Re: [Buildroot] [PATCH next 2/2] package/pkg-cargo: move CARGO_HOME into DL_DIR

["Yann E. MORIN" <yann.morin.1998@free.fr>]

Thomas, All,

On 2022-11-03 23:45 +0100, Thomas Petazzoni via buildroot spake thusly:
> CARGO_HOME is where Cargo stores its downloaded artefacts. See
> https://doc.rust-lang.org/cargo/reference/environment-variables.html:
> 
>   CARGO_HOME — Cargo maintains a local cache of the registry index and
>   of git checkouts of crates. By default these are stored under
>   $HOME/.cargo (%USERPROFILE%\.cargo on Windows), but this variable
>   overrides the location of this directory. Once a crate is cached it
>   is not removed by the clean command. For more details refer to the
>   guide.
> 
> We currently make it point to $(HOST_DIR)/share/cargo, but this has a
> number of drawbacks:
> 
>  (1) It is not shared between Buildroot builds. Each Buildroot build
>      will re-download the crates index, and the crates themselves,
>      unless of course the final vendored tarball is already there.
> 
>  (2) With BR2_PER_PACKAGE_DIRECTORIES=y, it is even worse: CARGO_HOME
>      is not even shared between packages, as $(HOST_DIR)/share/cargo
>      is per package. So each package in the build that needs vendoring
>      of Cargo crates will download the crates index and the crates in
>      its own CARGO_HOME location.
> 
> To solve this, this commit moves CARGO_HOME into $(DL_DIR), so that it
> is shared between builds and packages.
> 
> Even though not the best/most authoritative source,
> https://github.com/rust-lang/cargo/issues/6930 indicates that there is
> a lock when accessing CARGO_HOME, because a user even complains that
> this lock has even become more coarse-grained than it used to be
> (which for us is fine, it just means that two Cargo fetch operations
> from two different packages will be serialized, not a big deal).
> 
> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
> ---
>  package/pkg-cargo.mk | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/package/pkg-cargo.mk b/package/pkg-cargo.mk
> index 5923725f5e..c3d1e34309 100644
> --- a/package/pkg-cargo.mk
> +++ b/package/pkg-cargo.mk
> @@ -20,7 +20,7 @@
>  #
>  ################################################################################
>  
> -BR_CARGO_HOME = $(HOST_DIR)/share/cargo
> +BR_CARGO_HOME = $(DL_DIR)/.cargo

I am not a fan of dot-directories, because they get hidden, especially
since BR2_DL_DIR can be set in the environment and point to a long-lived
directory.

Instead, I would just call it 'cargo-home'. If that's OK, we can change
when applying...

Regards,
Yann E. MORIN.

>  PKG_COMMON_CARGO_ENV = \
>  	CARGO_HOME=$(BR_CARGO_HOME)
> -- 
> 2.38.1
> 
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