feat: enable cross-compilation of Linux binaries using Swift static SDK

Author: csjones

.github/workflows/release.yml

@@ -31,27 +31,26 @@ jobs:
           retention-days: 5
 
   linux-amd64:
-    name: Build Linux AMD64 binary
     runs-on: ubuntu-latest
     steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-        
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-        
-      - name: Build AMD64 binary
-        uses: docker/build-push-action@v5
+      - uses: actions/checkout@v4
+      - uses: docker/setup-buildx-action@v3
+      - uses: docker/build-push-action@v5
         with:
           context: .
           target: output
+          platforms: linux/amd64
+          build-args: |
+            SWIFT_SDK_ID=x86_64-swift-linux-musl
           outputs: type=local,dest=artifacts
-          
+      - name: Assert x86_64 binary
+        run: |
+          file artifacts/subtree
+          file artifacts/subtree | grep -q 'x86-64'  # fail if not x86_64
       - name: Strip and prepare binary
         run: |
           strip artifacts/subtree
           mv artifacts/subtree subtree_linux
-          
       - name: Upload AMD64 Artifact
         uses: actions/upload-artifact@v4
         with:
@@ -60,27 +59,26 @@ jobs:
           retention-days: 5
 
   linux-arm64:
-    name: Build Linux ARM64 binary
     runs-on: ubuntu-24.04-arm
     steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-        
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-        
-      - name: Build ARM64 binary
-        uses: docker/build-push-action@v5
+      - uses: actions/checkout@v4
+      - uses: docker/setup-buildx-action@v3
+      - uses: docker/build-push-action@v5
         with:
           context: .
           target: output
+          platforms: linux/arm64
+          build-args: |
+            SWIFT_SDK_ID=aarch64-swift-linux-musl
           outputs: type=local,dest=artifacts
-          
+      - name: Assert aarch64 binary
+        run: |
+          file artifacts/subtree
+          file artifacts/subtree | grep -q 'ARM aarch64'
       - name: Strip and prepare binary
         run: |
           strip artifacts/subtree
           mv artifacts/subtree subtree_linux_aarch64
-          
       - name: Upload ARM64 Artifact
         uses: actions/upload-artifact@v4
         with:

Dockerfile

@@ -1,28 +1,38 @@
 # syntax=docker/dockerfile:1
 
-# Base image and static SDK have to be updated together.
-# Native runner approach: Each job uses its own native architecture
-FROM swift:6.1 AS builder
+FROM --platform=$BUILDPLATFORM swift:6.1 AS builder
+ARG TARGETPLATFORM
+ARG SWIFT_SDK_ID
 WORKDIR /workspace
 
-# Install Swift static SDK for better portability
+# Install Swift static SDK (version must match base toolchain)
 RUN swift sdk install \
-	https://download.swift.org/swift-6.1-release/static-sdk/swift-6.1-RELEASE/swift-6.1-RELEASE_static-linux-0.0.1.artifactbundle.tar.gz \
-	--checksum 111c6f7d280a651208b8c74c0521dd99365d785c1976a6e23162f55f65379ac6
+  https://download.swift.org/swift-6.1-release/static-sdk/swift-6.1-RELEASE/swift-6.1-RELEASE_static-linux-0.0.1.artifactbundle.tar.gz \
+  --checksum 111c6f7d280a651208b8c74c0521dd99365d785c1976a6e23162f55f65379ac6
 
-# Copy source files
 COPY . /workspace
 
-# Build with static SDK - will compile for native architecture
-RUN swift build -c release --swift-sdk swift-6.1-RELEASE_static-linux-0.0.1 && \
-    find /workspace/.build -name "subtree" -type f -executable -exec cp {} /workspace/subtree \;
+# Install `file` utility
+RUN apt-get update && apt-get install -y --no-install-recommends file \
+ && rm -rf /var/lib/apt/lists/*
+
+# Map platform -> correct SDK ID if not provided
+RUN if [ -z "$SWIFT_SDK_ID" ]; then \
+      case "$TARGETPLATFORM" in \
+        linux/amd64) SWIFT_SDK_ID=x86_64-swift-linux-musl ;; \
+        linux/arm64) SWIFT_SDK_ID=aarch64-swift-linux-musl ;; \
+        *) echo "Unsupported TARGETPLATFORM: $TARGETPLATFORM" && exit 1 ;; \
+      esac; \
+    fi && \
+    echo "Using Swift SDK: $SWIFT_SDK_ID" && \
+    swift build -c release --swift-sdk "$SWIFT_SDK_ID" -Xswiftc -Osize && \
+    cp ".build/$SWIFT_SDK_ID/release/subtree" /workspace/subtree && \
+    file /workspace/subtree
 
-# Final minimal runtime image
 FROM scratch AS runner
 COPY --from=builder /workspace/subtree /usr/bin/subtree
-ENTRYPOINT [ "/usr/bin/subtree" ]
+ENTRYPOINT ["/usr/bin/subtree"]
 CMD ["--help"]
 
-# Output stage for CI extraction
 FROM scratch AS output
-COPY --from=builder /workspace/subtree /subtree
+COPY --from=builder /workspace/subtree /subtree