Fix race condition. Also make sure to flush output stream before writing the worker response

There was a race condition where a task could be cancelled with the
worker output streams still set to null, which would cause a null
pointer exception in the worker.

We also weren't always flushing the print stream before responding to
Bazel about the work request. This fixes that.

Author: jjudd

src/main/scala/higherkindness/rules_scala/common/worker/WorkerMain.scala

@@ -117,12 +117,18 @@ abstract class WorkerMain[S](stdin: InputStream = System.in, stdout: PrintStream
             // close them after the async work in the Future is all done.
             // If we do something synchronous with Using, then there's a race condition where the
             // streams can get closed before the Future is completed.
-            var outStream: ByteArrayOutputStream = null
-            var out: PrintStream = null
+            var maybeOutStream: Option[ByteArrayOutputStream] = None
+            var maybeOut: Option[PrintStream] = None
+
+            def flushOut(): Unit = {
+              maybeOut.map(_.flush())
+            }
 
             val workTask = CancellableTask {
-              outStream = new ByteArrayOutputStream
-              out = new PrintStream(outStream)
+              val outStream = new ByteArrayOutputStream()
+              val out = new PrintStream(outStream)
+              maybeOutStream = Some(outStream)
+              maybeOut = Some(out)
               try {
                 work(ctx, args, out, sandboxDir, verbosity)
                 0
@@ -137,24 +143,25 @@ abstract class WorkerMain[S](stdin: InputStream = System.in, stdout: PrintStream
               .andThen {
                 // Work task succeeded or failed in an expected way
                 case Success(code) =>
-                  out.flush()
-                  writeResponse(requestId, Some(outStream), Some(code))
+                  flushOut()
+                  writeResponse(requestId, maybeOutStream, Some(code))
                   logVerbose(s"WorkResponse $requestId sent with code $code")
 
                 case Failure(e: ExecutionException) =>
                   e.getCause() match {
                     // Task successfully cancelled
                     case cancelError: InterruptedException =>
+                      flushOut()
                       writeResponse(requestId, None, None, wasCancelled = true)
                       logVerbose(
                         s"Cancellation WorkResponse sent for request id: $requestId in response to an" +
                           " InterruptedException",
                       )
                     // Work task threw a non-fatal error
                     case e =>
-                      e.printStackTrace(out)
-                      out.flush()
-                      writeResponse(requestId, Some(outStream), Some(-1))
+                      maybeOut.map(e.printStackTrace(_))
+                      flushOut()
+                      writeResponse(requestId, maybeOutStream, Some(-1))
                       logVerbose(
                         "Encountered an uncaught exception that was wrapped in an ExecutionException while" +
                           s" proccessing the Future for WorkRequest $requestId. This usually means a non-fatal" +
@@ -165,6 +172,7 @@ abstract class WorkerMain[S](stdin: InputStream = System.in, stdout: PrintStream
 
                 // Task successfully cancelled
                 case Failure(e: CancellationException) =>
+                  flushOut()
                   writeResponse(requestId, None, None, wasCancelled = true)
                   logVerbose(
                     s"Cancellation WorkResponse sent for request id: $requestId in response to a" +
@@ -173,15 +181,15 @@ abstract class WorkerMain[S](stdin: InputStream = System.in, stdout: PrintStream
 
                 // Work task threw an uncaught exception
                 case Failure(e) =>
-                  e.printStackTrace(out)
-                  out.flush()
-                  writeResponse(requestId, Some(outStream), Some(-1))
+                  maybeOut.map(e.printStackTrace(_))
+                  flushOut()
+                  writeResponse(requestId, maybeOutStream, Some(-1))
                   logVerbose(s"Uncaught exception in Future while proccessing WorkRequest $requestId:")
                   e.printStackTrace(System.err)
               }(scala.concurrent.ExecutionContext.global)
               .andThen { case _ =>
-                out.close()
-                outStream.close()
+                maybeOut.map(_.close())
+                maybeOutStream.map(_.close())
               }(scala.concurrent.ExecutionContext.global)
 
             // putIfAbsent will return a non-null value if there was already a value in the map