Do not check for jvm_abi_gen_working_dir if the plugin is not used

Summary:
### Context
We are exploring the integration of the Kotlin Incremental Compiler into Buck for the incremental compilation of Kotlin source files. The goal is to improve build speed by only recompiling modified source files within a target, rather than rebuilding the entire target as is currently done.

Recently, I started noticing a continuous increase in compiler non-incremental fallbacks due to the absence of jvmAbiGenWorkingDir. This directory is an output of the jvm-abi-gen plugin, which is used solely for class ABI. Therefore, when the source-only ABI was set as the abi_generation_mode, this directory was never created, leading to a forced non-incremental mode.

### This Diff
This diff checks for jvm_abi_gen_working_dir only if the plugin is being used.

----
### Internal
[Doc](https://docs.google.com/document/d/1-zhGPkmt3I2os-kcIaFk9SvT6odt2AJWswSY-J_6AWs/edit?usp=sharing)|[Unidash](https://fburl.com/unidash/m84c5935)|[Code pointer](https://www.internalfb.com/code/fbsource/[adb2a0b14b76]/fbcode/buck2/prelude/kotlin/kotlincd_jar_creator.bzl?lines=98-107)

Differential Revision: D71127908

fbshipit-source-id: ad7b2337bf22d8d9ef97fb085ba8e7d849229b0f

Author: Iveta Jurčíková

toolchains/android/src/com/facebook/buck/jvm/kotlin/KotlincModeFactory.java

@@ -17,6 +17,7 @@
 import com.facebook.buck.jvm.kotlin.kotlinc.incremental.KotlincMode;
 import com.google.common.collect.ImmutableList;
 import java.util.Optional;
+import javax.annotation.Nullable;
 
 public class KotlincModeFactory {
   private static final Logger LOG = Logger.get(KotlincModeFactory.class);
@@ -52,11 +53,6 @@ public static KotlincMode create(
                   () ->
                       new IllegalStateException(
                           "incremental_state_dir/kotlinc_working_dir is not created"));
-      AbsPath jvmAbiGenWorkingDir =
-          extraParams
-              .getJvmAbiGenWorkingDir()
-              .orElseThrow(
-                  () -> new IllegalStateException("jvm_abi_gen_working_dir is not created"));
       ActionMetadata metadata =
           actionMetadata.orElseThrow(
               () -> new IllegalStateException("actionMetadata is not created"));
@@ -69,7 +65,18 @@ public static KotlincMode create(
           KotlinSourceChangesFactory.create(),
           ClasspathChangesFactory.create(metadata, classpathSnapshots),
           incrementalStateDir.resolve(kotlinClassUsageFile.getFileName()),
-          jvmAbiGenWorkingDir);
+          getJvmAbiGenWorkingDir(
+              extraParams.getShouldUseJvmAbiGen(), extraParams.getJvmAbiGenWorkingDir()));
+    }
+  }
+
+  private static @Nullable AbsPath getJvmAbiGenWorkingDir(
+      boolean shouldUseJvmAbiGen, Optional<AbsPath> jvmAbiGenWorkingDir) {
+    if (!shouldUseJvmAbiGen) {
+      return null;
     }
+
+    return jvmAbiGenWorkingDir.orElseThrow(
+        () -> new IllegalStateException("jvm_abi_gen_working_dir is not created"));
   }
 }

toolchains/android/src/com/facebook/buck/jvm/kotlin/buildtools/JvmCompilationConfigurationFactory.kt

@@ -59,12 +59,16 @@ internal class JvmCompilationConfigurationFactory(
                         forceNonIncrementalMode(true)
                       }
 
-                      if (mode.jvmAbiGenWorkingDir.toFile().exists().not()) {
+                      // targets with abi_generation_mode = "source_only" do not use jvm-abi-gen for
+                      // class abi generation so the directory is never created
+                      val jvmAbiGenWorkingDir = mode.jvmAbiGenWorkingDir
+                      if (jvmAbiGenWorkingDir != null &&
+                          jvmAbiGenWorkingDir.toFile().exists().not()) {
                         LOG.info(
-                            "Non-incremental compilation will be performed: ${mode.jvmAbiGenWorkingDir.fileName} not found")
+                            "Non-incremental compilation will be performed: ${jvmAbiGenWorkingDir.fileName} not found")
                         kotlinCDLoggingContext.addExtras(
                             JvmCompilationConfigurationFactory::class.java.simpleName,
-                            "Non-incremental compilation will be performed: ${mode.jvmAbiGenWorkingDir.fileName} not found")
+                            "Non-incremental compilation will be performed: ${jvmAbiGenWorkingDir.fileName} not found")
                         forceNonIncrementalMode(true)
                       }
 

toolchains/android/src/com/facebook/buck/jvm/kotlin/kotlinc/incremental/KotlincMode.kt

@@ -22,6 +22,6 @@ sealed interface KotlincMode {
       val kotlinSourceChanges: KotlinSourceChanges,
       val classpathChanges: ClasspathChanges,
       val kotlinClassUsageFile: AbsPath,
-      val jvmAbiGenWorkingDir: AbsPath
+      val jvmAbiGenWorkingDir: AbsPath?
   ) : KotlincMode
 }

toolchains/android/test/com/facebook/buck/jvm/kotlin/buildtools/JvmCompilationConfigurationFactoryTest.kt

@@ -163,7 +163,7 @@ internal class JvmCompilationConfigurationFactoryTest {
   }
 
   @Test
-  fun `when classpath changes can not be detected, incremental mode is forced`() {
+  fun `when classpath changes can not be detected, non-incremental mode is forced`() {
     jvmCompilationConfigurationFactory.create(
         createFakeIncrementalKotlincMode(ClasspathChanges.Unknown))
 
@@ -184,6 +184,15 @@ internal class JvmCompilationConfigurationFactoryTest {
         .forceNonIncrementalMode(true)
   }
 
+  @Test
+  fun `when jvmAbiGen output is null, non-incremental mode is forced`() {
+    jvmCompilationConfigurationFactory.create(
+        createFakeIncrementalKotlincMode(jvmAbiGenWorkingDir = null))
+
+    verify(classpathSnapshotBasedIncrementalJvmCompilationConfiguration, never())
+        .forceNonIncrementalMode(true)
+  }
+
   @Test
   fun `when jvmAbiGen working directory does not exist, non-incremental mode is forced`() {
     val noExistingFile = createNonExistingFileMock()
@@ -206,7 +215,7 @@ internal class JvmCompilationConfigurationFactoryTest {
   private fun createFakeIncrementalKotlincMode(
       classpathChanges: ClasspathChanges = createFakeClasspathChanges(),
       kotlinDepFile: AbsPath = createExistingFileMock(),
-      jvmAbiGenWorkingDir: AbsPath = createExistingFileMock()
+      jvmAbiGenWorkingDir: AbsPath? = createExistingFileMock()
   ): KotlincMode.Incremental {
     val rootProjectDir = AbsPath.get("/home/root")
     val buildDir = AbsPath.get("/home/root/buildDir")