Fix race condition on Windows by locking measurement file writes

src/main/scala/scoverage/Invoker.scala

@@ -5,10 +5,22 @@ import java.io.FileWriter
 /** @author Stephen Samuel */
 object Invoker {
 
+  /**
+   * We record that the given id has been invoked by appending its id to the coverage
+   * data file.
+   *
+   * This will happen concurrently on as many threads as the application is using,
+   * so this method is synchronized.
+   *
+   * This method is not thread-safe if the threads are in different JVMs. You may not
+   * use `scoverage` on multiple processes in parallel without risking corruption
+   * of the measurement file.
+   */
   def invoked(id: Int, path: String) = {
-    val writer = new FileWriter(path, true)
-    writer.append(id.toString)
-    writer.append(';')
-    writer.close()
+    this.synchronized {
+      val writer = new FileWriter(path, true)
+      writer.write(id.toString + ';')
+      writer.close()
+    }
   }
 }

src/test/scala/scoverage/InvokerConcurrencyTest.scala

@@ -0,0 +1,47 @@
+package scoverage
+
+import org.scalatest.FunSuite
+import org.scalatest.BeforeAndAfter
+import org.scalatest.OneInstancePerTest
+import java.io.File
+import scala.concurrent._
+import scala.concurrent.duration._
+import ExecutionContext.Implicits.global
+import scala.collection.breakOut
+
+/**
+ * Verify that [[Invoker.invoked()]] is thread-safe
+ */
+class InvokerConcurrencyTest extends FunSuite with BeforeAndAfter {
+
+  val measurementFile = new File("invoker-test.measurement")
+
+  before {
+    measurementFile.delete()
+    measurementFile.createNewFile()
+  }
+
+  test("calling Invoker.invoked on multiple threads does not corrupt the measurement file") {
+
+    val testIds: Set[Int] = (1 to 1000).toSet
+
+    // Create 1k "invoked" calls on the common thread pool, to stress test
+    // the method
+    val futures: List[Future[Unit]] = testIds.map { i: Int =>
+      future {
+        Invoker.invoked(i, measurementFile.toString)
+      }
+    }(breakOut)
+
+    futures.foreach(Await.result(_, 1.second))
+
+    // Now verify that the measurement file is not corrupted by loading it
+    val idsFromFile = IOUtils.invoked(measurementFile).toSet
+
+    idsFromFile === testIds
+  }
+
+  after {
+    measurementFile.delete()
+  }
+}