[ci][docs] updated fix for OpenMP conflicts (#2166)

* test: AppleClang on Travis

* updated fix for OpenMP multiple instances

* updated docs

* switch compiler back

.ci/test.sh

@@ -82,7 +82,8 @@ fi
 conda install -q -y -n $CONDA_ENV matplotlib numpy pandas psutil pytest python-graphviz scikit-learn scipy
 
 if [[ $OS_NAME == "macos" ]] && [[ $COMPILER == "clang" ]]; then
-    sudo ln -sf `ls -d "$(brew --cellar libomp)"/*/lib`/* $CONDA_PREFIX/lib || exit -1  # fix "OMP: Error #15: Initializing libiomp5.dylib, but found libomp.dylib already initialized." (OpenMP library conflict due to conda's MKL)
+    # fix "OMP: Error #15: Initializing libiomp5.dylib, but found libomp.dylib already initialized." (OpenMP library conflict due to conda's MKL)
+    for LIBOMP_ALIAS in libgomp.dylib libiomp5.dylib libomp.dylib; do sudo ln -sf "$(brew --cellar libomp)"/*/lib/libomp.dylib $CONDA_PREFIX/lib/$LIBOMP_ALIAS || exit -1; done
 fi
 
 if [[ $TASK == "sdist" ]]; then

docs/FAQ.rst

@@ -139,6 +139,20 @@ LightGBM
 
        ln -sf `ls -d "$(brew --cellar libomp)"/*/lib`/* $CONDA_PREFIX/lib
 
+   The described above fix worked fine before the release of OpenMP 8.0.0 version.
+   Starting from 8.0.0 version, Homebrew formula for OpenMP includes ``-DLIBOMP_INSTALL_ALIASES=OFF`` option which leads to that the fix doesn't work anymore.
+   However, you can create symlinks to library aliases manually:
+
+   ::
+
+       for LIBOMP_ALIAS in libgomp.dylib libiomp5.dylib libomp.dylib; do sudo ln -sf "$(brew --cellar libomp)"/*/lib/libomp.dylib $CONDA_PREFIX/lib/$LIBOMP_ALIAS; done
+
+   Another workaround would be removing MKL optimizations from Conda's packages completely:
+
+   ::
+
+       conda install nomkl
+
    If this is not your case, then you should find conflicting OpenMP library installations on your own and leave only one of them.
 
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