Rust code in main

Due to the current state of the Rust ecosystem, the MIR rules state that packages in main that contain Rust code should vendor their Rust dependencies rather than rely on the invididual package versions, see this PR for some background on the issue.

Vendoring Rust dependencies

It’s a simple matter of running cargo vendor where your on the top-level directory. Sadly, it’s not possible to exclude irrelevant dependencies during vendoring yet, so you might want to automate that step and add some post-processing to remove voluminous, unused dependencies, and/or the C code for some system libraries that could be statically linked.

Teaching dh-cargo about vendored sources

dh-cargo by default assumes the dependencies of the main project will be provided by packaged crates from our archive. Since our MIR policy is to vendor dependencies, you need to set the CARGO_VENDOR_DIR environment to wherever the vendored dependencies are stored, e.g. 

   1 export CARGO_VENDOR_DIR = vendor/

Note that even if you’re using the more manual steps, you’ll want to export this variable, as some scripts might expect it.

Modifying individual vendored crates

If you have to modify the contents of individual vendored crates, e.g. to remove pre-compiled files, this’ll change the checksum of the crate, and cargo will complain. So far, the best solution we’ve come up with is to do JSON surgery in the crate’s .cargo-checksum.json to remove the individual file checksums while keeping intact the individual crate checksum.

Eventually we should provide helper scripts to do so in the dh-cargo package, but meanwhile the following snippet does the trick: 

   1 [ -e /usr/bin/jq ] || (echo "jq is required to run this script. Try installing it with 'sudo apt install jq'" && exit 1);
   2 for dep in $(ls vendor_rust -1); do
   3     checksum_file="vendor_rust/${dep}/.cargo-checksum.json";
   4     a_files=$(cat ${checksum_file} | jq '.files | keys | map(select(.|test(".a$")))');
   5     if [ "$a_files" = "[]" ]; then
   6         continue;
   7     fi;
   8     pkg_checksum=$$(cat "${checksum_file}" | jq '.package');
   9     echo "{\"files\": {}, \"package\": ${pkg_checksum}}" >"${checksum_file}";
  10 done;

Cargo wrapper

The cargo package ships a wrapper script around the actual cargo binary that sets default options useful for packaging, and exposes custom subcommands. Therefore, it is advised to use it rather than the binary directly whenever you need to do manual calls. 

   1 CARGO := /usr/share/cargo/bin/cargo

dh-cargo

If your project is pure Rust code in a single crate, dh-cargo should be able to drive the build via the following snippet: 

   1 %:
   2     dh $@ --buildsystem=cargo

Sadly, dh cannot have multiple buildsystems at the same time, so for hybrid codebases you’ll need to trigger the build phases manually. Moreover, dh-cargo does not support building workspaces at the moment, at it was designed to work on source packages pulled directly from crates.io (which only ships individual crates), so there’s a likely chance it’d choke on it.

Jammy support

Support for vendored dependencies hasn’t been SRUed to Jammy yet, this is tracked in LP: #2028153.

Manual steps

`prepare-debian`

The wrapper expects a few things to be set up before the build: 

   1 override_dh_auto_configure:
   2     dh_auto_configure
   3     dh_auto_configure --buildsystem=cargo

If you’re targetting Jammy, you would want to run something like this instead: 

   1 DEB_HOST_GNU_TYPE=$(DEB_HOST_GNU_TYPE) DEB_HOST_RUST_TYPE=$(DEB_HOST_RUST_TYPE) \
   2 CARGO_HOME=$(CURDIR)/debian/cargo_home DEB_CARGO_CRATE=adsys_$(shell dpkg-parsechangelog --show-field Version) \
   3 $(CARGO) prepare-debian $(CARGO_VENDOR_DIR)

Tests

Due to the oddities of the Debian Rust ecosystem, by default dh-cargo does not run the tests but rather only does a build test. One needs an override to force it to run the tests. 

   1 override_dh_auto_test:
   2     dh_auto_test --buildsystem=cargo -- test --all

Build

dh-cargo does not build the code in the build stage, it’s all folded into the install stage.

Install

This should work out of the box for single-crate projects:

However, for more complex projects, you’ll need this somewhat gnarly invocation along those lines: 

   1     # Here we utilise a logical flaw in the cargo wrapper to our advantage:
   2     # when specifying DEB_CARGO_CRATE_IN_REGISTRY=1, the wrapper will
   3     # avoid adding the `--path` option, so that we can specify it ourselves
   4     DEB_HOST_GNU_TYPE=$(DEB_HOST_GNU_TYPE) \
   5     DEB_HOST_RUST_TYPE=$(DEB_HOST_RUST_TYPE) \
   6     CARGO_HOME=$(CURDIR)/debian/cargo_home \
   7     DEB_CARGO_CRATE=adsys_mount_0.1.0 \
   8     DEB_CARGO_CRATE_IN_REGISTRY=1 \
   9     DESTDIR=$(CURDIR)/debian/tmp \
  10         $(CARGO) install --path $(ADSYS_MOUNT)
  11     # Currently doesn't work because of https://github.com/rust-lang/cargo/issues/4101
  12     # combined with a lack of flexibility in the cargo wrapper. That means we
  13     # have to do it manually (with the build split out in dh_auto_build for good
  14     # measure, even though dh-cargo does it all in the install step)
  15     # dh_auto_install --buildsystem=cargo -- --path $(ADSYS_MOUNT)
  16

Rust vendored sources tracking

The tracking of embedded Rust vendored dependencies is done via a field in the source package, fittingly named XS-Vendored-Sources-Rust, composed of a comma-separated list of versioned crate names of the format CRATE@VERSION.

From version 28ubuntu1 on, the dh-cargo package contains a script that checks this field’s content against the contents of the vendor tree, and fails the build if it doesn’t match, outputting the expected value.

The script is available in /usr/share/cargo/bin/dh-cargo-vendored-sources

Note that the field can easily be fairly gigantic.

CategoryPackaging

RustCodeInMain (last edited 2024-10-08 14:42:42 by slyon)