Commits on Jun 7, 2022

1. Respect submodule update=none strategy in .gitmodules …

   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 rust-lang#4247.
   
   Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>

   

   jsitnicki committed Jun 7, 2022
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