Scoverage does not close source files after reading

[lefou]

I have the issue that our mill test suite fails in subsequent scoverage plugin tests on Windows. Between each run we clean the complete work directory, but we don't drop the JVM which runs the scalac-scoverage-plugin. As a result we can't clean our workdir because the source files, which were processed are still locked. Windows locks files as long as one process keeps it open. So my suspicion is, that somewhere in scoverage, the Scala source files were opened but not properly closed.
We get this message:

java.nio.file.FileSystemException: D:\a\mill\mill\target\worksources\mill\contrib\scoverage\HelloWorldTests#HelloWorld\core\src\Greet.scala: The process cannot access the file because it is being used by another process.

    sun.nio.fs.WindowsException.translateToIOException(WindowsException.java:86)
    sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:97)
    sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:102)
    sun.nio.fs.WindowsFileSystemProvider.implDelete(WindowsFileSystemProvider.java:269)
    sun.nio.fs.AbstractFileSystemProvider.delete(AbstractFileSystemProvider.java:103)
    java.nio.file.Files.delete(Files.java:1126)
    os.remove$.apply(FileOps.scala:285)
    os.remove$.apply(FileOps.scala:284)
    geny.Generator.$anonfun$foreach$1(Generator.scala:50)
    geny.Generator$Mapped.$anonfun$generate$4(Generator.scala:283)
    os.walk$stream$$anon$2$$anon$3.visitFile(ListOps.scala:249)
    os.walk$stream$$anon$2$$anon$3.visitFile(ListOps.scala:230)
    java.nio.file.Files.walkFileTree(Files.java:2670)
    os.walk$stream$$anon$2.generate(ListOps.scala:230)
    geny.Generator$Mapped.generate(Generator.scala:283)
    geny.Generator.foreach(Generator.scala:49)
    geny.Generator.foreach$(Generator.scala:49)
    geny.Generator$Mapped.foreach(Generator.scala:281)
    os.remove$all$.apply(FileOps.scala:294)
    mill.contrib.scoverage.HelloWorldTests.workspaceTest(HelloWorldTests.scala:67)
    mill.contrib.scoverage.HelloWorldTests.workspaceTest$(HelloWorldTests.scala:62)
    mill.contrib.scoverage.HelloWorldTests_2_12$.workspaceTest(HelloWorldTests.scala:191)
    mill.contrib.scoverage.HelloWorldTests.$anonfun$tests$30(HelloWorldTests.scala:74)

Of course the stack trace belongs to the point, where we try to delete the file. But as it seems to be already opened (and locked) it fails.

This can't be reproduced on non-Windows machines

After a chat on Gitter @ckipp01 probably located the suspected issue at:

scalac-scoverage-plugin/scalac-scoverage-plugin/src/main/scala/scoverage/report/CodeGrid.scala

Lines 77 to 83 in 8528c10

	private def source(mfile: MeasuredFile): String = {
	val src = sourceEncoding match {
	case Some(enc) => Source.fromFile(mfile.source, enc)
	case None => Source.fromFile(mfile.source)
	}
	src.mkString
	}

A naive fix could look like this:

@@ -79,7 +78,8 @@ class CodeGrid(mFile: MeasuredFile, sourceEncoding: Option[String]) {
       case Some(enc) => Source.fromFile(mfile.source, enc)
       case None      => Source.fromFile(mfile.source)
     }
-    src.mkString
+    try src.mkString
+    finally src.close()
   }

   private def spanStart(status: StatementStatus): String =