ci: build both musl targets on every pull request

The released Linux artifacts are `aarch64-unknown-linux-musl` and
`x86_64-unknown-linux-musl`, and nothing in CI built either: a break in the
argp stub, in the kernel-header paths or in the aarch64 `-lgcc` link flag
would have surfaced for the first time during a tag-triggered release. The
throwaway spike workflow used to cover this and was deleted with the spike.

Both legs build on a native runner, with no environment variables, which is
also what keeps `.cargo/config.toml` honest -- it has to carry the whole
recipe on its own. The assertions are `readelf`-based rather than a `file`
string, since rustc emits a static-PIE for x86_64 musl and spells it
differently from aarch64, and `codspeed exec-harness --version` /
`codspeed memtrack --version` answer only if both CLIs really are linked in.

Reported by Greptile on #531.

Refs COD-3440
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: moha-bekh

.github/workflows/ci.yml

@@ -148,6 +148,56 @@ jobs:
           mode: ${{ matrix.mode }}
           run: cargo codspeed run -p runner-shared
 
+  # The released Linux artifacts are musl, and nothing else here builds them, so
+  # a break in the argp stub, the kernel-header paths or the aarch64 `-lgcc`
+  # link flag would otherwise surface for the first time in a release.
+  musl-build:
+    runs-on: ${{ matrix.runner }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - runner: ubuntu-latest
+            target: x86_64-unknown-linux-musl
+          - runner: ubuntu-24.04-arm
+            target: aarch64-unknown-linux-musl
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          submodules: true
+      - uses: ./.github/actions/install-rust
+        with:
+          cache-key: ${{ matrix.target }}
+      - uses: ./.github/actions/install-bpf-deps
+      - name: Install the musl toolchain
+        run: |
+          sudo apt-get install -y musl-tools linux-libc-dev
+          rustup target add "${{ matrix.target }}"
+
+      # No environment variables: the whole recipe lives in `.cargo/config.toml`,
+      # and a plain build is what proves it still stands on its own.
+      - name: Build
+        run: cargo build --bin codspeed --target "${{ matrix.target }}"
+
+      - name: Assert the artifact is static and carries both subcommands
+        run: |
+          BIN=target/${{ matrix.target }}/debug/codspeed
+          file "$BIN"
+          # Asserted through readelf rather than a `file` string: rustc emits a
+          # static-PIE for x86_64 musl, which `file` spells differently from the
+          # aarch64 one. What matters is that nothing is loaded at runtime.
+          if readelf -d "$BIN" 2>/dev/null | grep -qE 'NEEDED|RPATH|RUNPATH'; then
+            echo "the musl binary has a dynamic dependency"
+            exit 1
+          fi
+          if readelf -lW "$BIN" 2>/dev/null | grep -q 'INTERP'; then
+            echo "the musl binary requests a dynamic loader"
+            exit 1
+          fi
+          # These answer only if the two CLIs really are linked in.
+          "$BIN" exec-harness --version
+          "$BIN" memtrack --version
+
   check:
     runs-on: ubuntu-latest
     if: always()
@@ -158,6 +208,7 @@ jobs:
       - basic-run-test
       - macos-basic-run-test
       - bpf-tests
+      - musl-build
       - benchmarks
     steps:
       - uses: re-actors/alls-green@release/v1