Better support SCM functionality

[mattseddon]

From #318 the basic user cases are as follows:

Proposal

Notion.

SCM view

1. View all directories and files that have changed when compared to HEAD or cache (Visibility / Situational awareness)
2. Checkout / commit and files or directories that have changed (Actions) - exact UX TBD.
3. Checkout / commit / push or pull the entire repository (Actions)

Statuses that we currently provide in the extension

Status	SCM View	Decorations Provided **	Sourced from	Notes
added	Y	Y	diff + list
deleted	Y	Y	diff + list
modified	Y	Y	diff + list + status
notInCache	Y	Y	diff + list
renamed	Y	Y	diff + list
stageModified	Y	Y	diff + list + status	For a detailed explanation of modified vs stageModified see #318 (comment)
untracked	Y	Y	git	this is untracked with respect to both git and dvc. We show these files because the user may want to dvc add them.
tracked	Y	Y	list	we decorate tracked because they are generally "git ignored" which will give them a "greyed out" decoration

** Where possible we match the git extension's decorations because we are trying to make the extension feel as native as possible. Our SCM integration is designed to show the user the state of the workspace with respect to the most recent commit.

Current approach (parallel CLI Commands)

name	command	reason
list	dvc list . --dvc-only -R --show-json	provides a list of all tracked files that we use for both decoration and SCM purposes. In the SCM view all files that we show must be tracked by DVC. We do this because we end up with untracked but modified (duplicates) items in the tree from diff if we do not
diff	dvc diff --show-json	we map the output of diff directly to the list output to set all added, deleted, renamed, notInCache. We use it in combination with the status output to determine the difference between modified and "stage modified"
status	dvc status --show-json	only used to determine the difference between modified and "stage modified"

We currently try to run all three of the above commands in parallel. If any of the commands fail then we will retry all three until they have all completed without error. We do this to best mitigate stale information ending up in the extension.

Issues with the current approach

1. It's still slow (General performance of trees #608)
2. We are unsure of the what the actual UI / UX should be (Decide on and implement UX for checkout / commit workflow in SCM view #609)
3. A lot of data is sent between the CLI and extension that is unused (example: in get-started-experiments after first running an experiment the output of diff contains ~80k "added" files, none of these files are tracked by dvc so we filter all of the records out)
4. We have issues running multiple commands in parallel (After reloading the window experiments part is stuck in the "loading" state #767 (comment)) <- this is particularly important because it means we cannot currently run the extension against get-started-experiments

Options for mitigation

#	option	pros	cons
1	Run commands sequentially	locks should no longer be an issue	even slower
2	Only rerun failed commands	also mitigates lock issue	involves more complicated logic, possibility of stale data
3	Make all 3 commands lockless	allows us to continue to run all commands in parallel	involves work from the CLI team and is only an interim solution, complicates internal of DVC
4	Combine commands into single command that the integration can run	limits the amount of data needing to be transferred between the cli and extension, should be faster, cuts out grouped retry logic	more effort required, unsure as to benefit to general users
5	Replace CLI calls with event driven architecture	eliminates the need to call the CLI, could serve multiple clients	requires even more work and is not a short or even medium term solution
6	Make commands "lightweight" (add --dvc-only)	would limit the amount of data being passed and could speed things up	unsure as to the benefit to general users, still requires effort, could still run into lock issues

My preference would be to start work on 4 as it would actually help us move towards 5.

[mattseddon]

cc @shcheklein @dberenbaum @efiop.

Chat about this next time we are all together.

[mattseddon]

Note: The native VS Code git extension gets all of the information that it needs from git status -z -u. It creates a child process to run the command in much the same way that we do.

If we are going to emulate what git does for status and diff then the correct command to update would be status. Here is what we can currently get from git:

IMO the current DVC implementation of status is doing something else and should be moved to be a subcommand e.g dvc stage status.

[mattseddon]

Other related issues / comments / discussions:

1. ls: optimize --dvc-only dvc#5712
2. status: fails whenever an experiment is running dvc#6133
3. status: takes too long to get status dvc#6543 (comment)
4. status: it's slow dvc#5594
5. status: add --dvc-only & --outs-only flags dvc#5895
6. status: what should be the default behavior and distinction to diff? dvc#5936
7. diff: consider including modified git-tracked files dvc#3385

I think the amount of information lying around actually muddies the waters.

[mattseddon]

Having discussed with @efiop and @dberenbaum in the 21/09/14 cross team meeting we came to the following conclusion for next steps:

1. We need a single command.
2. The best candidate is status (to match up with git).
3. It should have an option to display paths only tracked by DVC (--dvc-only).
4. It should have an option (or separate command) to show stage information (dvc stage status).
5. It will have all the same available statuses as diff with a further status used to separate files between modified against cache and modified against HEAD.

Points to discuss:

1. Should the current dvc status be migrated to a separate command (e.g dvc stage status) or should there be a flag added to condense the information to what we need?
2. What will the new status be named?
3. How will the new status be displayed in the output (to distinguish from modified)? The way that git handles this is shown above (Better support SCM functionality #772 (comment))

I will start a document now in notion here. We can continue the discussion there.

[dberenbaum]

Thanks, @mattseddon! Looks great.

We actually have a proposal template in Notion. I didn't want to ask you to do that much work, but you basically filled the template already in the doc you created, so I transferred your text into a proposal in https://www.notion.so/iterative/Consolidate-repo-status-ed3cd60f706f4fcaba1d3f3cac1498e9.

I'd like to flesh it out with some DVC requirements since this work should also be about improving the user experience within DVC. Take a look and let me know if/when it's okay to start adding on to the proposal.