.908.v2.git.1616723016659.gitgitgadget@xxxxxxxxx) 2. Shared Configuration Across Submodules (https://lore.kernel.org/git/20210408233936.533342-1-emilyshaffer@xxxxxxxxxx) Edge Cases, Mess Recovery, & Power Users ------------------------------- - Unstaged Changes in Submodules At Commit Time -- Related Changes (Single Branch) If a user has unstaged changes in multiple submodules and runs 'git commit --all' from the superproject, they should be presented with an editor which contains commit message drafts for each modified branch, including the superproject, separated by scissors or some other delineator. After providing a commit message, Git should perform each submodule commit, then finally perform the superproject commit based on the submodules' new commit IDs and apply the proposed superproject commit message. What doesn't already work: * "git commit --recurse-submodules" that lets me write a commit message with scissors dividing things in each repository -- Unrelated Changes (Separating Into Multiple Branches) If a user has unstaged changes in multiple submodules and only wants to commit some of them, and runs 'git add --patch' from the superproject, they should be walked through 'git add --patch' for each submodule first. However, since this could be a lengthy process, we need to think carefully about how the UX should look compared to the existing `git add --patch` UX for single-repo projects. What doesn't already work: * "git add --patch" that recurses through submodule hunks as well - Recovering from Exploratory Changes with 'git restore' and 'git reset' When a user has checked out some historical commit in at least one submodule for the purpose of exploration/investigation, it should be easy to reset the entire tree back to the state defined by the superproject commit. Running git restore (or git reset) from the superproject should recurse by running git checkout on each submodule - and when there are no untracked changes in the submodule, it can do this without asking for user intervention or approval. What doesn't already work: * Add some tests for good restore/reset behavior and make them pass - Multiple Commits on a Superproject Branch Generally, one superproject commit should represent one feature, where that one feature may consist of multiple submodule commits. It could be thought of similarly to a merge commit, which brings a stack of related changes into the history and summarizes them a single commit, without squashing or losing history. So a user who has two commits in one superproject branch is working on two features, one of which depends on the other. Reordering those commits should involve replaying the commits in each submodule associated with each superproject commit: superproject submodule superproject submodule A ---------> a1 B ----------> b1 | | | | | a2 | b2 | | | | | a3 A ----------> a1 | | rebase | | B ---------> b1 =====> | a2 | | | | | b2 | a3 | | | | o o o o | | | | o o o o | | | | - Branching in a Submodule In addition to the 'git status' warning, users should also receive a warning like detached-HEAD when switching branches in the submodule without a superproject commit - "the branch you are leaving behind is not tracked by any superproject commit". Users who are just working in and pushing from a single submodule may find this warning annoying, so it should be clear how to disable that warning per-submodule. - Worktrees When a user runs 'git worktree add' from the superproject, each submodule in the new worktree should also be created as a worktree of the corresponding submodule in the original project. What doesn't already work: * worktrees and submodules getting along - submodules are now freshly cloned when creating a superproject worktree - git clone --reference [--dissociate] When cloning with an alternate directory, submodules should also try to use object stores associated with the referenced project instead of cloning from their remotes right away. It is unclear how much of this works today. What doesn't already work: * Writing some tests and making them pass
