fetch-submodules fallback; add remove-submodules

[dhalbert]

Fixes #7786.

The partial clone method of fetching submodules introduced in #7763 does not work for git older than version 2.36.0. (I had hoped it was earlier than that.)

• Add a fallback to the older method, which does a shallow clone of depth 1 for each submodule.
• Add a new target make remove-submodules, which cleans out the currently fetch submodules, so you can start from scratch and do make fetch-submodules again. This is useful to switch between fetch-submodule methods, and also to recover from changes in submodules, such as the replacement of one submodule for another.

Tested with git 2.30.0 (depth 1 shallow clone) and 2.40.0 (blobless partial clone).

[RetiredWizard]

Does placing the || between the statements cause the statements to be executed until one is successful (i.e. not False)? If so, I'm curious how the old method executes both statement 2 and statement 3 in the cases that the new method (statement 1) fails? I'm sure I don't understand how make processes the new line but I would have expected statement 1 || (statement 2 && statement 3) || echo Error Message

Also, for the last month or so I've been having to run the fetch-submodules multiples times because cloning one or more of the submodules fails, will that cause a failover to the old method if it happens during the execution of the new fetch method? That's probably not a terrible thing if the answer is yes...

[RetiredWizard]

Oh nevermind, I think your statement 2 is a new method and I see the && on the third method....

Well I was getting closer 😁 Thanks as always @dhalbert

[dhalbert]

The old way was this:

# This update will fail because the commits we need aren't the latest on the
# branch. We can ignore that though because we fix it with the second command.
# (Only works for git servers that allow sha fetches.)
.PHONY: fetch-submodules
fetch-submodules:
	git submodule update --init -N --depth 1 || true
	git submodule foreach 'git fetch --tags --depth 1 origin $$sha1 && git checkout -q $$sha1'

Note that there is an expectation that the first line will fail, hence the || true, so that the make target will continue. If by chance it succeeds, that git submodule update --init was sufficient for all of the fetches.

I just got it all to be on one line so I didn't have to use \ or write a script.

Also, for the last month or so I've been having to run the fetch-submodules multiples times because cloning one or more of the submodules fails, will that cause a failover to the old method if it happens during the execution of the new fetch method? That's probably not a terrible thing if the answer is yes...

That would be true, I was assuming it would not fail due to external causes. But the new remove-submodules target will help you start over.