GH-50242: [Release] Fix flaky libLLVM load failure in macOS verify-rc build

dev/release/verify-release-candidate.sh

@@ -460,6 +460,47 @@ test_and_install_cpp() {
     DEFAULT_DEPENDENCY_SOURCE="AUTO"
   fi
 
+  # TEMP - remove before merge: VERIFY_RC_DEBUG=1 probes the libLLVM @rpath cause.
+  # set +e: this is expected to abort (exit 134) and must not kill the script
+  if [ "${VERIFY_RC_DEBUG:-0}" -gt 0 ] && [ "$(uname)" = "Darwin" ] && [ -n "${CONDA_PREFIX:-}" ]; then
+    set +e
+    echo "===== VERIFY_RC_DEBUG: libLLVM @rpath probe ====="
+    echo "CONDA_PREFIX=${CONDA_PREFIX}"
+    echo "ARROW_HOME=${ARROW_HOME:-<unset>}"
+
+    envlib=$(ls -d "${CONDA_PREFIX}"/lib/libLLVM.*.dylib 2>/dev/null | head -1)
+    echo "env libLLVM           : ${envlib:-MISSING}"
+    if [ -n "${envlib}" ]; then
+      otool -L "${envlib}" >/dev/null 2>&1 && echo "env libLLVM loadable  : yes"
+    fi
+
+    # env binary resolving into the env = hardlinked (safe) vs. into pkgs = vulnerable
+    for tool in llvm-link llvm-ranlib; do
+      envln="${CONDA_PREFIX}/bin/${tool}"
+      [ -e "${envln}" ] || continue
+      real=$(python3 -c 'import os,sys;print(os.path.realpath(sys.argv[1]))' "${envln}" 2>/dev/null)
+      echo "env ${tool} realpath: ${real}"
+      case "${real}" in
+        "${CONDA_PREFIX}"/*) echo "  -> INTO env (safe)" ;;
+        *)                   echo "  -> pkgs cache (VULNERABLE)" ;;
+      esac
+    done
+
+    # controlled comparison on the pkgs binary (its @loader_path is always the pkgs cache)
+    pkgbin=$(ls -d "${CONDA_PREFIX}"/../../pkgs/llvm-tools-*/bin/llvm-link-* 2>/dev/null | head -1)
+    echo "pkgs llvm-link        : ${pkgbin:-MISSING}"
+    if [ -n "${pkgbin}" ]; then
+      DYLD_LIBRARY_PATH="${ARROW_HOME}/lib" "${pkgbin}" --version >/dev/null 2>&1
+      echo "A) no conda lib                      : exit=$?  (134 or 0, pkgs binary varies by run)"
+      DYLD_LIBRARY_PATH="${CONDA_PREFIX}/lib:${ARROW_HOME}/lib" "${pkgbin}" --version >/dev/null 2>&1
+      echo "B) conda lib via DYLD_LIBRARY_PATH   : exit=$?  (expect 0)"
+      DYLD_LIBRARY_PATH="${ARROW_HOME}/lib" DYLD_FALLBACK_LIBRARY_PATH="${CONDA_PREFIX}/lib" "${pkgbin}" --version >/dev/null 2>&1
+      echo "C) conda lib via DYLD_FALLBACK (fix) : exit=$?  (expect 0)"
+    fi
+    echo "===== end VERIFY_RC_DEBUG ====="
+    set -e
+  fi
+
   mkdir -p $ARROW_TMPDIR/cpp-build
   pushd $ARROW_TMPDIR/cpp-build
 
@@ -519,7 +560,14 @@ test_and_install_cpp() {
     ${ARROW_CMAKE_OPTIONS:-} \
     ${ARROW_SOURCE_DIR}/cpp
   export CMAKE_BUILD_PARALLEL_LEVEL=${CMAKE_BUILD_PARALLEL_LEVEL:-${NPROC}}
-  cmake --build . --target install
+  # Expose conda's lib on the *fallback* path so the LLVM tools (llvm-link,
+  # llvm-ranlib) find libLLVM.*.dylib on flaky @rpath miss on macOS. (Fallback is searched last,
+  # so it doesn't shadow system libs like libiconv during the build or leak into tests.)
+  if [ "$(uname)" = "Darwin" ] && [ "${USE_CONDA}" -gt 0 ] && [ -n "${CONDA_PREFIX:-}" ]; then
+    DYLD_FALLBACK_LIBRARY_PATH="${CONDA_PREFIX}/lib:${DYLD_FALLBACK_LIBRARY_PATH:-/usr/local/lib:/usr/lib}" cmake --build . --target install
+  else
+    cmake --build . --target install
+  fi
 
   if [ ${TEST_CPP} -gt 0 ]; then
     LD_LIBRARY_PATH=$PWD/release:$LD_LIBRARY_PATH ctest \
@@ -779,10 +827,10 @@ test_source_distribution() {
 
   if [ "$(uname)" == "Darwin" ]; then
     NPROC=$(sysctl -n hw.ncpu)
-    export DYLD_LIBRARY_PATH=$ARROW_HOME/lib:${DYLD_LIBRARY_PATH:-}
+    export DYLD_LIBRARY_PATH=$ARROW_HOME/lib${DYLD_LIBRARY_PATH:+:${DYLD_LIBRARY_PATH}}
   else
     NPROC=$(nproc)
-    export LD_LIBRARY_PATH=$ARROW_HOME/lib:${LD_LIBRARY_PATH:-}
+    export LD_LIBRARY_PATH=$ARROW_HOME/lib${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}
   fi
 
   pushd $ARROW_SOURCE_DIR

dev/tasks/tasks.yml

@@ -298,6 +298,9 @@ tasks:
     ci: github
     template: verify-rc/github.macos.yml
     params:
+      # TEMP - remove before merge: enable the libLLVM @rpath probe
+      env:
+        VERIFY_RC_DEBUG: 1
       target: {{ target }}
       use_conda: True
       github_runner: "macos-15-intel"