Upload core files etc upon CI failure (#3405)

.github/workflows/build.yml

@@ -247,11 +247,20 @@ jobs:
           --with-dune=$GITHUB_WORKSPACE/ocaml-414/_install/bin/dune \
           ${{ matrix.config }}
 
+    - name: Setup for saving core files (not for macOS at the moment)
+      if: matrix.os != 'macos-latest'
+      run: |
+        sudo mkdir /cores
+        sudo chmod 777 /cores
+        # Core filenames will be of the form executable.pid.timestamp:
+        sudo bash -c 'echo "/cores/%e.%p.%t" > /proc/sys/kernel/core_pattern'
+
     - name: Build, install and test Flambda backend
       working-directory: flambda_backend
       run: |
         if [ $run_testsuite = true ]; then target=ci; else target=compiler; fi
         export PATH=$GITHUB_WORKSPACE/ocaml-414/_install/bin:$PATH
+        ulimit -c unlimited
         make $target \
           || (if [ $expected_fail = true ]; then exit 0; else exit 1; fi);
       env:
@@ -265,6 +274,25 @@ jobs:
       if: matrix.check_arch == true
       run: |
         PATH=$GITHUB_WORKSPACE/ocaml-414/_install/bin:$PATH make check_all_arches
+
+    - uses: actions/upload-artifact@v3
+      if: ${{ failure() }} && matrix.os != 'macos-latest'
+      with:
+        name: cores
+        path: /cores
+
+    - uses: actions/upload-artifact@v3
+      if: ${{ failure() }} && matrix.os != 'macos-latest'
+      with:
+        name: _build
+        path: $GITHUB_WORKSPACE/_build
+
+    - uses: actions/upload-artifact@v3
+      if: ${{ failure() }} && matrix.os != 'macos-latest'
+      with:
+        name: _runtest
+        path: $GITHUB_WORKSPACE/_runtest
+
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
   cancel-in-progress: true