Validate checksum for Python installers

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -12,7 +12,6 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
 # Source this rc script to prepare the environment for macos builds
 
 # show original open file limit values
@@ -56,10 +55,14 @@ export PYTHONPATH=/Library/Python/3.4/site-packages
 
 # Install Python 3.7
 time curl -O https://www.python.org/ftp/python/3.7.0/python-3.7.0-macosx10.9.pkg
+download_hash=$(md5sum python-3.7.0-macosx10.9.pkg | awk '{print $1}')
+if [[ download_hash != 'ae0717a02efea3b0eb34aadc680dc498' ]]; then echo "Incorrect hash!"; exit 1; fi
 time sudo installer -pkg ./python-3.7.0-macosx10.9.pkg -target /
 
 # Install Python 3.8
 time curl -O https://www.python.org/ftp/python/3.8.0/python-3.8.0-macosx10.9.pkg
+download_hash=$(md5sum python-3.8.0-macosx10.9.pkg | awk '{print $1}')
+if [[ download_hash != 'f5f9ae9f416170c6355cab7256bb75b5' ]]; then echo "Incorrect hash!"; exit 1; fi
 time sudo installer -pkg ./python-3.8.0-macosx10.9.pkg -target /
 
 # set xcode version for Obj-C tests
@@ -85,3 +88,4 @@ ln -s /tmpfs/DerivedData ~/Library/Developer/Xcode/DerivedData
 
 # PHP tests currently require using an older version of PHPUnit
 ln -sf /usr/local/bin/phpunit-5.7 /usr/local/bin/phpunit
+