Fix CI build break on Mac with LLVM 10.0.1 (#1245)

There's a bug in the current Homebrew llvm 10.0.1 where
it fails to link because of missing libxml2.tbd. It's fixed
upstream in LLVM, but not yet fixed in Homebrew. Hopefully
a subsequent 10.0.x or 11.x will fix it, but for now, just
fall back to llvm@9.

Also issue a warning in the build scripts if this condition
is detected, so if a user encounters it on their system, they
have a hint about how to address it.

Signed-off-by: Larry Gritz <[email protected]>

Author: lgritz

.github/workflows/ci.yml

@@ -242,13 +242,20 @@ jobs:
           CMAKE_CXX_STANDARD: 14
           PYTHON_VERSION: 3.8
           LLVM_BC_GENERATOR: /usr/bin/clang++
+          LLVMBREWVER: "@9"
+            # ^^ There's a bug in the current Homebrew llvm 10.0.1 where
+            # it fails to link because of missing libxml2.tbd. It's fixed
+            # upstream in LLVM, but not yet fixed in Homebrew. Hopefully
+            # a subsequent 10.0.x or 11.x will fix it, but for now, just
+            # fall back to llvm@9.
           MY_CMAKE_FLAGS: -DLLVM_BC_GENERATOR=/usr/bin/clang++
             # ^^ Force bitcode compiles to use the system clang compiler by
             # preemptively telling it the LLVM_BC_GENERATOR, to avoid some
             # library mismatches we tend to see on GH CI's Mac images.
         run: |
             source src/build-scripts/ci-startup.bash
             source src/build-scripts/install_homebrew_deps.bash
+            brew uninstall llvm
             OPENIMAGEIO_CMAKE_FLAGS="-DOIIO_BUILD_TESTS=0 -DUSE_OPENGL=0"
             source src/build-scripts/build_openimageio.bash
             source src/build-scripts/ci-build-and-test.bash

src/build-scripts/install_homebrew_deps.bash

@@ -56,4 +56,6 @@ fi
 export PATH=/usr/local/opt/qt5/bin:$PATH
 export PATH=/usr/local/opt/python/libexec/bin:$PATH
 export PYTHONPATH=/usr/local/lib/python${PYTHON_VERSION}/site-packages:$PYTHONPATH
-export PATH=/usr/local/opt/llvm/bin:$PATH
+export PATH=/usr/local/opt/llvm${LLVMBREWVER}/bin:$PATH
+export LLVM_DIRECTORY=/usr/local/opt/llvm${LLVMBREWVER}
+export LLVM_ROOT=/usr/local/opt/llvm${LLVMBREWVER}

src/cmake/externalpackages.cmake

@@ -255,6 +255,17 @@ if (LLVM_VERSION VERSION_GREATER_EQUAL 10.0.0 AND
              "icc >= 7, MSVS >= 2017. "
              "If you must use C++11, you need to build against LLVM 9 or earlier.")
 endif ()
+if (APPLE AND LLVM_VERSION VERSION_EQUAL 10.0.1 AND EXISTS "/usr/local/Cellar/llvm")
+    message (WARNING
+             "${ColorYellow}If you are using LLVM 10.0.1 installed by Homebrew, "
+             "please note that a known bug in LLVM may produce a link error where "
+             "it says it can't find libxml2.tbd. If you encounter this, please "
+             "try downgrading to LLVM 9: \n"
+             "    brew uninstall llvm \n"
+             "    brew install llvm@9 \n"
+             "    export LLVM_DIRECTORY=/usr/local/opt/llvm@9 "
+             "${ColorReset}\n")
+endif ()
 
 checked_find_package (partio)