False-positive diag::err_undeclared_use_of_module possibly due to long paths

[froody]

I recently started using clang-19.1.7 with bazel and I'm seeing errors sometimes like:

external/grpc+/src/core/load_balancing/oob_backend_metric.cc:30:10: error: module grpc+//:oob_backend_metric does not depend on a module exporting 'google/protobuf/duration.upb.h'
   30 | #include "google/protobuf/duration.upb.h"
      |          ^

When I look at the clang invocation I see:

-fmodule-map-file=bazel-out/k8-fastbuild/bin/external/protobuf+/src/google/protobuf/duration_proto.upb.cppmap

and the contents of that file is below. I suspect that the overuse of ../ in the paths generated by bazel is creating very long paths inside clang resulting in truncation, but I'm not sure how to validate this.

Can someone familiar with module maps shed any light on this?

duration_proto.upb.cppmap:

module "protobuf+//src/google/protobuf:duration_proto.upb" {
  export *
  textual header "../../../../../../../../bazel-out/k8-fastbuild/bin/external/protobuf+/src/google/protobuf/_virtual_imports/duration_proto/google/protobuf/duration.upb.h"
  use "protobuf+//src/google/protobuf:duration_proto.upb_minitable"
  use "protobuf+//upb:generated_code_support__only_for_generated_code_do_not_use__i_give_permission_to_break_me"
  use "crosstool"
}
extern module "protobuf+//src/google/protobuf:duration_proto.upb_minitable" "../../../../../../../../bazel-out/k8-fastbuild/bin/external/protobuf+/src/google/protobuf/duration_proto.upb_minitable.cppmap"
extern module "protobuf+//upb:generated_code_support__only_for_generated_code_do_not_use__i_give_permission_to_break_me" "../../../../../../../../bazel-out/k8-fastbuild/bin/external/protobuf+/upb/generated_code_support__only_for_generated_code_do_not_use__i_give_permission_to_break_me.cppmap"
extern module "crosstool" "../../../../../../../../bazel-out/k8-fastbuild/bin/external/toolchains_llvm++llvm+llvm_toolchain/module-x86_64-linux.modulemap"

[toprakmurat]

Before implementing a full solution, you can test if path length is indeed the issue by creating shorter symlinks:

Example:

# Create a shorter symlink to test
ln -s bazel-out/k8-fastbuild/bin/external/protobuf+/src/google/protobuf/ short_protobuf

# Temporarily modify your module map to use the shorter path
# Replace the long path with something like:
# textual header "short_protobuf/duration.upb.h"

Also, It would be helpful if you could also share what you have tried so far and what didn’t work.

[froody]

It turns out this error above was not path length but rather a -isystem vs -iquote due to a bazel change.

I did however verify a similar issue on macos was fixed by shortening the paths. I basically renamed the directory containing the .cppmap files to a shorter name and replaced all the references in the .cppmap files, and then I was able to successfully compile.

I'm happy to recompile clang and investigate myself, I just don't know where to start adding logging/etc.