cargo.eclass: add trivial crate overrides

Updating vulnerable (or otherwise outdated) crates in Rust ebuilds
is painful. Generally speaking, there are 5 options:

- Run `cargo update` to fetch new versions from the web.
  This is obviously not suitable for use in Portage.
- Patch the software via Portage to accept a non-vulnerable crate.
  This is a reasonable option when the package is not too complex
  but still requires significant developer effort and some familiarity
  with Cargo. In the case of complex patches this may not be feasible,
  or require the generation of a dependency tarball.
- [patch] the source (repository) in Cargo.toml. This enables the
  targeting of specific crates, but does not allow the replacement
  of only a specific version in the depgraph.
- [replace] a particular crate:version in the Cargo.toml. This
  enables the targeting of a particular version with an arbitrary
  path however the replacement crate must *have the same version*
  as the one being overridden.
- `paths = [...]` overrides: pass an array of paths to directories that
  contain a Cargo.toml. Cargo will override any crate with the same package name
  arbitrarily, ignoring the lock file and versions; typically used for testing.
  Is applied via ${CARGO_HOME}/config.toml (i.e. globally)

This commit:

- Implements the `paths` overrides, which will work even when
  Cargo is configured to use a vendored directory. This is not a 'smart'
  replacement and care must be taken to ensure that all versions of
  the crate in use are compatible (`cargo tree` will help).

- Provides a helper which runs `cargo --update --offline` against
  ${ECARGO_VENDOR} (where ${CRATES} are unpacked). This enables the
  replacement of vulnerable versions in ${CRATES}. It is up to the
  consumer to ensure that only the desired crates are being replaced
  and that package behaviour does not change.

Resources:
- https://doc.rust-lang.org/cargo/reference/overriding-dependencies.html
- https://github.com/rust-lang/cargo/issues/3308

Signed-off-by: Matt Jolly <kangie@gentoo.org>

1 files changed, 94 insertions, 0 deletions

diff --git a/eclass/cargo.eclass b/eclass/cargo.eclass
index 5a3820267544..02b048732f7f 100644
--- a/eclass/cargo.eclass
+++ b/eclass/cargo.eclass
@@ -7,6 +7,7 @@
 # @AUTHOR:
 # Doug Goldstein <cardoe@gentoo.org>
 # Georgy Yakovlev <gyakovlev@gentoo.org>
+# Matt Jolly <kangie@gentoo.org>
 # @SUPPORTED_EAPIS: 8
 # @PROVIDES: rust
 # @BLURB: common functions and variables for cargo builds
@@ -51,6 +52,10 @@ case ${EAPI} in
 		;;
 esac
 
+if [[ -n ${CRATE_PATHS_OVERRIDE} ]]; then
+	CRATES="${CRATES} ${CRATE_PATHS_OVERRIDE}"
+fi
+
 inherit flag-o-matic multiprocessing rust rust-toolchain toolchain-funcs
 
 IUSE="${IUSE} debug"
@@ -81,6 +86,41 @@ ECARGO_VENDOR="${ECARGO_HOME}/gentoo"
 # SRC_URI="${CARGO_CRATE_URIS}"
 # @CODE
 
+# @ECLASS_VARIABLE: CRATE_PATHS_OVERRIDE
+# @DEFAULT_UNSET
+# @PRE_INHERIT
+# @DESCRIPTION:
+# Bash string containing crates that will be used to override
+# dependencies via generated `paths = ['/path/to/crate']` configuration.
+# This is not "smart", _all crates_ which match the `Cargo.toml`
+# for a given crate/path will be overridden, ignoring lockfiles,
+# version constraints, etc.
+#
+# This should be used as a last resort where (e.g.) you are
+# bootstrapping Rust and need to override a vendored crate
+# with a newer version, and all versions in use are compatible.
+#
+# Crate names and versions must be separated by a `@`;
+# multiple crates are separated by a space or newline.
+# Crates in CRATE_PATHS_OVERRIDE are implicitly added to CRATES;
+# they do not need to be listed.
+#
+# Example:
+# @CODE
+# CRATES="
+# 	foo@1.2.3
+# "
+#
+# CRATE_PATHS_OVERRIDE="
+# 	openssl@0.10.35
+# 	openssl-sys@0.9.65
+# "
+#
+# inherit cargo
+# ...
+# SRC_URI="${CARGO_CRATE_URIS}"
+# @CODE
+
 # @ECLASS_VARIABLE: GIT_CRATES
 # @DEFAULT_UNSET
 # @PRE_INHERIT
@@ -277,6 +317,26 @@ cargo_crate_uris() {
 	echo "${CARGO_CRATE_URIS}"
 }
 
+# @FUNCTION: _cargo_gen_override_paths_config
+# @INTERNAL
+# @DESCRIPTION:
+# Generate the TOML content for overriding crates globally using the package manager.
+# This is called from within cargo_gen_config to insert the appropriate snippet
+# into the generated config.toml. Does not support git crates.
+_cargo_gen_override_paths_config() {
+	if [[ ! ${#CRATE_PATHS_OVERRIDE[@]} -gt 0 ]]; then
+		return
+	fi
+	local content override path
+	content=( 'paths = [' )
+	for override in ${CRATE_PATHS_OVERRIDE}; do
+		local path="${ECARGO_VENDOR}/${override//@/-}"
+		content+=( "'${path}'," )
+	done
+	content+=( ']' )
+	printf "%s\n" "${content[@]}"
+}
+
 # @FUNCTION: cargo_gen_config
 # @DESCRIPTION:
 # Generate the $CARGO_HOME/config.toml necessary to use our local registry and settings.
@@ -293,6 +353,8 @@ cargo_gen_config() {
 	mkdir -p "${ECARGO_HOME}" || die
 
 	cat > "${ECARGO_HOME}/config.toml" <<- _EOF_ || die "Failed to create cargo config"
+	$(_cargo_gen_override_paths_config)
+
 	[source.gentoo]
 	directory = "${ECARGO_VENDOR}"
 
@@ -311,6 +373,7 @@ cargo_gen_config() {
 	verbose = true
 	$([[ "${NOCOLOR}" = true || "${NOCOLOR}" = yes ]] && echo "color = 'never'")
 	$(_cargo_gen_git_config)
+
 	_EOF_
 
 	export CARGO_HOME="${ECARGO_HOME}"
@@ -358,6 +421,37 @@ cargo_target_dir() {
 	echo "${CARGO_TARGET_DIR:-target}/$(rust_abi)/$(usex debug debug release)"
 }
 
+# @FUNCTION: cargo_update_crates
+# @USAGE:
+# @DESCRIPTION:
+# Helper function to call `cargo update --offline` with the given Cargo.toml.
+# This will update Cargo.{toml,lock}. This should provide a straightforward
+# approach to updating vulnerable crates in a package.
+#
+# To use: replace any vulnerable crates in ${CRATES} with updated (and compatible)
+# versions, then call `cargo_update_crates` in src_prepare. If Cargo.toml is not
+# in the root of ${S}, pass the path to the Cargo.toml as the first argument.
+# It is up to the ebuild to ensure that the updated crates are compatible with the
+# package and that no unexpected breakage occurs.
+cargo_update_crates () {
+	debug-print-function ${FUNCNAME} "$@"
+
+	if [[ -z ${CARGO} ]]; then
+		die "CARGO is not set; was rust_pkg_setup run?"
+	fi
+
+	local path=${1:-"${S}/Cargo.toml"}
+	if [[ $# -gt 1 ]]; then
+		die "Usage: cargo_update_crates [path_to_Cargo.toml]"
+	fi
+	[[ -f ${path} ]] || die "${path} does not exist"
+
+	set -- "${CARGO}" update --offline --manifest-path "${path}"
+	einfo "${@}"
+	# This is overkill (we're not using rustflags (etc) here) but it's safe.
+	cargo_env "${@}" || die "Failed to update crates"
+}
+
 # @FUNCTION: cargo_src_unpack
 # @DESCRIPTION:
 # Unpacks the package and the cargo registry.