gapic-generator-java exits with `IllegalStateException` when there's no service in protos

[JoeWang1127]

The generator should return normally with no-op when there's no service in protos.

For example: when generating from google/cloud/alloydb/connectors/v1alpha, the generator exited with the following error:

Exception in thread "main" java.lang.IllegalStateException: No services found to generate
        at com.google.common.base.Preconditions.checkState(Preconditions.java:512)
        at com.google.api.generator.gapic.protoparser.Parser.parse(Parser.java:172)
        at com.google.api.generator.gapic.Generator.generateGapic(Generator.java:30)
        at com.google.api.generator.Main.main(Main.java:28)
--java_gapic_out: protoc-gen-java_gapic: Plugin failed with status code 1.

[suztomo]

I would implement the script in a way that it would not call gapic-generator-java if there's no need to call it.

[JoeWang1127]

I would implement the script in a way that it would not call gapic-generator-java if there's no need to call it.

I'm currently working on this feature.

However, generate_library.sh can't decide whether to invoke generator or not. This information is stored in BUILD.bazel: if there's no java_gapic_library target, then generate_library.sh should not call the generator.

The grpc java plugin is implemented in a way that if there's no service to generate, it will exit normally without generating any code. I think the generator (which is also a protoc plugin) should do the same.

[blakeli0]

We discussed this in the project meeting and I agree with Joe. In addition, we have other use cases that can benefit from this change. For example, as @mpeddada1 mentioned in #2048, we may need the generator to generate reflect-config.json for proto-only modules.

[blakeli0]

This is working as intended, changing the type from bug to feature request.

[diegomarquezp]

cc: @lqiu96 @JoeWang1127 @alicejli
We will address this issue by strictly causing a no-op (no output is generated) whenever no services are generated. We will leave the noop + reflect-config.json as a follow up to be addressed separately (#2048)

Some considerations:

• Update documentation to explain that the generator will not fail but simply no-op
• Add a log statement (stderr) indicating that no services will be generated