Add support for Scoverage (#571)

* Add initial work for ScoverageModule

* style: Move package scoverage from lib to contrib

Suggested by @lefou
> I think, it would be better to add under `mill.contrib.scoverage`.

Pull request: #571

* Initial changes to non-hardcoded scoverage version

* Using task context api to specify dataDir

Now measurement data will be written to:
PROJECT_ROOT/out/MODULE/scoverage/data/

and the html report will be written to:
PROJECT_ROOT/out/MODULE/scoverage/data/htmlReport/

* Remove wild card imports in scoverage

Also remove scoverage dependency from build.sc

* Move htmlReport into worker

Based on what I've seen in
scalalib, scalajslib, scalanativelib, playlib, and twirllib modules.

Still need to add tests

* Add basic docs + tests for scoverage

I still am working on testing the actual generation of
reports.

* Use cross-module for scoverage worker

Now we can support multiple versions of scoverage
by adding them to the crossmodule list.

Also now running the local publish script succeeds.

* Add scoverage to ci tests

* Add detailed ScoverageModule documentation

* Test scoverage dataDir

* Remove <pre> tags in scaladoc

* Add scoverage dependency in less hacky way

* Modify scoverage tests to check classpaths

* Put docs in alphabetical order

* Test classpaths for scoverage runtime

* Remove abstract def test: ScoverageTests

* Construct classloader differently

* Revert "Construct classloader differently"

This reverts commit fccf9a94cc38fb9e2be58a9ff90b00b65f339db6.

* Revert "Construct classloader differently"

Also fixes unfound error in html report

This reverts commit fccf9a94cc38fb9e2be58a9ff90b00b65f339db6.

* Fix classpath for scoverage worker

11 files changed, 375 insertions, 5 deletions

diff --git a/build.sc b/build.sc
index fd5d4e29..215e8d37 100755
--- a/build.sc
+++ b/build.sc
@@ -287,6 +287,31 @@ object contrib extends MillModule {
     def moduleDeps = Seq(scalalib)
   }
 
+  object scoverage extends MillModule {
+    def moduleDeps = Seq(scalalib, scoverage.api)
+
+    def testArgs = T {
+      val mapping = Map(
+        "MILL_SCOVERAGE_REPORT_WORKER_1_3_1" -> worker("1.3.1").compile().classes.path
+      )
+      scalalib.worker.testArgs() ++
+        scalalib.backgroundwrapper.testArgs() ++
+        (for ((k, v) <- mapping) yield s"-D$k=$v")
+    }
+
+    object api extends MillApiModule {
+      def moduleDeps = Seq(scalalib)
+    }
+
+    object worker extends Cross[WorkerModule]("1.3.1")
+
+    class WorkerModule(scoverageVersion: String) extends MillApiModule {
+      def moduleDeps = Seq(scoverage.api)
+
+      def ivyDeps = Agg(ivy"org.scoverage::scalac-scoverage-plugin:${scoverageVersion}")
+    }
+  }
+
   object buildinfo extends MillModule {
     def moduleDeps = Seq(scalalib)
     // why do I need this?
@@ -436,7 +461,7 @@ def launcherScript(shellJvmArgs: Seq[String],
 }
 
 object dev extends MillModule{
-  def moduleDeps = Seq(scalalib, scalajslib, scalanativelib, contrib.scalapblib, contrib.tut)
+  def moduleDeps = Seq(scalalib, scalajslib, scalanativelib, contrib.scalapblib, contrib.tut, contrib.scoverage)
 
   def forkArgs =
     (
diff --git a/ci/test-mill-0.sh b/ci/test-mill-0.sh
index fa8d7604..83c361b1 100755
--- a/ci/test-mill-0.sh
+++ b/ci/test-mill-0.sh
@@ -6,4 +6,5 @@ set -eux
 git clean -xdf
 
 # Run tests
-mill -i all {main,scalalib,scalajslib,contrib.twirllib,contrib.playlib,main.client,contrib.scalapblib,contrib.flyway}.test
+
+mill -i all {main,scalalib,scalajslib,contrib.twirllib,contrib.playlib,main.client,contrib.scalapblib,contrib.flyway,contrib.scoverage}.test
\ No newline at end of file
diff --git a/ci/test-mill-bootstrap.sh b/ci/test-mill-bootstrap.sh
index f95c0646..80086df2 100755
--- a/ci/test-mill-bootstrap.sh
+++ b/ci/test-mill-bootstrap.sh
@@ -27,4 +27,4 @@ git clean -xdf
 rm -rf ~/.mill
 
 # Use second build to run tests using Mill
-~/mill-2 -i all {main,scalalib,scalajslib,contrib.twirllib,contrib.playlib,contrib.scalapblib}.test
+~/mill-2 -i all {main,scalalib,scalajslib,contrib.twirllib,contrib.playlib,contrib.scalapblib,contrib.scoverage}.test
diff --git a/contrib/playlib/src/mill/playlib/RouterModule.scala b/contrib/playlib/src/mill/playlib/RouterModule.scala
index abf3082b..62a4c16b 100644
--- a/contrib/playlib/src/mill/playlib/RouterModule.scala
+++ b/contrib/playlib/src/mill/playlib/RouterModule.scala
@@ -97,4 +97,4 @@ trait RouterModule extends ScalaModule with Version {
   override def generatedSources = T {
     super.generatedSources() ++ routerClasses()
   }
-}
\ No newline at end of file
+}
diff --git a/contrib/scoverage/api/src/ScoverageReportWorkerApi.scala b/contrib/scoverage/api/src/ScoverageReportWorkerApi.scala
new file mode 100644
index 00000000..d74e1275
--- /dev/null
+++ b/contrib/scoverage/api/src/ScoverageReportWorkerApi.scala
@@ -0,0 +1,7 @@
+package mill.contrib.scoverage.api
+
+import mill.eval.PathRef
+
+trait ScoverageReportWorkerApi {
+  def htmlReport(sources: Seq[PathRef], dataDir: String, selfDir: String): Unit
+}
diff --git a/contrib/scoverage/src/ScoverageModule.scala b/contrib/scoverage/src/ScoverageModule.scala
new file mode 100644
index 00000000..b96afa34
--- /dev/null
+++ b/contrib/scoverage/src/ScoverageModule.scala
@@ -0,0 +1,122 @@
+package mill
+package contrib
+package scoverage
+
+import coursier.{Cache, MavenRepository}
+import mill.api.Result
+import mill.eval.PathRef
+import mill.util.Ctx
+import mill.scalalib.{DepSyntax, JavaModule, Lib, ScalaModule, TestModule, Dep}
+import mill.moduledefs.Cacher
+
+
+/** Adds targets to a [[mill.scalalib.ScalaModule]] to create test coverage reports.
+ *
+ * This module allows you to generate code coverage reports for Scala projects with
+ * [[https://github.com/scoverage Scoverage]] via the
+ * [[https://github.com/scoverage/scalac-scoverage-plugin scoverage compiler plugin]].
+ *
+ * To declare a module for which you want to generate coverage reports you can
+ * Extends the `mill.contrib.scoverage.ScoverageModule` trait when defining your
+ * Module. Additionally, you must define a submodule that extends the
+ * `ScoverageTests` trait that belongs to your instance of `ScoverageModule`.
+ *
+ * {{{
+ * // You have to replace VERSION
+ * import $ivy.`com.lihaoyi::mill-contrib-buildinfo:VERSION`
+ * import mill.contrib.scoverage.ScoverageModule
+ *
+ * Object foo extends ScoverageModule  {
+ *   def scalaVersion = "2.11.8"
+ *   def scoverageVersion = "1.3.1"
+ *
+ *   object test extends ScoverageTests {
+ *     def ivyDeps = Agg(ivy"org.scalatest::scalatest:3.0.5")
+ *     def testFrameworks = Seq("org.scalatest.tools.Framework")
+ *   }
+ * }
+ * }}}
+ *
+ * In addition to the normal tasks available to your Scala module, Scoverage
+ * Modules introduce a few new tasks and changes the behavior of an existing one.
+ *
+ * - mill foo.scoverage.compile      # compiles your module with test instrumentation
+ *                                 # (you don't have to run this manually, running the test task will force its invocation)
+ *
+ * - mill foo.test                   # tests your project and collects metrics on code coverage
+ * - mill foo.scoverage.htmlReport   # uses the metrics collected by a previous test run to generate a coverage report in html format
+ *
+ * The measurement data is available at `out/foo/scoverage/data/`,
+ * And the html report is saved in `out/foo/scoverage/htmlReport/`.
+ */
+trait ScoverageModule extends ScalaModule { outer: ScalaModule =>
+  def scoverageVersion: T[String]
+  private def scoverageRuntimeDep = T {
+    ivy"org.scoverage::scalac-scoverage-runtime:${outer.scoverageVersion()}"
+  }
+  private def scoveragePluginDep = T {
+    ivy"org.scoverage::scalac-scoverage-plugin:${outer.scoverageVersion()}"
+  }
+
+  private def toolsClasspath = T {
+    scoverageReportWorkerClasspath() ++ scoverageClasspath()
+  }
+
+  def scoverageClasspath = T {
+    Lib.resolveDependencies(
+      Seq(Cache.ivy2Local, MavenRepository("https://repo1.maven.org/maven2")),
+      Lib.depToDependency(_, outer.scalaVersion()),
+      Seq(scoveragePluginDep()),
+      ctx = Some(implicitly[mill.util.Ctx.Log])
+    )
+  }
+
+  def scoverageReportWorkerClasspath = T {
+    val workerKey = "MILL_SCOVERAGE_REPORT_WORKER_" + scoverageVersion().replace(".", "_")
+    mill.modules.Util.millProjectModule(
+      workerKey,
+      s"mill-contrib-scoverage-worker-${outer.scoverageVersion()}",
+      repositories,
+      resolveFilter = _.toString.contains("mill-contrib-scoverage-worker")
+    )
+  }
+
+  object scoverage extends ScalaModule {
+    def selfDir = T { T.ctx().dest / os.up / os.up }
+    def dataDir = T { selfDir() / "data" }
+
+    def sources = outer.sources
+    def resources = outer.resources
+    def scalaVersion = outer.scalaVersion()
+    def compileIvyDeps = outer.compileIvyDeps()
+    def ivyDeps = outer.ivyDeps() ++ Agg(outer.scoverageRuntimeDep())
+    def scalacPluginIvyDeps = outer.scalacPluginIvyDeps() ++ Agg(outer.scoveragePluginDep())
+    def scalacOptions = outer.scalacOptions() ++
+      Seq(s"-P:scoverage:dataDir:${dataDir()}")
+
+    def htmlReport() = T.command {
+      ScoverageReportWorkerApi
+        .scoverageReportWorker()
+        .bridge(toolsClasspath().map(_.path))
+        .htmlReport(sources(), dataDir().toString, selfDir().toString)
+    }
+  }
+
+  trait ScoverageTests extends outer.Tests {
+    override def upstreamAssemblyClasspath = T {
+      super.upstreamAssemblyClasspath() ++
+      resolveDeps(T.task{Agg(outer.scoverageRuntimeDep())})()
+    }
+    override def compileClasspath = T {
+      super.compileClasspath() ++
+      resolveDeps(T.task{Agg(outer.scoverageRuntimeDep())})()
+    }
+    override def runClasspath = T {
+      super.runClasspath() ++
+      resolveDeps(T.task{Agg(outer.scoverageRuntimeDep())})()
+    }
+
+    // Need the sources compiled with scoverage instrumentation to run.
+    override def moduleDeps: Seq[JavaModule] = Seq(outer.scoverage)
+  }
+}
diff --git a/contrib/scoverage/src/ScoverageReportWorker.scala b/contrib/scoverage/src/ScoverageReportWorker.scala
new file mode 100644
index 00000000..1aaa31ad
--- /dev/null
+++ b/contrib/scoverage/src/ScoverageReportWorker.scala
@@ -0,0 +1,39 @@
+package mill.contrib.scoverage
+
+import mill.{Agg, T}
+import mill.api.{ClassLoader, Ctx, Result}
+import mill.define.{Discover, ExternalModule, Worker}
+import mill.eval.PathRef
+
+class ScoverageReportWorker {
+  private var scoverageInstanceCache = Option.empty[(Long, api.ScoverageReportWorkerApi)]
+
+  def bridge(classpath: Agg[os.Path])
+                    (implicit ctx: Ctx) = {
+    val classloaderSig =
+      classpath.map(p => p.toString().hashCode + os.mtime(p)).sum
+    scoverageInstanceCache match {
+      case Some((sig, bridge)) if sig == classloaderSig => bridge
+      case _ =>
+        val toolsClassPath = classpath.map(_.toIO.toURI.toURL).toVector
+        ctx.log.debug("Loading classes from\n"+toolsClassPath.mkString("\n"))
+        val cl = ClassLoader.create(
+          toolsClassPath,
+          getClass.getClassLoader
+        )
+        val bridge = cl
+          .loadClass("mill.contrib.scoverage.worker.ScoverageReportWorkerImpl")
+          .getDeclaredConstructor()
+          .newInstance()
+          .asInstanceOf[api.ScoverageReportWorkerApi]
+        scoverageInstanceCache = Some((classloaderSig, bridge))
+        bridge
+    }
+  }
+}
+
+object ScoverageReportWorkerApi extends ExternalModule {
+
+  def scoverageReportWorker = T.worker { new ScoverageReportWorker() }
+  lazy val millDiscover = Discover[this.type]
+}
diff --git a/contrib/scoverage/test/resources/hello-world/core/src/Greet.scala b/contrib/scoverage/test/resources/hello-world/core/src/Greet.scala
new file mode 100644
index 00000000..608becc9
--- /dev/null
+++ b/contrib/scoverage/test/resources/hello-world/core/src/Greet.scala
@@ -0,0 +1,6 @@
+object Greet {
+  def greet(name: String, prefix: Option[String]): String = prefix match {
+    case Some(p) => s"Hello, ${p} ${name}!"
+    case None => s"Hello, ${name}!"
+  }
+}
diff --git a/contrib/scoverage/test/src/HelloWorldTests.scala b/contrib/scoverage/test/src/HelloWorldTests.scala
new file mode 100644
index 00000000..98a4201c
--- /dev/null
+++ b/contrib/scoverage/test/src/HelloWorldTests.scala
@@ -0,0 +1,107 @@
+package mill.contrib.scoverage
+
+import mill._
+import mill.api.Result
+import mill.scalalib._
+import mill.util.{TestEvaluator, TestUtil}
+import utest._
+import utest.framework.TestPath
+
+object HelloWorldTests extends utest.TestSuite {
+  val resourcePath = os.pwd / 'contrib / 'scoverage / 'test / 'resources / "hello-world"
+  trait HelloBase extends TestUtil.BaseModule {
+    def millSourcePath =  TestUtil.getSrcPathBase() / millOuterCtx.enclosing.split('.')
+  }
+
+  object HelloWorld extends HelloBase {
+    object core extends ScoverageModule {
+      def scalaVersion = "2.12.4"
+      def scoverageVersion = "1.3.1"
+
+      object test extends ScoverageTests {
+        override def ivyDeps = Agg(ivy"org.scalatest::scalatest:3.0.5")
+        def testFrameworks = Seq("org.scalatest.tools.Framework")
+      }
+    }
+  }
+
+  def workspaceTest[T](m: TestUtil.BaseModule, resourcePath: os.Path = resourcePath)
+                      (t: TestEvaluator => T)
+                      (implicit tp: TestPath): T = {
+    val eval = new TestEvaluator(m)
+    os.remove.all(m.millSourcePath)
+    os.remove.all(eval.outPath)
+    os.makeDir.all(m.millSourcePath / os.up)
+    os.copy(resourcePath, m.millSourcePath)
+    t(eval)
+  }
+
+  def tests: utest.Tests = utest.Tests {
+    "HelloWorld" - {
+      "core" - {
+        "scoverageVersion" - workspaceTest(HelloWorld) { eval =>
+          val Right((result, evalCount)) = eval.apply(HelloWorld.core.scoverageVersion)
+
+          assert(
+            result == "1.3.1",
+            evalCount > 0
+          )
+        }
+        "scoverage" - {
+          "ivyDeps" - workspaceTest(HelloWorld) { eval =>
+            val Right((result, evalCount)) =
+              eval.apply(HelloWorld.core.scoverage.ivyDeps)
+
+            assert(
+              result == Agg(ivy"org.scoverage::scalac-scoverage-runtime:1.3.1"),
+              evalCount > 0
+            )
+          }
+          "scalacPluginIvyDeps" - workspaceTest(HelloWorld) { eval =>
+            val Right((result, evalCount)) =
+              eval.apply(HelloWorld.core.scoverage.scalacPluginIvyDeps)
+
+            assert(
+              result == Agg(ivy"org.scoverage::scalac-scoverage-plugin:1.3.1"),
+              evalCount > 0
+            )
+          }
+          "dataDir" - workspaceTest(HelloWorld) { eval =>
+            val Right((result, evalCount)) = eval.apply(HelloWorld.core.scoverage.dataDir)
+
+            assert(
+              result.toString.endsWith("mill/target/workspace/mill/contrib/scoverage/HelloWorldTests/eval/HelloWorld/core/scoverage/dataDir/core/scoverage/data"),
+              evalCount > 0
+            )
+          }
+        }
+        "test" - {
+          "upstreamAssemblyClasspath" - workspaceTest(HelloWorld) { eval =>
+            val Right((result, evalCount)) = eval.apply(HelloWorld.core.scoverage.upstreamAssemblyClasspath)
+
+            assert(
+              result.map(_.toString).exists(_.contains("scalac-scoverage-runtime")),
+              evalCount > 0
+            )
+          }
+          "compileClasspath" - workspaceTest(HelloWorld) { eval =>
+            val Right((result, evalCount)) = eval.apply(HelloWorld.core.scoverage.compileClasspath)
+
+            assert(
+              result.map(_.toString).exists(_.contains("scalac-scoverage-runtime")),
+              evalCount > 0
+            )
+          }
+          "runClasspath" - TestUtil.disableInJava9OrAbove(workspaceTest(HelloWorld) { eval =>
+            val Right((result, evalCount)) = eval.apply(HelloWorld.core.scoverage.runClasspath)
+
+            assert(
+              result.map(_.toString).exists(_.contains("scalac-scoverage-runtime")),
+              evalCount > 0
+            )
+          })
+        }
+      }
+    }
+  }
+}
diff --git a/contrib/scoverage/worker/1.3.1/src/ScoverageReportWorkerImpl.scala b/contrib/scoverage/worker/1.3.1/src/ScoverageReportWorkerImpl.scala
new file mode 100644
index 00000000..44f506f7
--- /dev/null
+++ b/contrib/scoverage/worker/1.3.1/src/ScoverageReportWorkerImpl.scala
@@ -0,0 +1,21 @@
+package mill.contrib.scoverage.worker
+
+import mill.contrib.scoverage.api.ScoverageReportWorkerApi
+import mill.eval.PathRef
+import _root_.scoverage.Serializer.{ coverageFile, deserialize }
+import _root_.scoverage.IOUtils.{ findMeasurementFiles, invoked }
+import _root_.scoverage.report.ScoverageHtmlWriter
+
+class ScoverageReportWorkerImpl extends ScoverageReportWorkerApi {
+  def htmlReport(sources: Seq[PathRef], dataDir: String, selfDir: String) = {
+    val coverageFileObj = coverageFile(dataDir)
+    val coverage = deserialize(coverageFileObj)
+    coverage(invoked(findMeasurementFiles(dataDir)))
+    val Seq(PathRef(sourceFolderPath, _, _)) = sources
+    val sourceFolders = Seq(sourceFolderPath.toIO)
+    val htmlFolder = new java.io.File(s"${selfDir}/htmlReport")
+    htmlFolder.mkdir()
+    new ScoverageHtmlWriter(sourceFolders, htmlFolder, None)
+      .write(coverage)
+  }
+}
diff --git a/docs/pages/9 - Contrib Modules.md b/docs/pages/9 - Contrib Modules.md
index eca61be3..c3d4751a 100644
--- a/docs/pages/9 - Contrib Modules.md
+++ b/docs/pages/9 - Contrib Modules.md
@@ -543,6 +543,49 @@ object example extends ScalaPBModule {
 }
 ```
 
+
+## Scoverage
+
+This module allows you to generate code coverage reports for Scala projects with
+[Scoverage](https://github.com/scoverage) via the
+[scalac-scoverage-plugin](https://github.com/scoverage/scalac-scoverage-plugin).
+
+To declare a module for which you want to generate coverage reports you can
+extends the `mill.contrib.scoverage.ScoverageModule` trait when defining your
+module. Additionally, you must define a submodule that extends the
+`ScoverageTests` trait that belongs to your instance of `ScoverageModule`.
+
+```scala
+// You have to replace VERSION
+import $ivy.`com.lihaoyi::mill-contrib-buildinfo:VERSION`
+import mill.contrib.scoverage.ScoverageModule
+
+object foo extends ScoverageModule  {
+  def scalaVersion = "2.11.8"
+  def scoverageVersion = "1.3.1"
+
+  object test extends ScoverageTests {
+    def ivyDeps = Agg(ivy"org.scalatest::scalatest:3.0.5")
+    def testFrameworks = Seq("org.scalatest.tools.Framework")
+  }
+}
+```
+
+In addition to the normal tasks available to your Scala module, Scoverage
+modules introduce a few new tasks and changes the behavior of an existing one.
+
+```
+mill foo.scoverage.compile      # compiles your module with test instrumentation
+                                # (you don't have to run this manually, running the test task will force its invocation)
+
+mill foo.test                   # tests your project and collects metrics on code coverage
+mill foo.scoverage.htmlReport   # uses the metrics collected by a previous test run to generate a coverage report in html format
+```
+
+The measurement data is available at `out/foo/scoverage/data/`,
+and the html report is saved in `out/foo/scoverage/htmlReport/`.
+
+
 ## TestNG
 
 Provides support for [TestNG](https://testng.org/doc/index.html).
@@ -722,4 +765,3 @@ These imports will always be added to every template.  You don't need to list th
 
 ### Example
 There's an [example project](https://github.com/lihaoyi/cask/tree/master/example/twirl)
-