We are facing an issue where mockgen -prog_only ... is exiting successfully, but outputting an empty file.

In our repo's build system (Bazel), the call to mockgen -prog_only is a cacheable build action. Not returning a failure in the case mentioned becomes problematic because Bazel caches the build action's output whenever the underlying command succeeds (returned an exit code 0). Meaning when mockgen fails to write to stdout and does not exit with a failure, we end up uploading an incorrect build artifact to our cache.