miner: refactor update loop to remove race condition

The go routine handling the downloader events handling
vars in parallel with the parent routine, causing a
race condition.

This change, though ugly, remove the condition while
still allowing the downloader event subscription to be
closed when the miner has no further use for it (ie DoneEvent).

miner/miner.go

@@ -91,7 +91,7 @@ func (miner *Miner) update() {
 	}()
 
 	miningRequested := false
-	canStart := true
+	isDownloading := false
 
 	// handleDownloaderEvents handles the downloader events. Please be aware that this is a one-shot type of logic.
 	// As soon as `Done` has been broadcast subsequent downloader events are not handled, and the event subscription is closed.
@@ -103,20 +103,20 @@ func (miner *Miner) update() {
 		case downloader.StartEvent:
 			wasMining := miner.Mining()
 			miner.worker.stop()
-			canStart = false
+			isDownloading = true
 			if wasMining {
 				// Resume mining after sync was finished
 				miningRequested = true
 				log.Info("Mining aborted due to sync")
 			}
 		case downloader.FailedEvent:
-			canStart = true
+			isDownloading = false
 			if miningRequested {
 				miner.SetEtherbase(miner.coinbase)
 				miner.worker.start()
 			}
 		case downloader.DoneEvent:
-			canStart = true
+			isDownloading = false
 			if miningRequested {
 				miner.SetEtherbase(miner.coinbase)
 				miner.worker.start()
@@ -125,20 +125,37 @@ func (miner *Miner) update() {
 		}
 	}
 
-	// Handle downloader events while subscription is open.
-	go func() {
-		for ev := range downloaderEvents.Chan() {
+	// withDownloaderState loop handles channeled events while the miner cares about the downloader events.
+	// This loop will be broken once the downloader emits a DoneEvent, signaling a successful sync.
+	// The following unnamed loop should be equivalent to this loop, but without the downloader event handling.
+withDownloaderState:
+	for {
+		select {
+		case ev := <-downloaderEvents.Chan():
 			if ev == nil {
-				return
+				break withDownloaderState
 			}
 			handleDownloaderEvents(ev)
+		case addr := <-miner.startCh:
+			if !isDownloading {
+				miner.SetEtherbase(addr)
+				miner.worker.start()
+			}
+			miningRequested = true
+		case <-miner.stopCh:
+			miningRequested = false
+			miner.worker.stop()
+		case <-miner.exitCh:
+			miner.worker.close()
+			return
 		}
-	}()
+	}
 
+	// Same loop as above, minus downloader event handling.
 	for {
 		select {
 		case addr := <-miner.startCh:
-			if canStart {
+			if !isDownloading {
 				miner.SetEtherbase(addr)
 				miner.worker.start()
 			}