Improve CI fork-PR build disk handling

- Free extra unused SDKs on GitHub-hosted runners
- Fail fast with a clear message when a fork-PR VM is too small
- Drop the no-op /mnt bind-mount (same filesystem as root)

Author: cjdsellers

.github/actions/common-setup/action.yml

@@ -78,6 +78,30 @@ runs:
         docker-images: true
         swap-storage: true
 
+    # Extra unused SDKs beyond jlumbroso; only GitHub-hosted runners are disk-bound
+    - name: Free additional disk space (Linux)
+      if: >-
+        inputs.free-disk-space == 'true'
+        && runner.os == 'Linux'
+        && runner.environment == 'github-hosted'
+      shell: bash
+      run: |
+        echo "=== Disk usage before extra cleanup ==="
+        df -h /
+        for path in \
+          /usr/share/swift \
+          /usr/local/lib/swift \
+          /usr/local/share/powershell \
+          /usr/local/share/boost \
+          /usr/local/share/chromium \
+          /opt/google/chrome \
+          /opt/microsoft \
+          /usr/local/julia*; do
+          sudo rm -rf $path || true
+        done
+        echo "=== Disk usage after extra cleanup ==="
+        df -h /
+
     - name: Free disk space (macOS)
       if: inputs.free-disk-space == 'true' && runner.os == 'macOS'
       shell: bash

.github/workflows/build.yml

@@ -263,19 +263,6 @@ jobs:
       - name: Verify immutable CI inputs (post-checkout)
         run: bash scripts/ci/verify-ci-inputs.sh post-checkout
 
-      # Fork-PR runners keep the workspace on the small root fs; bind the target
-      # dir to the larger /mnt disk (path stays workspace-relative for rust-cache).
-      - name: Back Cargo target with /mnt ephemeral disk (fork PRs)
-        if: github.event_name == 'pull_request' && github.event.pull_request.head.repo.fork
-        run: |
-          target="${{ github.workspace }}/target/build-linux-x86"
-          backing="/mnt/cargo-target/build-linux-x86"
-          sudo mkdir -p "$backing"
-          sudo chown "$(id -u):$(id -g)" "$backing"
-          mkdir -p "$target"
-          sudo mount --bind "$backing" "$target"
-          df -h / /mnt
-
       - name: Common setup
         uses: ./.github/actions/common-setup
         with:
@@ -309,6 +296,26 @@ jobs:
       - name: Cached test data
         uses: ./.github/actions/common-test-data
 
+      # Fork-PR runners vary in disk size; fail early with a clear message
+      # rather than a cryptic linker Bus error when a small VM runs out.
+      - name: Check available disk space
+        if: >-
+          needs.plan.outputs.run-rust-tests == 'true'
+          && github.event_name == 'pull_request'
+          && github.event.pull_request.head.repo.fork
+        run: |
+          echo "::group::Disk space before Rust tests"
+          df -h /
+          echo "::endgroup::"
+          avail_gb=$(($(df -Pk / | awk 'NR==2 {print $4}') / 1024 / 1024))
+          echo "Available on / : ${avail_gb}G"
+          if [ "${avail_gb}" -lt 25 ]; then
+            echo "::error::Only ${avail_gb}G free, runner too small for the Rust build. Re-run for a larger VM."
+            exit 1
+          elif [ "${avail_gb}" -lt 40 ]; then
+            echo "::warning::Only ${avail_gb}G free, the Rust build may run out of space, re-run if it fails."
+          fi
+
       - name: Run Rust tests (core)
         if: needs.plan.outputs.run-rust-tests == 'true'
         run: make cargo-test-core EXTRA_FEATURES="capnp,hypersync" NEXTEST_PROFILE=ci