Respect submodule update=none strategy in .gitmodules

[jsitnicki]

Git lets users define the default update/checkout strategy for a submodule
by setting the submodule.<name>.update key in .gitmodules file.

If the update strategy is none, the submodule will be skipped during
update. It will not be fetched and checked out:

1. foo is a big git repo

/tmp $ git init foo
Initialized empty Git repository in /tmp/foo/.git/
/tmp $ dd if=/dev/zero of=foo/big bs=1000M count=1
1+0 records in
1+0 records out
1048576000 bytes (1.0 GB, 1000 MiB) copied, 0.482087 s, 2.2 GB/s
/tmp $ git -C foo add big
/tmp $ git -C foo commit -m 'I am big'
[main (root-commit) 84fb533] I am big
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 big

2. bar is a repo with a big submodule with update=none

/tmp $ git init bar
Initialized empty Git repository in /tmp/bar/.git/
/tmp $ git -C bar submodule add file:///tmp/foo foo
Cloning into '/tmp/bar/foo'...
remote: Enumerating objects: 3, done.
remote: Counting objects: 100% (3/3), done.
remote: Total 3 (delta 0), reused 1 (delta 0), pack-reused 0
Receiving objects: 100% (3/3), 995.50 KiB | 338.00 KiB/s, done.
/tmp $ git -C bar config --file .gitmodules submodule.foo.update none
/tmp $ cat bar/.gitmodules
[submodule "foo"]
        path = foo
        url = file:///tmp/foo
        update = none
/tmp $ git -C bar commit --all -m 'I have a big submodule with update=none'
[main (root-commit) 6c355ea] I have a big submodule not updated by default
 2 files changed, 4 insertions(+)
 create mode 100644 .gitmodules
 create mode 160000 foo

3. baz is a clone of bar, notice foo submodule gets skipped

/tmp $ git clone --recurse-submodules file:///tmp/bar baz
Cloning into 'baz'...
remote: Enumerating objects: 3, done.
remote: Counting objects: 100% (3/3), done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 0), reused 0 (delta 0), pack-reused 0
Receiving objects: 100% (3/3), done.
Submodule 'foo' (file:///tmp/foo) registered for path 'foo'
Skipping submodule 'foo'
/tmp $ git -C baz submodule update --init
Skipping submodule 'foo'
/tmp $

Cargo, on the other hand, ignores the submodule update strategy set in
.gitmodules properties when updating dependencies. Such behavior can
be considered against the wish of the crate publisher.

4. bar is now a lib with a big submodule with update disabled

/tmp $ cargo init --lib bar
     Created library package
/tmp $ git -C bar add .
/tmp $ git -C bar commit -m 'I am a lib with a big submodule but update=none'
[main eb07cf7] I am a lib with a big submodule but update=none
 3 files changed, 18 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 Cargo.toml
 create mode 100644 src/lib.rs
/tmp $

5. qux depends on bar, notice bar's submodules are fetched

/tmp $ cargo init qux && cd qux
     Created binary (application) package
/tmp/qux $ echo -e '[dependencies.bar]\ngit = "file:///tmp/bar"' >> Cargo.toml
/tmp/qux $ time cargo update
    Updating git repository `file:///tmp/bar`
    Updating git submodule `file:///tmp/foo`

real    0m22.182s
user    0m20.402s
sys     0m1.714s
/tmp/qux $

Fix it by checking if a Git repository submodule should be updated when
cargo processes dependencies.

6. With the change applied, submodules with update=none are skipped

/tmp/qux $ cargo cache -a > /dev/null
/tmp/qux $ time ~/src/cargo/target/debug/cargo update
    Updating git repository `file:///tmp/bar`
    Skipping git submodule `file:///tmp/foo`

real    0m0.029s
user    0m0.021s
sys     0m0.008s
/tmp/qux $

Fixes #4247.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @ehuss (or someone else) soon.

Please see the contribution instructions for more information.

[Muscraft]

It might be a good idea to include a test for this change. That way future changes wouldn't revert this one. I'd look at this test or this test for inspiration

[jsitnicki]

Thanks for the pointers @Muscraft. Let me do that.

[jsitnicki]

v2:

• added test for skipping a submodule of a dependency, when its update strategy is none
• fix up sign off in the commit message

PTAL

[jsitnicki]

v3:

• explain to the user why the git submodule was skipped

[epage]

Thanks!

@bors r+

[bors]

📌 Commit 26031b9 has been approved by epage

[bors]

⌛ Testing commit 26031b9 with merge bc3b99d...

[bors]

☀️ Test successful - checks-actions
Approved by: epage
Pushing bc3b99d to master...