legacy proto_library does not work with incompatible_generated_protos_in_virtual_imports

[katre]

Reproduction: https://gist.github.com/katre/8a13e84a4ef27b766fd572fe99ce33a6

$ USE_BAZEL_VERSION=0.29.0rc7 bazelisk build //:lib_proto 
INFO: Analyzed target //:lib_proto (2 packages loaded, 17 targets configured).
INFO: Found 1 target...
INFO: From Generating Descriptor Set proto_library //:useful_proto:
external/com_google_protobuf: warning: directory does not exist.
ERROR: /usr/local/google/home/jcater/repos/demo/BUILD.bazel:1:1: Couldn't build file lib_proto-descriptor-set.proto.bin: Generating Descriptor Set proto_library //:lib_proto failed (Exit 1) protoc failed: error executing command bazel-out/host/bin/external/com_google_protobuf/protoc '--proto_path=external/com_google_protobuf' '--descriptor_set_out=bazel-out/k8-fastbuild/bin/lib_proto-descriptor-set.proto.bin' ... (remaining 7 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox
external/com_google_protobuf: warning: directory does not exist.
google/protobuf/duration.proto: File not found.
useful.proto:5:1: Import "google/protobuf/duration.proto" was not found or had errors.
useful.proto:10:3: "google.protobuf.Duration" is not defined.
lib.proto:5:1: Import "useful.proto" was not found or had errors.
lib.proto:8:3: "very.useful.VeryUseful" is not defined.
Target //:lib_proto failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 1.147s, Critical Path: 0.19s
INFO: 1 process: 1 linux-sandbox.
FAILED: Build did NOT complete successfully

[katre]

Running a bisect, because I've identified two endpoints.

• ce65de4 - Build works correctly (with the experimental flag disabled)
• 7e83721 - Build fails (with the experimental flag disabled)

$ bazel-bisect ce65de4ee8e006137c8d927fa474384c0d940cb2 209175ff8ffeb05628ed8a187dd414a3d2935c55^ build --experimental_generated_protos_in_virtual_imports=false //:lib_proto

[katre]

Bisect indicates that 6a667c6 is the culprit: assigning to @lberki to figure out a safe rollback.