Attempt to fix race by removing request from book keeping before responding to Bazel

We were previously responding to Bazel about the request before removing
the request from book keeping. That leaves a window for Bazel to send us
another request with the same request ID before we've removed the
original request from the worker's book keeping.

Author: jjudd

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

@@ -69,6 +69,16 @@ abstract class WorkerMain[S](stdin: InputStream = System.in, stdout: PrintStream
           maybeExitCode: Option[Int],
           wasCancelled: Boolean = false,
         ): Unit = {
+          // Remove the request from our book keeping right before we respond to Bazel. If
+          // we respond to Bazel about the request before removing it,then there is a race:
+          // Bazel could make a request with the same requestId to this worker before the
+          // requestId is removed from the worker's book keeping.
+          //
+          // Ideally Bazel will not send a request to this worker with the same requestId
+          // as another request before we've responded to the original request. If that
+          // happens, then there's a race regardless of what we do.
+          activeRequests.remove(requestId)
+
           // Defined here so all writes to stdout are synchronized
           stdout.synchronized {
             val builder = WorkerProtocol.WorkResponse.newBuilder
@@ -88,8 +98,6 @@ abstract class WorkerMain[S](stdin: InputStream = System.in, stdout: PrintStream
               .build()
               .writeDelimitedTo(stdout)
           }
-
-          activeRequests.remove(requestId)
         }
 
         /**