[gRPC] Android duplicate classes with lite/protolite protobuf dependencies

[Jaakkko]

Describe the bug
Android projects that use kotlinx-rpc can run into duplicate-class failures when another library brings the Android lite/protolite protobuf stack.

Firebase Firestore is one concrete example of that broader problem. It is a useful minimal reproducer because the current kotlinx-rpc Android/JVM dependency graph brings full protobuf/gRPC artifacts while Firestore brings the Android lite/protolite protobuf stack.

The issue is reproducible in samples/grpc-kmp-app with a single dependency addition to shared/build.gradle.kts, but the underlying problem appears to be the general Android dependency shape rather than anything specific to that sample.

Observed conflict shape:

• kotlinx-rpc brings grpc-protobuf, protobuf-java, protobuf-java-util, and proto-google-common-protos
• Firestore brings grpc-protobuf-lite, protobuf-javalite, and protolite-well-known-types
• Android packaging fails because these artifacts contain overlapping com.google.protobuf.* and com.google.api.* classes

To Reproduce

1. Kotlin version
   2.3.10

2. Gradle version
   9.4.0

3. OS (Or at least KMP platform)
   macOS, Android target

4. Minimal reproducer in code
   Start from samples/grpc-kmp-app as a minimal reproducer for the general Android compatibility issue.

   Baseline:

   • ./gradlew :androidApp:checkDebugDuplicateClasses
   • result: success

Then add Firestore to shared/build.gradle.kts to introduce a representative lite/protolite protobuf dependency:

kotlin {
    // ...
    sourceSets {
        androidMain.dependencies {
            implementation("com.google.firebase:firebase-firestore:26.2.0")
        }
        commonMain.dependencies {
            api(libs.kotlinx.rpc.grpc.core)
            api(libs.kotlinx.rpc.protobuf.core)
        }
    }
}

Then run:

./gradlew :androidApp:checkDebugDuplicateClasses

5. Error description
   The sample already contains this exclusion in androidApp/build.gradle.kts:

   configurations.all {
       exclude(group = "com.google.protobuf", module = "protobuf-javalite")
   }

   Even with that exclusion, the build fails with duplicate classes such as:

   Duplicate class com.google.api.Advice found in modules
   proto-google-common-protos-2.63.2.jar
   and protolite-well-known-types-18.0.1.aar
   

6. Dependency paths
   protobuf-java is brought in via:

   com.google.protobuf:protobuf-java:4.34.0
   \--- com.google.protobuf:protobuf-java-util:4.34.0
        \--- org.jetbrains.kotlinx:kotlinx-rpc-protobuf-api-jvm:0.11.0-grpc-188
   

   and also via:

   com.google.protobuf:protobuf-java:3.25.8 -> 4.34.0
   +--- com.google.api.grpc:proto-google-common-protos:2.63.2
   |    \--- io.grpc:grpc-protobuf:1.80.0
   |         \--- org.jetbrains.kotlinx:kotlinx-rpc-grpc-core-jvm:0.11.0-grpc-188
   \--- io.grpc:grpc-protobuf:1.80.0
   

   proto-google-common-protos is brought in via:

   com.google.api.grpc:proto-google-common-protos:2.63.2
   \--- io.grpc:grpc-protobuf:1.80.0
        \--- org.jetbrains.kotlinx:kotlinx-rpc-grpc-core-jvm:0.11.0-grpc-188
   

Expected behavior
Adding Firestore to an Android project that uses kotlinx-rpc gRPC/protobuf artifacts should not immediately produce duplicate-class failures.

At minimum, one of these should be true:

• kotlinx-rpc publishes an Android-safe dependency graph that can coexist with Firebase/Firestore
• kotlinx-rpc documents a supported exclusion/alignment strategy for Android apps using Firestore
• kotlinx-rpc avoids pulling full protobuf/common-protos artifacts into the Android runtime path when they are not required

Additional context
Things tried during investigation:

• Forcing protobuf-javalite to Firestore-compatible versions did not fix the issue.
  The failing duplicate set changed, but the build still failed.

• Excluding only protobuf-java did not fix the issue.
  The failure then surfaced as proto-google-common-protos vs protolite-well-known-types.

• Excluding both protobuf-java and proto-google-common-protos did allow the sample to pass:

  allprojects {
      configurations.configureEach {
          exclude(group = "com.google.protobuf", module = "protobuf-java")
          exclude(group = "com.google.api.grpc", module = "proto-google-common-protos")
      }
  }

  With those exclusions:

  • :androidApp:checkDebugDuplicateClasses succeeded
  • :androidApp:assembleDebug succeeded

• However, that workaround still looks risky for general use because protobuf-java-util remains on the runtime classpath via kotlinx-rpc-protobuf-api-jvm.

• In one limited unary-call path, generated request/response marshallers appeared to use the plain WireEncoder / WireDecoder path rather than protobuf-java-util, so that specific path looked plausibly safe.
  But this does not prove safety for broader usage, especially with streaming calls, well-known types, or other protobuf utility-heavy features.

So the current state seems to be:

• the sample is a minimal reproducible example for the Android dependency conflict
• there is a build-level workaround using exclusions
• but the workaround is not clearly supported or guaranteed safe for all kotlinx-rpc Android use cases