Make sure Bazel generated files get overwritten (#4205)

build.sh

@@ -122,8 +122,13 @@ else
       --target=$ROOT_DIR/python/ray/pyarrow_files pyarrow==0.12.0.RAY \
       --find-links https://s3-us-west-2.amazonaws.com/arrow-wheels/9357dc130789ee42f8181d8724bee1d5d1509060/index.html
   bazel build //:ray_pkg -c opt
-  # Copy files and skip existing files
-  cp -r -n $ROOT_DIR/bazel-genfiles/ray_pkg/ray $ROOT_DIR/python || true
+  # Copy files and keep them writeable. This is a workaround, as Bazel
+  # marks all generated files non-writeable. If we would just copy them
+  # over without adding write permission, the copy would fail the next time.
+  # TODO(pcm): It would be great to have a solution here that does not
+  # require us to copy the files.
+  find $ROOT_DIR/bazel-genfiles/ray_pkg/ -exec chmod +w {} \;
+  cp -r $ROOT_DIR/bazel-genfiles/ray_pkg/ray $ROOT_DIR/python || true
 fi
 
 popd