feat(compiler): Allow multiple plugins of the same type in a project

Separate the generation of the factory classes from the generation of
the service loader files, as the service loader files must contain all
factories of a specfic type, and KSP's `CodeGenerator` cannot append to
previously generated files.

Signed-off-by: Martin Nonnenmacher <martin.nonnenmacher@bosch.com>

plugins/compiler/src/main/kotlin/PluginFactoryGenerator.kt

@@ -20,7 +20,6 @@
 package org.ossreviewtoolkit.plugins.compiler
 
 import com.google.devtools.ksp.processing.CodeGenerator
-import com.google.devtools.ksp.processing.Dependencies
 
 import com.squareup.kotlinpoet.ClassName
 import com.squareup.kotlinpoet.CodeBlock
@@ -39,9 +38,9 @@ import org.ossreviewtoolkit.plugins.api.PluginOptionType
 import org.ossreviewtoolkit.plugins.api.Secret
 
 class PluginFactoryGenerator(private val codeGenerator: CodeGenerator) {
-    fun generate(pluginSpec: PluginSpec) {
+    fun generate(pluginSpec: PluginSpec): ServiceLoaderSpec {
         val generatedFactory = generateFactoryClass(pluginSpec)
-        generateServiceLoaderFile(pluginSpec, generatedFactory)
+        return ServiceLoaderSpec(pluginSpec, generatedFactory)
     }
 
     /**
@@ -188,19 +187,4 @@ class PluginFactoryGenerator(private val codeGenerator: CodeGenerator) {
                 """.trimIndent()
             )
         }.build()
-
-    /**
-     * Generate a service loader file for the [generatedFactory].
-     */
-    private fun generateServiceLoaderFile(pluginSpec: PluginSpec, generatedFactory: TypeSpec) {
-        codeGenerator.createNewFileByPath(
-            dependencies = Dependencies(aggregating = true, *listOfNotNull(pluginSpec.containingFile).toTypedArray()),
-            path = "META-INF/services/${pluginSpec.factory.qualifiedName}",
-            extensionName = ""
-        ).use { output ->
-            output.writer().use { writer ->
-                writer.write("${pluginSpec.packageName}.${generatedFactory.name}\n")
-            }
-        }
-    }
 }

plugins/compiler/src/main/kotlin/PluginProcessor.kt

@@ -40,6 +40,7 @@ class PluginProcessor(codeGenerator: CodeGenerator) : SymbolProcessor {
     private val specFactory = PluginSpecFactory()
     private val factoryGenerator = PluginFactoryGenerator(codeGenerator)
     private val jsonGenerator = JsonSpecGenerator(codeGenerator)
+    private val serviceLoaderGenerator = ServiceLoaderGenerator(codeGenerator)
 
     /**
      * Process all classes annotated with [OrtPlugin] to generate plugin factories for them.
@@ -54,6 +55,8 @@ class PluginProcessor(codeGenerator: CodeGenerator) : SymbolProcessor {
             "Could not get qualified name of OrtPlugin annotation."
         }
 
+        val serviceLoaderSpecs = mutableListOf<ServiceLoaderSpec>()
+
         resolver.getSymbolsWithAnnotation(ortPluginClassName).forEach { pluginClass ->
             require(pluginClass is KSClassDeclaration) {
                 "Annotated element $pluginClass is not a class."
@@ -68,10 +71,12 @@ class PluginProcessor(codeGenerator: CodeGenerator) : SymbolProcessor {
             checkExtendsPluginClass(pluginClass, pluginParentClass)
 
             val pluginSpec = specFactory.create(pluginAnnotation, pluginClass, pluginFactoryClass)
-            factoryGenerator.generate(pluginSpec)
+            serviceLoaderSpecs += factoryGenerator.generate(pluginSpec)
             jsonGenerator.generate(pluginSpec)
         }
 
+        serviceLoaderGenerator.generate(serviceLoaderSpecs)
+
         invoked = true
 
         return emptyList()

plugins/compiler/src/main/kotlin/PluginSpec.kt

@@ -22,6 +22,7 @@ package org.ossreviewtoolkit.plugins.compiler
 import com.google.devtools.ksp.symbol.KSFile
 
 import com.squareup.kotlinpoet.TypeName
+import com.squareup.kotlinpoet.TypeSpec
 
 import org.ossreviewtoolkit.plugins.api.PluginDescriptor
 
@@ -52,3 +53,11 @@ data class PluginFactorySpec(
     val typeName: TypeName,
     val qualifiedName: String
 )
+
+/**
+ * A specification for a service loader, used to generate service loader files.
+ */
+data class ServiceLoaderSpec(
+    val pluginSpec: PluginSpec,
+    val factory: TypeSpec
+)

plugins/compiler/src/main/kotlin/ServiceLoaderGenerator.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2024 The ORT Project Authors (see <https://github.com/oss-review-toolkit/ort/blob/main/NOTICE>)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ * License-Filename: LICENSE
+ */
+
+package org.ossreviewtoolkit.plugins.compiler
+
+import com.google.devtools.ksp.processing.CodeGenerator
+import com.google.devtools.ksp.processing.Dependencies
+
+/**
+ * A class to generate service loader files for plugin factories.
+ */
+class ServiceLoaderGenerator(private val codeGenerator: CodeGenerator) {
+    fun generate(serviceLoaderSpecs: List<ServiceLoaderSpec>) {
+        serviceLoaderSpecs.groupBy { it.pluginSpec.factory.qualifiedName }.forEach { (factoryName, specs) ->
+            val containingFiles = specs.mapNotNull { it.pluginSpec.containingFile }
+
+            codeGenerator.createNewFileByPath(
+                dependencies = Dependencies(
+                    aggregating = true,
+                    *containingFiles.toTypedArray()
+                ),
+                path = "META-INF/services/$factoryName",
+                extensionName = ""
+            ).use { output ->
+                output.writer().use { writer ->
+                    specs.forEach { spec ->
+                        writer.write("${spec.pluginSpec.packageName}.${spec.factory.name}\n")
+                    }
+                }
+            }
+        }
+    }
+}