Add empty proto file test

* Add empty proto file test
* Add a preamble to `foo-messages.proto`
* Don't unconditionally add a dependency on `GRPCProtobuf` if there are
  no services
* Add the new test case code generation to `dev/protos/generate.sh`
* depend on `grpc-swift` `main` to pick up empty file generation
  changes.

Package.swift

@@ -31,7 +31,7 @@ let products: [Product] = [
 let dependencies: [Package.Dependency] = [
   .package(
     url: "https://github.com/grpc/grpc-swift.git",
-    exact: "2.0.0-beta.2"
+    branch: "main"
   ),
   .package(
     url: "https://github.com/apple/swift-protobuf.git",

Sources/GRPCProtobufCodeGen/ProtobufCodeGenParser.swift

@@ -98,9 +98,11 @@ package struct ProtobufCodeGenParser {
 
 extension ProtobufCodeGenParser {
   fileprivate func codeDependencies(file: FileDescriptor) -> [Dependency] {
-    var codeDependencies: [Dependency] = [
-      Dependency(module: "GRPCProtobuf", accessLevel: .internal)
-    ]
+    var codeDependencies: [Dependency] = []
+
+    if file.services.count > 0 {
+      codeDependencies.append(Dependency(module: "GRPCProtobuf", accessLevel: .internal))
+    }
 
     // If there's a dependency on a bundled proto then add the SwiftProtobuf import.
     //

Tests/GRPCProtobufCodeGenTests/ProtobufCodeGeneratorTests.swift

@@ -19,41 +19,43 @@ import GRPCProtobufCodeGen
 import SwiftProtobufPluginLibrary
 import Testing
 
-struct ProtobufCodeGeneratorTests: UsesDescriptorSet {
-  static let descriptorSetName = "test-service"
-  static let fileDescriptorName = "test-service"
+struct ProtobufCodeGeneratorTests {
+  @Suite("Self-contained service (test-service.proto)")
+  struct TestService: UsesDescriptorSet {
+    static let descriptorSetName = "test-service"
+    static let fileDescriptorName = "test-service"
 
-  @Test("Generate", arguments: [SourceGenerator.Config.AccessLevel.internal, .public, .package])
-  func generate(accessLevel: SourceGenerator.Config.AccessLevel) throws {
-    let generator = ProtobufCodeGenerator(
-      config: SourceGenerator.Config(
-        accessLevel: accessLevel,
-        accessLevelOnImports: false,
-        client: true,
-        server: true,
-        indentation: 2
+    @Test("Generate", arguments: [SourceGenerator.Config.AccessLevel.internal, .public, .package])
+    func generate(accessLevel: SourceGenerator.Config.AccessLevel) throws {
+      let generator = ProtobufCodeGenerator(
+        config: SourceGenerator.Config(
+          accessLevel: accessLevel,
+          accessLevelOnImports: false,
+          client: true,
+          server: true,
+          indentation: 2
+        )
       )
-    )
 
-    let access: String
-    switch accessLevel {
-    case .public:
-      access = "public"
-    case .internal:
-      access = "internal"
-    case .package:
-      access = "package"
-    default:
-      fatalError()
-    }
+      let access: String
+      switch accessLevel {
+      case .public:
+        access = "public"
+      case .internal:
+        access = "internal"
+      case .package:
+        access = "package"
+      default:
+        fatalError()
+      }
 
-    let generated = try generator.generateCode(
-      fileDescriptor: Self.fileDescriptor,
-      protoFileModuleMappings: ProtoFileToModuleMappings(),
-      extraModuleImports: []
-    )
+      let generated = try generator.generateCode(
+        fileDescriptor: Self.fileDescriptor,
+        protoFileModuleMappings: ProtoFileToModuleMappings(),
+        extraModuleImports: []
+      )
 
-    let expected = """
+      let expected = """
       /// Leading trivia.
 
       // DO NOT EDIT.
@@ -1058,6 +1060,49 @@ struct ProtobufCodeGeneratorTests: UsesDescriptorSet {
       }
       """
 
-    #expect(generated == expected)
+      #expect(generated == expected)
+    }
+  }
+
+  @Suite("File-without-services (foo-messages.proto)")
+  struct NoServices: UsesDescriptorSet {
+    static let descriptorSetName = "foo-messages"
+    static let fileDescriptorName = "foo-messages"
+
+    @Test("Generate")
+    func generate() throws {
+      let generator = ProtobufCodeGenerator(
+        config: SourceGenerator.Config(
+          accessLevel: .public,
+          accessLevelOnImports: false,
+          client: true,
+          server: true,
+          indentation: 2
+        )
+      )
+
+      let generated = try generator.generateCode(
+        fileDescriptor: Self.fileDescriptor,
+        protoFileModuleMappings: ProtoFileToModuleMappings(),
+        extraModuleImports: []
+      )
+
+      let expected = """
+      /// Leading trivia.
+
+      // DO NOT EDIT.
+      // swift-format-ignore-file
+      //
+      // Generated by the gRPC Swift generator plugin for the protocol buffer compiler.
+      // Source: foo-messages.proto
+      //
+      // For information on using the generated types, please see the documentation:
+      //   https://github.com/grpc/grpc-swift
+
+      // This file contained no services.
+      """
+
+      #expect(generated == expected)
+    }
   }
 }

dev/protos/generate.sh

@@ -119,6 +119,17 @@ function generate_foo_service_descriptor_set {
     --include_imports
 }
 
+function generate_foo_messages_descriptor_set {
+  local proto proto_path output
+  proto="$here/local/foo-messages.proto"
+  proto_path="$(dirname "$proto")"
+  output="$root/Tests/GRPCProtobufCodeGenTests/Generated/foo-messages.pb"
+
+  invoke_protoc --descriptor_set_out="$output" "$proto" -I "$proto_path" \
+    --include_source_info \
+    --include_imports
+}
+
 function generate_bar_service_descriptor_set {
   local proto proto_path output
   proto="$here/local/bar-service.proto"
@@ -152,5 +163,6 @@ generate_error_service
 # Descriptor sets for tests
 generate_test_service_descriptor_set
 generate_foo_service_descriptor_set
+generate_foo_messages_descriptor_set
 generate_bar_service_descriptor_set
 generate_wkt_service_descriptor_set

dev/protos/local/foo-messages.proto

@@ -1,3 +1,4 @@
+// Leading trivia.
 syntax = "proto3";
 
 package foo;