Exclude plugin marker dependencies from graph

Plugin marker dependencies are an artifact of the way plugins are resolved in Gradle, and duplicate the actual plugin dependency.
These add no value to the graph and should be excluded by default.

Fixes #111

Author: bigdaz

plugin-test/src/test/groovy/org/gradle/github/dependencygraph/PluginDependencyExtractorTest.groovy

@@ -124,61 +124,29 @@ class PluginDependencyExtractorTest extends BaseExtractorTest {
         // Settings plugin
         Map<String, Map> pluginDependencies = settingsPluginsAreSupported()
             ? [
-                "my.settings.plugin:my.settings.plugin.gradle.plugin:1.0": [
-                    relationship: "direct",
-                    dependencies: ["com.example:settingPlugin:1.0"]
-                ],
                 "com.example:settingPlugin:1.0"                          : [
-                    relationship: "indirect",
+                    relationship: "direct",
                     dependencies: ["org.test:foo:1.0"]
                 ],
                 "org.test:foo:1.0"                                       : [
                     relationship: "indirect",
                     dependencies: []
-                ],
-
-                // The project plugins are resolved at build level without any transitive deps
-                "my.project.plugin1:my.project.plugin1.gradle.plugin:1.0": [
-                    relationship: "direct",
-                    dependencies: []
-                ],
-                "my.project.plugin2:my.project.plugin2.gradle.plugin:1.0": [
-                    relationship: "direct",
-                    dependencies: []
-                ]
-            ]
-            : [
-                // The project plugins are resolved at build level without any transitive deps
-                "my.project.plugin1:my.project.plugin1.gradle.plugin:1.0": [
-                    relationship: "direct",
-                    dependencies: []
-                ],
-                "my.project.plugin2:my.project.plugin2.gradle.plugin:1.0": [
-                    relationship: "direct",
-                    dependencies: []
                 ]
             ]
+            : [:]
 
         // Plugin 1
         pluginDependencies.putAll([
-            "my.project.plugin1:my.project.plugin1.gradle.plugin:1.0": [
-                relationship: "direct",
-                dependencies: ["com.example:plugin1:1.0"]
-            ],
             "com.example:plugin1:1.0"                                : [
-                relationship: "indirect",
+                relationship: "direct",
                 dependencies: []
             ]
         ])
 
         // Plugin 2
         pluginDependencies.putAll([
-            "my.project.plugin2:my.project.plugin2.gradle.plugin:1.0": [
-                relationship: "direct",
-                dependencies: ["com.example:plugin2:1.0"]
-            ],
             "com.example:plugin2:1.0"                                : [
-                relationship: "indirect",
+                relationship: "direct",
                 dependencies: ["org.test:bar:1.0"]
             ],
             "org.test:bar:1.0"                                       : [

plugin-test/src/test/groovy/org/gradle/github/dependencygraph/SampleProjectDependencyExtractorTest.groovy

@@ -26,7 +26,6 @@ class SampleProjectDependencyExtractorTest extends BaseExtractorTest {
         def manifestDependencies = manifest.resolved
 
         [   // plugin dependencies
-            "com.gradle.enterprise:com.gradle.enterprise.gradle.plugin:3.12.6",
             "com.gradle:gradle-enterprise-gradle-plugin:3.12.6",
             "com.diffplug.spotless:spotless-plugin-gradle:4.5.1",
             "com.diffplug.durian:durian-core:1.2.0",

plugin-test/src/test/groovy/org/gradle/github/dependencygraph/SingleProjectDependencyExtractorTest.groovy

@@ -365,13 +365,8 @@ class SingleProjectDependencyExtractorTest extends BaseExtractorTest {
         def manifest = gitHubManifest()
         manifest.sourceFile == "settings.gradle"
         manifest.assertResolved([
-            "my.project.plugin:my.project.plugin.gradle.plugin:1.0": [
-                package_url : purlFor("my.project.plugin", "my.project.plugin.gradle.plugin", "1.0"),
-                dependencies: ["com.example:plugin:1.0"]
-            ],
-            "com.example:plugin:1.0"                               : [
-                package_url : purlFor("com.example", "plugin", "1.0"),
-                relationship: "indirect"
+            "com.example:plugin:1.0": [
+                package_url : purlFor("com.example", "plugin", "1.0")
             ]
         ])
     }
@@ -401,13 +396,8 @@ class SingleProjectDependencyExtractorTest extends BaseExtractorTest {
         def manifest = gitHubManifest()
         manifest.sourceFile == "settings.gradle"
         manifest.assertResolved([
-            "my.settings.plugin:my.settings.plugin.gradle.plugin:1.0": [
-                package_url : purlFor("my.settings.plugin", "my.settings.plugin.gradle.plugin", "1.0"),
-                dependencies: ["com.example:plugin:1.0"]
-            ],
-            "com.example:plugin:1.0"                                 : [
-                package_url : purlFor("com.example", "plugin", "1.0"),
-                relationship: "indirect"
+            "com.example:plugin:1.0": [
+                package_url : purlFor("com.example", "plugin", "1.0")
             ]
         ])
     }

plugin/src/main/kotlin/org/gradle/dependencygraph/extractor/DependencyExtractor.kt

@@ -1,5 +1,6 @@
 package org.gradle.dependencygraph.extractor
 
+import org.gradle.api.artifacts.component.ModuleComponentIdentifier
 import org.gradle.api.artifacts.component.ProjectComponentIdentifier
 import org.gradle.api.artifacts.result.ResolvedComponentResult
 import org.gradle.api.artifacts.result.ResolvedDependencyResult
@@ -230,7 +231,31 @@ abstract class DependencyExtractor :
     }
 
     private fun getResolvedDependencies(component: ResolvedComponentResult): List<ResolvedComponentResult> {
-        return component.dependencies.filterIsInstance<ResolvedDependencyResult>().map { it.selected }.filter { it != component }
+        return component.dependencies
+            .filterIsInstance<ResolvedDependencyResult>()
+            .map { it.selected }
+            .mapNotNull { traversePluginMarker(it) }
+            .filter { it != component }
+    }
+
+    /**
+     * If the rawComponent represents a plugin marker artifact, then use the target plugin dependency instead.
+     * For a plugin marker with no dependencies return null, so it can be filtered from the list.
+     */
+    private fun traversePluginMarker(rawComponent: ResolvedComponentResult): ResolvedComponentResult? {
+        if (rawComponent.id is ModuleComponentIdentifier) {
+            val componentId = rawComponent.id as ModuleComponentIdentifier
+            if (componentId.module == componentId.group + ".gradle.plugin") {
+                if (rawComponent.dependencies.isEmpty()) {
+                    return null
+                }
+                if (rawComponent.dependencies.size == 1) {
+                    val pluginDep = rawComponent.dependencies.iterator().next() as? ResolvedDependencyResult
+                    return pluginDep?.selected
+                }
+            }
+        }
+        return rawComponent
     }
 
     private fun createComponentNode(componentId: String, origin: DependencyOrigin, isDirectDependency: Boolean, component: ResolvedComponentResult, repositoryLookup: RepositoryUrlLookup): ResolvedDependency {