Provide support for imported files and Well-Known types in gRPC

[gregboero]

Describe the bug
An error occurred (see stack trace in step 5) during Buf generation due to the use of an import statement in a .proto file.

To Reproduce
Steps to reproduce the behavior:

1. Kotlin version: 2.2.0

2. Gradle version: 9.0.0

3. OS (or at least the KMP platform): Windows

4. Minimal reproducer in code:

   • Clone the [grpc-app](https://github.com/Kotlin/kotlinx-rpc/tree/main/samples/grpc-app) sample project.
   • In the image-recognizer.proto file, add: import "google/protobuf/empty.proto";
   • Create a custom Buf configuration file (buf.yaml):

     version: v2
     deps:
       # Common Protobuf types.
       - buf.build/googleapis/googleapis
     lint:
       use:
         # Indicates that all the default rules should be used.
         # See https://buf.build/docs/lint/rules for more details.
         - STANDARD
       except:
         - PACKAGE_DIRECTORY_MATCH
         - RPC_REQUEST_RESPONSE_UNIQUE
         - RPC_REQUEST_STANDARD_NAME
         - RPC_RESPONSE_STANDARD_NAME
     breaking:
       use:
         # Indicates that breaking change detection should be done on a file level.
         # See https://buf.build/docs/breaking/rules for more details.
         - FILE
     modules:
       - path: proto

   • Update the build.gradle.kts file:

     rpc {
         grpc {
             buf {
                 configFile = file("src/main/proto/buf.yaml")
             }
         }
     }

   • Run the Gradle task bufGenerateMain.

5. Error description:

   Execution failed for task ':bufGenerateMain'.
   > Process buf finished:
     - Buf Arguments: [...\build.buf\buf\1.55.1\d04c47a7d4c47f7fd91922b6f4fa746b8623b134\buf-1.55.1-windows-x86_64.exe, generate, --output, ...\build\protoBuild\generated\main, --config, ...\src\main\proto\buf.yaml]
     - Exit code: 1
     - Stdout: No output
     - Stderr:
         WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
         WARNING: sun.misc.Unsafe::arrayBaseOffset has been called by com.google.protobuf.UnsafeUtil$MemoryAccessor (file:/.../caches/modules-2/files-2.1/org.jetbrains.kotlinx/protoc-gen-kotlin-multiplatform/0.10.0-grpc-121/7b0ffaa90f70e4658a81a10a7de2c0da8536aaeb/protoc-gen-kotlin-multiplatform-0.10.0-grpc-121-all.jar)
         WARNING: Please consider reporting this to the maintainers of class com.google.protobuf.UnsafeUtil$MemoryAccessor
         WARNING: sun.misc.Unsafe::arrayBaseOffset will be removed in a future release
         Exception in thread "main" java.lang.IllegalStateException: Unknown dependency google/protobuf/empty.proto for image-recognizer.proto proto file, wrong topological order
           at kotlinx.rpc.protobuf.ProtoToModelInterpreter.toModel(ProtoToModelInterpreter.kt:39)
           at kotlinx.rpc.protobuf.ProtoToModelInterpreter.interpretProtocRequest(ProtoToModelInterpreter.kt:30)
           at kotlinx.rpc.protobuf.RpcProtobufPlugin.generateKotlinFiles(RpcProtobufPlugin.kt:104)
           at kotlinx.rpc.protobuf.RpcProtobufPlugin.run(RpcProtobufPlugin.kt:75)
           at kotlinx.rpc.protobuf.MainKt.main(Main.kt:13)
           at kotlinx.rpc.protobuf.MainKt.main(Main.kt)
   

Expected behavior
The files should be generated successfully.

Additional context
No error occurs when using the Buf CLI directly with only the Java buf plugin.

[Mr3zee]

Hey @gregboero ! This is a known restriction for now.
The declarations in com.google.protobuf are called Well-Known Types, and we don't support then yet, but we'll be quite soon together with KMP version of gRPC (but not earlier)

I'll rename the ticket to better indicate the problem