feat: gapic-generator-java to perform a no-op when no services are detected (#2460)

Fixes #2050

Adds behavior to gracefully perform a NOOP if no services are contained
in the generation request.


## Confimation in hermetic build scripts

From `generate_library.sh` against `google/cloud/alloydb/connectors/v1`
```
+ /usr/local/google/home/diegomarquezp/Desktop/sdk2/sdk3/sdk-platform-java/library_generation/output/protobuf-25.2/bin/protoc --experimental_allow_proto3_optional --plugin=protoc-gen-java_gapic=/usr/local/google/home/diegomarquezp/.pyenv/versions/3.11.0/lib/python3.11/site-packages/library_generation/gapic-generator-java-wrapper --java_gapic_out=metadata:/usr/local/google/home/diegomarquezp/Desktop/sdk2/sdk3/sdk-platform-java/library_generation/output/temp_preprocessed/java_gapic_srcjar_raw.srcjar.zip --java_gapic_opt=transport=grpc,rest-numeric-enums,grpc-service-config=,gapic-config=,api-service-config=google/cloud/alloydb/connectors/v1/connectors_v1.yaml google/cloud/alloydb/connectors/v1/resources.proto google/cloud/common_resources.proto
Apr 05, 2024 9:33:22 PM com.google.api.generator.gapic.protoparser.Parser parse
WARNING: No services found to generate. This will produce an empty zip file
Apr 05, 2024 9:33:22 PM com.google.api.generator.gapic.composer.ClientLibraryPackageInfoComposer generatePackageInfo
WARNING: Generating empty package info since no services were found
+ did_generate_gapic=true
+ zipinfo -t /usr/local/google/home/diegomarquezp/Desktop/sdk2/sdk3/sdk-platform-java/library_generation/output/temp_preprocessed/java_gapic_srcjar_raw.srcjar.zip
Empty zipfile. 
+ did_generate_gapic=false
+ [[ false == \t\r\u\e ]]

```

I made some changes to library_generation but I moved them to a follow
up PR (#2599) so the
checks can pass here.

Author: diegomarquezp

gapic-generator-java/src/main/java/com/google/api/generator/Main.java

@@ -14,6 +14,8 @@
 
 package com.google.api.generator;
 
+import static com.google.api.generator.gapic.protowriter.Writer.EMPTY_RESPONSE;
+
 import com.google.api.generator.gapic.Generator;
 import com.google.protobuf.ExtensionRegistry;
 import com.google.protobuf.compiler.PluginProtos.CodeGeneratorRequest;
@@ -26,6 +28,8 @@ public static void main(String[] args) throws IOException {
     ProtoRegistry.registerAllExtensions(registry);
     CodeGeneratorRequest request = CodeGeneratorRequest.parseFrom(System.in, registry);
     CodeGeneratorResponse response = Generator.generateGapic(request);
-    response.writeTo(System.out);
+    if (response != EMPTY_RESPONSE) {
+      response.writeTo(System.out);
+    }
   }
 }

gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/ClientLibraryPackageInfoComposer.java

@@ -29,11 +29,15 @@
 import com.google.api.generator.gapic.model.GapicPackageInfo;
 import com.google.api.generator.gapic.model.Sample;
 import com.google.api.generator.gapic.model.Service;
-import com.google.common.base.Preconditions;
 import com.google.common.base.Strings;
+import java.util.logging.Logger;
 import javax.annotation.Generated;
 
 public class ClientLibraryPackageInfoComposer {
+
+  private static final Logger LOGGER =
+      Logger.getLogger(ClientLibraryPackageInfoComposer.class.getName());
+
   private static final String DIVIDER = "=======================";
 
   private static final String PACKAGE_INFO_DESCRIPTION =
@@ -44,7 +48,10 @@ public class ClientLibraryPackageInfoComposer {
   private static final String SERVICE_DESCRIPTION_HEADER_PATTERN = "Service Description: %s";
 
   public static GapicPackageInfo generatePackageInfo(GapicContext context) {
-    Preconditions.checkState(!context.services().isEmpty(), "No services found to generate");
+    if (!context.containsServices()) {
+      LOGGER.warning("Generating empty package info since no services were found");
+      return null;
+    }
     // Pick some service's package, as we assume they are all the same.
     String libraryPakkage = context.services().get(0).pakkage();
 

gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/Composer.java

@@ -53,6 +53,9 @@ public static List<GapicClass> composeServiceClasses(GapicContext context) {
   }
 
   public static GapicPackageInfo composePackageInfo(GapicContext context) {
+    if (!context.containsServices()) {
+      return null;
+    }
     return addApacheLicense(ClientLibraryPackageInfoComposer.generatePackageInfo(context));
   }
 

gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/GapicContext.java

@@ -21,6 +21,7 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 import java.util.TreeMap;
 import java.util.stream.Collectors;
@@ -32,6 +33,16 @@ public abstract class GapicContext {
   // it iteratively as we generate client methods.
   private GapicMetadata gapicMetadata = defaultGapicMetadata();
 
+  public static final GapicContext EMPTY =
+      builder()
+          .setServices(Collections.emptyList())
+          .setMessages(Collections.emptyMap())
+          .setServiceConfig(GapicServiceConfig.create(Optional.empty()))
+          .setResourceNames(Collections.emptyMap())
+          .setHelperResourceNames(Collections.emptySet())
+          .setTransport(Transport.GRPC)
+          .build();
+
   // Maps the message name (as it appears in the protobuf) to Messages.
   public abstract ImmutableMap<String, Message> messages();
 
@@ -59,6 +70,10 @@ public GapicMetadata gapicMetadata() {
   @Nullable
   public abstract com.google.api.Service serviceYamlProto();
 
+  public boolean containsServices() {
+    return !services().isEmpty();
+  }
+
   public boolean hasServiceYamlProto() {
     return serviceYamlProto() != null;
   }

gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/Parser.java

@@ -79,14 +79,18 @@
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Map.Entry;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
 import java.util.function.Function;
+import java.util.logging.Logger;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
 
 public class Parser {
+
+  private static final Logger LOGGER = Logger.getLogger(Parser.class.getName());
   private static final String COMMA = ",";
   private static final String COLON = ":";
   private static final String DEFAULT_PORT = "443";
@@ -175,7 +179,10 @@ public static GapicContext parse(CodeGeneratorRequest request) {
             mixinServices,
             transport);
 
-    Preconditions.checkState(!services.isEmpty(), "No services found to generate");
+    if (services.isEmpty()) {
+      LOGGER.warning("No services found to generate. This will cause a no-op (no files generated)");
+      return GapicContext.EMPTY;
+    }
 
     // TODO(vam-google): Figure out whether we should keep this allowlist or bring
     // back the unused resource names for all APIs.
@@ -1102,7 +1109,8 @@ private static Map<String, FileDescriptor> getFilesToGenerate(CodeGeneratorReque
     return fileDescriptors;
   }
 
-  private static String parseServiceJavaPackage(CodeGeneratorRequest request) {
+  @VisibleForTesting
+  static String parseServiceJavaPackage(CodeGeneratorRequest request) {
     Map<String, Integer> javaPackageCount = new HashMap<>();
     Map<String, FileDescriptor> fileDescriptors = getFilesToGenerate(request);
     for (String fileToGenerate : request.getFileToGenerateList()) {
@@ -1135,13 +1143,12 @@ private static String parseServiceJavaPackage(CodeGeneratorRequest request) {
       processedJavaPackageCount = javaPackageCount;
     }
 
-    String finalJavaPackage =
-        processedJavaPackageCount.entrySet().stream()
-            .max(Map.Entry.comparingByValue())
-            .get()
-            .getKey();
-    Preconditions.checkState(
-        !Strings.isNullOrEmpty(finalJavaPackage), "No service Java package found");
+    String finalJavaPackage = "";
+    Optional<Entry<String, Integer>> finalPackageEntry =
+        processedJavaPackageCount.entrySet().stream().max(Map.Entry.comparingByValue());
+    if (finalPackageEntry.isPresent()) {
+      finalJavaPackage = finalPackageEntry.get().getKey();
+    }
     return finalJavaPackage;
   }
 

gapic-generator-java/src/main/java/com/google/api/generator/gapic/protowriter/Writer.java

@@ -36,29 +36,29 @@
 import java.util.jar.JarOutputStream;
 
 public class Writer {
-  static class GapicWriterException extends RuntimeException {
-    public GapicWriterException(String errorMessage) {
-      super(errorMessage);
-    }
 
+  static class GapicWriterException extends RuntimeException {
     public GapicWriterException(String errorMessage, Throwable cause) {
       super(errorMessage, cause);
     }
   }
 
-  public static CodeGeneratorResponse write(
+  public static final CodeGeneratorResponse EMPTY_RESPONSE = null;
+
+  @VisibleForTesting
+  protected static CodeGeneratorResponse write(
       GapicContext context,
       List<GapicClass> clazzes,
       GapicPackageInfo gapicPackageInfo,
       List<ReflectConfig> reflectConfigInfo,
-      String outputFilePath) {
-    ByteString.Output output = ByteString.newOutput();
+      String outputFilePath,
+      JarOutputStream jos,
+      ByteString.Output output)
+      throws IOException {
     JavaWriterVisitor codeWriter = new JavaWriterVisitor();
-    JarOutputStream jos;
-    try {
-      jos = new JarOutputStream(output);
-    } catch (IOException e) {
-      throw new GapicWriterException(e.getMessage(), e);
+
+    if (!context.containsServices()) {
+      return EMPTY_RESPONSE;
     }
 
     for (GapicClass gapicClazz : clazzes) {
@@ -72,12 +72,8 @@ public static CodeGeneratorResponse write(
     writeMetadataFile(context, writePackageInfo(gapicPackageInfo, codeWriter, jos), jos);
     writeReflectConfigFile(gapicPackageInfo.packageInfo().pakkage(), reflectConfigInfo, jos);
 
-    try {
-      jos.finish();
-      jos.flush();
-    } catch (IOException e) {
-      throw new GapicWriterException(e.getMessage(), e);
-    }
+    jos.finish();
+    jos.flush();
 
     CodeGeneratorResponse.Builder response = CodeGeneratorResponse.newBuilder();
     response
@@ -88,6 +84,23 @@ public static CodeGeneratorResponse write(
     return response.build();
   }
 
+  public static CodeGeneratorResponse write(
+      GapicContext context,
+      List<GapicClass> clazzes,
+      GapicPackageInfo gapicPackageInfo,
+      List<ReflectConfig> reflectConfigInfo,
+      String outputFilePath) {
+    ByteString.Output output = ByteString.newOutput();
+    CodeGeneratorResponse response;
+    try (JarOutputStream jos = new JarOutputStream(output)) {
+      response =
+          write(context, clazzes, gapicPackageInfo, reflectConfigInfo, outputFilePath, jos, output);
+    } catch (IOException e) {
+      throw new GapicWriterException(e.getMessage(), e);
+    }
+    return response;
+  }
+
   @VisibleForTesting
   static void writeReflectConfigFile(
       String pakkage, List<ReflectConfig> reflectConfigInfo, JarOutputStream jos) {
@@ -167,7 +180,8 @@ private static void writeSamples(
     }
   }
 
-  private static String writePackageInfo(
+  @VisibleForTesting
+  static String writePackageInfo(
       GapicPackageInfo gapicPackageInfo, JavaWriterVisitor codeWriter, JarOutputStream jos) {
     PackageInfoDefinition packageInfo = gapicPackageInfo.packageInfo();
     packageInfo.accept(codeWriter);

gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/ClientLibraryPackageInfoComposerTest.java

@@ -14,6 +14,8 @@
 
 package com.google.api.generator.gapic.composer;
 
+import static org.junit.jupiter.api.Assertions.assertNull;
+
 import com.google.api.generator.engine.writer.JavaWriterVisitor;
 import com.google.api.generator.gapic.model.GapicContext;
 import com.google.api.generator.gapic.model.GapicPackageInfo;
@@ -39,4 +41,9 @@ void composePackageInfo_showcase() {
             GoldenFileWriter.getGoldenDir(this.getClass()), "ShowcaseWithEchoPackageInfo.golden");
     Assert.assertCodeEquals(goldenFilePath, visitor.write());
   }
+
+  @Test
+  void testGeneratePackageInfo_noServices_returnsNullPackageInfo() {
+    assertNull(ClientLibraryPackageInfoComposer.generatePackageInfo(GapicContext.EMPTY));
+  }
 }

gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/ComposerTest.java

@@ -14,8 +14,10 @@
 
 package com.google.api.generator.gapic.composer;
 
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import com.google.api.generator.engine.ast.ClassDefinition;
 import com.google.api.generator.engine.ast.ScopeNode;
@@ -35,6 +37,7 @@
 import java.nio.file.Paths;
 import java.util.Arrays;
 import java.util.List;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 class ComposerTest {
@@ -53,8 +56,13 @@ class ComposerTest {
           .build();
   private List<Sample> ListofSamples = Arrays.asList(new Sample[] {sample});
 
+  @BeforeEach
+  void initialSanityCheck() {
+    assertTrue(context.containsServices());
+  }
+
   @Test
-  void gapicClass_addApacheLicense() {
+  public void gapicClass_addApacheLicense_validInput_succeeds() {
     ClassDefinition classDef =
         ClassDefinition.builder()
             .setPackageString("com.google.showcase.v1beta1.stub")
@@ -84,14 +92,14 @@ void composeSamples_showcase() {
     assertFalse(composedSamples.isEmpty());
     for (Sample sample : composedSamples) {
       assertEquals(
-          "File header should be APACHE",
           Arrays.asList(CommentComposer.APACHE_LICENSE_COMMENT),
-          sample.fileHeader());
+          sample.fileHeader(),
+          "File header should be APACHE");
       assertEquals(
-          "ApiShortName should be Localhost7469",
           "Localhost7469",
-          sample.regionTag().apiShortName());
-      assertEquals("ApiVersion should be V1Beta1", "V1Beta1", sample.regionTag().apiVersion());
+          sample.regionTag().apiShortName(),
+          "ApiShortName should be Localhost7469");
+      assertEquals("V1Beta1", sample.regionTag().apiVersion(), "ApiVersion should be V1Beta1");
     }
   }
 
@@ -120,10 +128,10 @@ void composeSamples_parseProtoPackage() {
 
     for (Sample sample : composedSamples) {
       assertEquals(
-          "ApiShortName should be Accessapproval",
+          "Accessapproval",
           sample.regionTag().apiShortName(),
-          "Accessapproval");
-      assertEquals("ApiVersion should be V1", sample.regionTag().apiVersion(), "V1");
+          "ApiShortName should be Accessapproval");
+      assertEquals("V1", sample.regionTag().apiVersion(), "ApiVersion should be V1");
     }
 
     protoPack = "google.cloud.vision.v1p1beta1";
@@ -136,8 +144,8 @@ void composeSamples_parseProtoPackage() {
     assertFalse(composedSamples.isEmpty());
 
     for (Sample sample : composedSamples) {
-      assertEquals("ApiShortName should be Vision", sample.regionTag().apiShortName(), "Vision");
-      assertEquals("ApiVersion should be V1P1Beta1", sample.regionTag().apiVersion(), "V1P1Beta1");
+      assertEquals("Vision", sample.regionTag().apiShortName(), "ApiShortName should be Vision");
+      assertEquals("V1P1Beta1", sample.regionTag().apiVersion(), "ApiVersion should be V1P1Beta1");
     }
 
     protoPack = "google.cloud.vision";
@@ -149,11 +157,21 @@ void composeSamples_parseProtoPackage() {
     assertFalse(composedSamples.isEmpty());
 
     for (Sample sample : composedSamples) {
-      assertEquals("ApiShortName should be Vision", sample.regionTag().apiShortName(), "Vision");
-      assertEquals("ApiVersion should be empty", sample.regionTag().apiVersion(), "");
+      assertEquals("Vision", sample.regionTag().apiShortName(), "ApiShortName should be Vision");
+      assertTrue(sample.regionTag().apiVersion().isEmpty(), "ApiVersion should be empty");
     }
   }
 
+  @Test
+  void testEmptyGapicContext_doesNotThrow() {
+    assertTrue(Composer.composeServiceClasses(GapicContext.EMPTY).isEmpty());
+  }
+
+  @Test
+  void testComposePackageInfo_emptyGapicContext_returnsNull() {
+    assertNull(Composer.composePackageInfo(GapicContext.EMPTY));
+  }
+
   private List<GapicClass> getTestClassListFromService(Service testService) {
     GapicClass testClass =
         GrpcServiceCallableFactoryClassComposer.instance()