[SPARK-11092] [DOCS] Add source links to scaladoc generation

Modify the SBT build script to include GitHub source links for generated Scaladocs, on releases only (no snapshots).

Author: Jakob Odersky <jodersky@gmail.com>

Closes #9110 from jodersky/unidoc.

project/SparkBuild.scala

@@ -156,6 +156,10 @@ object SparkBuild extends PomBuild {
 
     javacOptions in Compile ++= Seq("-encoding", "UTF-8"),
 
+    scalacOptions in Compile ++= Seq(
+      "-sourcepath", (baseDirectory in ThisBuild).value.getAbsolutePath  // Required for relative source links in scaladoc
+    ),
+
     // Implements -Xfatal-warnings, ignoring deprecation warnings.
     // Code snippet taken from https://issues.scala-lang.org/browse/SI-8410.
     compile in Compile := {
@@ -489,6 +493,8 @@ object Unidoc {
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/hive/test")))
   }
 
+  val unidocSourceBase = settingKey[String]("Base URL of source links in Scaladoc.")
+
   lazy val settings = scalaJavaUnidocSettings ++ Seq (
     publish := {},
 
@@ -531,8 +537,19 @@ object Unidoc {
       "-noqualifier", "java.lang"
     ),
 
-    // Group similar methods together based on the @group annotation.
-    scalacOptions in (ScalaUnidoc, unidoc) ++= Seq("-groups")
+    // Use GitHub repository for Scaladoc source linke
+    unidocSourceBase := s"https://github.com/apache/spark/tree/v${version.value}",
+
+    scalacOptions in (ScalaUnidoc, unidoc) ++= Seq(
+      "-groups" // Group similar methods together based on the @group annotation.
+    ) ++ (
+      // Add links to sources when generating Scaladoc for a non-snapshot release
+      if (!isSnapshot.value) {
+        Opts.doc.sourceUrl(unidocSourceBase.value + "€{FILE_PATH}.scala")
+      } else {
+        Seq()
+      }
+    )
   )
 }