GH-49930: [CI][C++] Pin MinGW MSYS2 packages to unblock CI

.github/workflows/cpp.yml

@@ -366,6 +366,51 @@ jobs:
       - name: Setup MSYS2
         shell: msys2 {0}
         run: ci/scripts/msys2_setup.sh cpp
+      - name: Pin MSYS2 packages
+        # Temporary workarounds for upstream issues, both with concrete exit
+        # criteria.
+        # - MINGW64: gcc 16 makes the intermittent __emutls race in ThreadPool's
+        #   `thread_local current_thread_pool_` (#49272) crash on every run. The
+        #   list also pins C++ packages rebuilt at pkgrel 2 since they reference
+        #   libstdc++ symbols missing from 15.2. Remove once #49462 lands. CLANG64
+        #   uses libc++ and is unaffected.
+        # - MINGW64 + CLANG64 jobs: aws-sdk-cpp 1.11.801 dropped Content-Md5 on
+        #   DeleteObjects, which the bundled MinIO (RELEASE.2024-09-13) still
+        #   requires. Remove once MinIO is bumped or the SDK keeps sending MD5.
+        shell: msys2 {0}
+        run: |
+          set -ex
+          base="https://repo.msys2.org/mingw"
+          case "${{ matrix.msystem_lower }}" in
+            mingw64)
+              urls=(
+                "$base/mingw64/mingw-w64-x86_64-gcc-libs-15.2.0-14-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-gcc-15.2.0-14-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-aws-crt-cpp-0.38.4-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-boost-libs-1.91.0-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-boost-1.91.0-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-ccache-4.13.2-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-clang-libs-22.1.4-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-clang-22.1.4-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-cmake-4.3.2-2-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-icu-78.3-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-llvm-libs-22.1.4-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-llvm-tools-22.1.4-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-llvm-22.1.4-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-tbb-2022.3.0-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-thrift-0.22.0-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-zstd-1.5.7-1-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-gflags-2.2.2-7-any.pkg.tar.zst"
+                "$base/mingw64/mingw-w64-x86_64-aws-sdk-cpp-1.11.479-1-any.pkg.tar.zst"
+              )
+              ;;
+            clang64)
+              urls=(
+                "$base/clang64/mingw-w64-clang-x86_64-aws-sdk-cpp-1.11.479-1-any.pkg.tar.zst"
+              )
+              ;;
+          esac
+          pacman -U --noconfirm "${urls[@]}"
       - name: Cache ccache
         uses: actions/cache@v5
         with:
@@ -386,6 +431,8 @@ jobs:
           # https://github.com/apache/arrow/issues/48593
           ci/scripts/download_tz_database.sh
       - name: Download MinIO
+        # Pinned in tandem with aws-sdk-cpp 1.11.479 (see "Pin MSYS2
+        # packages" step). Bump this version and remove the SDK pin together.
         shell: msys2 {0}
         run: |
           mkdir -p /usr/local/bin