Update Gradle dokka configuration to make sure "source" button is visible in all API docs (#3960)

Author: whyoleg

gradle/dokka.gradle.kts

@@ -37,14 +37,31 @@ tasks.withType(DokkaTaskPartial::class).configureEach {
     }
 }
 
-// Custom configuration for MPP modules
-tasks.withType(DokkaTaskPartial::class).configureEach {
-    dokkaSourceSets.configureEach {
-        sourceLink {
-            val relPath = rootProject.projectDir.toPath().relativize(projectDir.toPath())
-            localDirectory.set(projectDir.resolve("src"))
-            remoteUrl.set(URL("https://github.com/kotlin/kotlinx.coroutines/tree/master/$relPath/src"))
-            remoteLineSuffix.set("#L")
+fun GradleDokkaSourceSetBuilder.makeLinkMapping(projectDir: File) {
+    sourceLink {
+        val relPath = rootProject.projectDir.toPath().relativize(projectDir.toPath())
+        localDirectory.set(projectDir.resolve("src"))
+        remoteUrl.set(URL("https://github.com/kotlin/kotlinx.coroutines/tree/master/$relPath/src"))
+        remoteLineSuffix.set("#L")
+    }
+}
+
+if (project.isMultiplatform) {
+    // Configuration for MPP modules
+    tasks.withType(DokkaTaskPartial::class).configureEach {
+        // sources in MPP are located in moduleDir/PLATFORM/src,
+        // where PLATFORM could be jvm, js, jdk8, concurrent, etc
+        // configuration happens in buildSrc/src/main/kotlin/SourceSetsKt.configureMultiplatform
+        dokkaSourceSets.matching { it.name.endsWith("Main") }.configureEach {
+            val platform = name.dropLast(4)
+            makeLinkMapping(project.file(platform))
+        }
+    }
+} else {
+    // Configuration for JVM modules
+    tasks.withType(DokkaTaskPartial::class).configureEach {
+        dokkaSourceSets.named("main") {
+            makeLinkMapping(projectDir)
         }
     }
 }