Modularise CI workflow and validate outputs for binary size checks.

.github/actions/build-with-patched-std/action.yml

@@ -0,0 +1,45 @@
+# Github composite action to build a single-source-file test binary with an
+# already-checked-out version of Rust's stdlib, that will be patched with a
+# given revision of the backtrace crate.
+
+name: Build with patched std
+description: >
+  Build a binary with a version of std that's had a specific revision of
+  backtrace patched in.
+inputs:
+  backtrace-commit:
+    description: The git commit of backtrace to patch in to std
+    required: true
+  main-rs:
+    description: The (single) source code file to compile
+    required: true
+  rustc-dir:
+    description: The root directory of the rustc repo
+    required: true
+outputs:
+  test-binary-size:
+    description: The size in bytes of the built test binary
+    value: ${{ steps.measure.outputs.test-binary-size }}
+runs:
+  using: "composite"
+  steps:
+    - shell: bash
+      id: measure
+      env:
+        RUSTC_FLAGS: -Copt-level=s -Cstrip=symbols
+        RUSTC_BUILD_DIR: build/x86_64-unknown-linux-gnu
+      working-directory: ${{ inputs.rustc-dir }}
+      run: |
+        rm -rf "$RUSTC_BUILD_DIR/stage0-std"
+
+        (cd library/backtrace && git checkout ${{ inputs.backtrace-commit }})
+        git add library/backtrace
+
+        python3 x.py build library --stage 0
+
+        TEMP_BUILD_OUTPUT=$(mktemp test-binary-XXXXXXXX)
+        "$RUSTC_BUILD_DIR/stage0-sysroot/bin/rustc" $RUSTC_FLAGS "${{ inputs.main-rs }}" -o "$TEMP_BUILD_OUTPUT"
+        BINARY_SIZE=$(stat -c '%s' "$TEMP_BUILD_OUTPUT")
+        rm "$TEMP_BUILD_OUTPUT"
+
+        echo "test-binary-size=$BINARY_SIZE" >> "$GITHUB_OUTPUT"

.github/workflows/check-binary-size.yml

@@ -15,51 +15,88 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       pull-requests: write
+    env:
+      # This cannot be used as a context variable in the 'uses' key later. If it
+      # changes, update those steps too.
+      BACKTRACE_DIR: backtrace
+      RUSTC_DIR: rustc
+      TEST_MAIN_RS: foo.rs
+      BASE_COMMIT: ${{ github.event.pull_request.base.sha }}
+      HEAD_COMMIT: ${{ github.event.pull_request.head.sha }}
     steps:
       - name: Print info
         run: |
-          echo "Current SHA: ${{ github.event.pull_request.head.sha }}"
-          echo "Base SHA: ${{ github.event.pull_request.base.sha }}"
+          echo "Current SHA: $HEAD_COMMIT"
+          echo "Base SHA: $BASE_COMMIT"
+      # Note: the backtrace source that's cloned here is NOT the version to be
+      # patched in to std. It's cloned here to access the Github action for
+      # building the test binary and measuring its size.
+      - name: Clone backtrace to access Github action
+        uses: actions/checkout@v3
+        with:
+          path: ${{ env.BACKTRACE_DIR }}
       - name: Clone Rustc
         uses: actions/checkout@v3
         with:
           repository: rust-lang/rust
-          fetch-depth: 1
-      - name: Fetch backtrace
-        run: git submodule update --init library/backtrace
-      - name: Create hello world program that uses backtrace
-        run: printf "fn main() { panic!(); }" > foo.rs
-      - name: Build binary with base version of backtrace
+          path: ${{ env.RUSTC_DIR }}
+      - name: Set up std repository and backtrace submodule for size test
+        working-directory: ${{ env.RUSTC_DIR }}
         run: |
-          printf "[llvm]\ndownload-ci-llvm = true\n\n[rust]\nincremental = false\n" > config.toml
+          # Bootstrap config
+          cat <<EOF > config.toml
+          [llvm]
+          download-ci-llvm = true
+          [rust]
+          incremental = false
+          EOF
+
+          # Test program source
+          cat <<EOF > $TEST_MAIN_RS
+          fn main() {
+            panic!();
+          }
+          EOF
+
+          git submodule update --init library/backtrace
+
           cd library/backtrace
           git remote add head-pr https://github.com/${{ github.event.pull_request.head.repo.full_name }}
           git fetch --all
-          git checkout ${{ github.event.pull_request.base.sha }}
-          cd ../..
-          git add library/backtrace
-          python3 x.py build library --stage 0
-          ./build/x86_64-unknown-linux-gnu/stage0-sysroot/bin/rustc -O foo.rs -o binary-reference
+      - name: Build binary with base version of backtrace
+        uses: ./backtrace/.github/actions/build-with-patched-std
+        with:
+          backtrace-commit: $BASE_COMMIT
+          main-rs: ${{ env.TEST_MAIN_RS }}
+          rustc-dir: ${{ env.RUSTC_DIR }}
+        id: size-reference
       - name: Build binary with PR version of backtrace
-        run: |
-          cd library/backtrace
-          git checkout ${{ github.event.pull_request.head.sha }}
-          cd ../..
-          git add library/backtrace
-          rm -rf build/x86_64-unknown-linux-gnu/stage0-std
-          python3 x.py build library --stage 0
-          ./build/x86_64-unknown-linux-gnu/stage0-sysroot/bin/rustc -O foo.rs -o binary-updated
-      - name: Display binary size
-        run: |
-          ls -la binary-*
-          echo "SIZE_REFERENCE=$(stat -c '%s' binary-reference)" >> "$GITHUB_ENV"
-          echo "SIZE_UPDATED=$(stat -c '%s' binary-updated)" >> "$GITHUB_ENV"
+        uses: ./backtrace/.github/actions/build-with-patched-std
+        with:
+          backtrace-commit: $HEAD_COMMIT
+          main-rs: ${{ env.TEST_MAIN_RS }}
+          rustc-dir: ${{ env.RUSTC_DIR }}
+        id: size-updated
       - name: Post a PR comment if the size has changed
         uses: actions/github-script@v6
+        env:
+          SIZE_REFERENCE: ${{ steps.size-reference.outputs.test-binary-size }}
+          SIZE_UPDATED: ${{ steps.size-updated.outputs.test-binary-size }}
         with:
           script: |
             const reference = process.env.SIZE_REFERENCE;
             const updated = process.env.SIZE_UPDATED;
+
+            if (!(reference > 0)) {
+              core.setFailed(`Reference size invalid: ${reference}`);
+              return;
+            }
+
+            if (!(updated > 0)) {
+              core.setFailed(`Updated size invalid: ${updated}`);
+              return;
+            }
+
             const diff = updated - reference;
             const plus = diff > 0 ? "+" : "";
             const diff_str = `${plus}${diff}B`;