[GHA] use system ROOT (#901)

* download ROOT as opposed to building it due to continuing problems on GitHub Actions, see #899
* export ROOT environment and Vdt_ROOT to GITHUB_ENV (the latter is needed for the ROOT version we use)
* add CMP0074 to SIRF to be able to use Vdt_ROOT (pending SyneRBI/SIRF#1261)

.github/workflows/c-cpp.yml

@@ -59,11 +59,11 @@ jobs:
           DEVEL_BUILD: "OFF"
         - os: ubuntu-latest
           compiler_version: 11
-          EXTRA_BUILD_FLAGS: "-DUSE_ITK=ON -DBUILD_CIL=ON -DUSE_SYSTEM_ACE=ON -DUSE_SYSTEM_Armadillo=OFF -DUSE_SYSTEM_Boost=ON -DUSE_SYSTEM_FFTW3=ON -DUSE_SYSTEM_HDF5=ON -DBUILD_siemens_to_ismrmrd=ON -DUSE_SYSTEM_SWIG=ON -DUSE_ROOT:BOOL=ON"
+          EXTRA_BUILD_FLAGS: "-DUSE_ITK=ON -DBUILD_CIL=ON -DUSE_SYSTEM_ACE=ON -DUSE_SYSTEM_Armadillo=OFF -DUSE_SYSTEM_Boost=ON -DUSE_SYSTEM_FFTW3=ON -DUSE_SYSTEM_HDF5=ON -DBUILD_siemens_to_ismrmrd=ON -DUSE_SYSTEM_SWIG=ON -DUSE_ROOT:BOOL=ON -DUSE_SYSTEM_ROOT=ON -DSIRF_EXTRA_CMAKE_ARGS=-DCMAKE_POLICY_DEFAULT_CMP0074=NEW" # note: SIRF needs CMP0074 with older versions of pre-built ROOT (see below)
           DEVEL_BUILD: "ON"
         - os: ubuntu-latest
           compiler_version: 11
-          EXTRA_BUILD_FLAGS: "-DUSE_ITK=ON -DBUILD_CIL=ON -DUSE_SYSTEM_ACE=ON -DUSE_SYSTEM_Armadillo=OFF -DUSE_SYSTEM_Boost=ON -DUSE_SYSTEM_FFTW3=ON -DUSE_SYSTEM_HDF5=ON -DBUILD_siemens_to_ismrmrd=ON -DUSE_SYSTEM_SWIG=ON -DUSE_ROOT:BOOL=ON"
+          EXTRA_BUILD_FLAGS: "-DUSE_ITK=ON -DBUILD_CIL=ON -DUSE_SYSTEM_ACE=ON -DUSE_SYSTEM_Armadillo=OFF -DUSE_SYSTEM_Boost=ON -DUSE_SYSTEM_FFTW3=ON -DUSE_SYSTEM_HDF5=ON -DBUILD_siemens_to_ismrmrd=ON -DUSE_SYSTEM_SWIG=ON -DUSE_ROOT:BOOL=ON -DUSE_SYSTEM_ROOT=ON -DSIRF_EXTRA_CMAKE_ARGS=-DCMAKE_POLICY_DEFAULT_CMP0074=NEW" # note: SIRF needs CMP0074 with older versions of pre-built ROOT (see below)
           DEVEL_BUILD: "OFF"
       fail-fast: false # don't terminate matrix if one job fails
     steps:
@@ -81,6 +81,41 @@ jobs:
         sudo ./build_gadgetron-ubuntu.sh
         sudo ./build_python-ubuntu.sh
         echo "python-version=$(python3 --version | awk '{print $2}')" >> $GITHUB_OUTPUT
+
+    - name: optionally install ROOT
+      env:
+        EXTRA_BUILD_FLAGS: ${{ matrix.EXTRA_BUILD_FLAGS }}
+      run: |
+        sh -ex
+        if expr match "${EXTRA_BUILD_FLAGS}" ".*-DUSE_SYSTEM_ROOT=ON.*" >/dev/null
+        then
+            # Install ROOT (warning: currently only valid on Ubuntu)
+            ROOT_file=root_v6.28.12.Linux-ubuntu20-x86_64-gcc9.4.tar.gz
+            wget https://root.cern/download/"$ROOT_file"
+            tar -xzf "$ROOT_file"
+            rm "$ROOT_file"
+            source root/bin/thisroot.sh
+            echo ROOT_DIR="$ROOTSYS/cmake" >> $GITHUB_ENV
+            echo ROOTSYS="$ROOTSYS" >> $GITHUB_ENV
+            echo
+            # work-around problem with problem with Vdt for ROOT version pre-6.30.04 as per https://github.com/root-project/root/pull/14178
+            echo Vdt_ROOT="$ROOTSYS" >> $GITHUB_ENV
+            # thisroot modifies the following
+            case ${{matrix.os}} in
+             (ubuntu*)
+                echo LD_LIBRARY_PATH="${CMAKE_INSTALL_PREFIX}/lib:$LD_LIBRARY_PATH" >> $GITHUB_ENV
+                echo PATH="$PATH" >> $GITHUB_ENV
+                ;;
+              (macOS*) # future-proof
+                echo DYLD_FALLBACK_LIBRARY_PATH="${CMAKE_INSTALL_PREFIX}/lib:$DYLD_FALLBACK_LIBRARY_PATH" >> $GITHUB_ENV
+                echo PATH="$PATH" >> $GITHUB_ENV
+                ;;
+              (windows*) # future-proof
+                echo PATH="${CMAKE_INSTALL_PREFIX}/lib:${CMAKE_INSTALL_PREFIX}/bin:$PATH" >> $GITHUB_ENV
+                ;;
+            esac
+        fi
+
     - uses: actions/setup-python@v5
       with:
         python-version: '${{ steps.deps.outputs.python-version }}'