Write the error output from Protobuf parse exceptions

core/src/main/scala/higherkindness/mu/rpc/srcgen/avro/AvroSrcGenerator.scala

@@ -112,6 +112,7 @@ final case class AvroSrcGenerator(
       s"${protocol.getNamespace.replace('.', '/')}/${protocol.getName}$ScalaFileExtension"
 
     val schemaGenerator = if (protocol.getMessages.isEmpty) adtGenerator else mainGenerator
+
     val schemaLines = schemaGenerator
       .protocolToStrings(protocol)
       .mkString
@@ -163,7 +164,7 @@ final case class AvroSrcGenerator(
     outputPath -> outputCode
   }
 
-  def validateRequest(request: Schema): ErrorsOr[String] = {
+  private def validateRequest(request: Schema): ErrorsOr[String] = {
     val requestArgs = request.getFields.asScala
     requestArgs.toList match {
       case Nil =>
@@ -177,7 +178,7 @@ final case class AvroSrcGenerator(
     }
   }
 
-  def validateResponse(response: Schema): ErrorsOr[String] = {
+  private def validateResponse(response: Schema): ErrorsOr[String] = {
     response.getType match {
       case Schema.Type.NULL =>
         EmptyType.validNel

core/src/main/scala/higherkindness/mu/rpc/srcgen/proto/ProtoSrcGenerator.scala

@@ -42,7 +42,9 @@ import higherkindness.skeuomorph.protobuf.{ProtobufF, Protocol}
 
 object ProtoSrcGenerator {
 
-  final case class ProtobufSrcGenException(message: String) extends NoStackTrace
+  final case class ProtobufSrcGenException(message: String) extends NoStackTrace {
+    override def toString: String = s"ProtoBufSrcGenException: $message"
+  }
 
   def build(
       compressionTypeGen: CompressionTypeGen,

core/src/test/resources/avro/Invalid.avdl

@@ -0,0 +1,19 @@
+@namespace("foo.bar")
+protocol MyGreeterService {
+
+  record HelloRequest {
+    string arg1;
+    union { null, string } arg2;
+    array<string> arg3;
+  }
+
+  record HelloResponse {
+    string arg1;
+    union { null, string } arg2;
+    array<string>  arg3;
+  }
+
+  string sayHelloAvro(foo.bar.HelloRequest arg);
+
+  void sayNothingAvro();
+}

core/src/test/resources/proto/broken.proto

@@ -0,0 +1,6 @@
+syntax = "proto3";
+
+message Broken {
+    string name = 1;
+    string nick = 2;
+}

core/src/test/scala/higherkindness/mu/rpc/srcgen/AvroSrcGenTests.scala

@@ -16,11 +16,18 @@
 
 package higherkindness.mu.rpc.srcgen
 
+import cats.data.Validated
 import cats.data.Validated.Valid
 
 import scala.io._
 import higherkindness.mu.rpc.srcgen.AvroScalaGeneratorArbitrary._
-import higherkindness.mu.rpc.srcgen.Model.ScalaBigDecimalTaggedGen
+import higherkindness.mu.rpc.srcgen.Model.SerializationType.Avro
+import higherkindness.mu.rpc.srcgen.Model.{
+  BigDecimalAvroMarshallers,
+  NoCompressionGen,
+  ScalaBigDecimalTaggedGen,
+  UseIdiomaticEndpoints
+}
 import higherkindness.mu.rpc.srcgen.avro._
 import org.scalacheck.Prop.forAll
 import org.scalatest._
@@ -37,6 +44,29 @@ class AvroSrcGenTests extends AnyWordSpec with Matchers with OneInstancePerTest
         forAll { scenario: Scenario => test(scenario) }
       }
     }
+
+    "return a non-empty list of errors instead of generating code from an invalid IDL file" in {
+      val response = {
+        AvroSrcGenerator(
+          List(BigDecimalAvroMarshallers),
+          ScalaBigDecimalTaggedGen,
+          NoCompressionGen,
+          UseIdiomaticEndpoints.trueV
+        ).generateFrom(
+          Source.fromInputStream(getClass.getResourceAsStream("/avro/Invalid.avdl")).mkString,
+          Avro
+        )
+      }
+
+      response shouldBe Some(
+        (
+          "foo/bar/MyGreeterService.scala",
+          Validated.invalidNel(
+            "RPC method response parameter has non-record response type 'STRING'"
+          )
+        )
+      )
+    }
   }
 
   private def test(scenario: Scenario): Boolean = {

core/src/test/scala/higherkindness/mu/rpc/srcgen/ProtoSrcGenTests.scala

@@ -26,6 +26,7 @@ import higherkindness.mu.rpc.srcgen.Model.{
   UseIdiomaticEndpoints
 }
 import higherkindness.mu.rpc.srcgen.proto.ProtoSrcGenerator
+import higherkindness.mu.rpc.srcgen.proto.ProtoSrcGenerator.ProtobufSrcGenException
 import org.scalatest._
 import org.scalatest.matchers.should.Matchers
 import org.scalatest.wordspec.AnyWordSpec
@@ -75,6 +76,22 @@ class ProtoSrcGenTests extends AnyWordSpec with Matchers with OneInstancePerTest
       result shouldBe Some(("com/proto/book.scala", expectedFileContent))
     }
 
+    "throw an exception on an incorrect Protobuf schema" in {
+      assertThrows[ProtobufSrcGenException] {
+        ProtoSrcGenerator
+          .build(
+            NoCompressionGen,
+            UseIdiomaticEndpoints(false),
+            MonixObservable,
+            new java.io.File(".")
+          )
+          .generateFrom(
+            files = Set(protoFile("broken")),
+            serializationType = SerializationType.Protobuf
+          )
+      }
+    }
+
   }
 
   def bookExpectation(streamOf: String => String): String =