eth/downloader: detailed comment for the race corner case

eth/downloader/downloader.go

@@ -522,9 +522,7 @@ out:
 					peer.Promote()
 					peer.SetIdle()
 					glog.V(logger.Detail).Infof("%s: delivered %d blocks", peer, len(blockPack.blocks))
-					if atomic.LoadInt32(&d.processing) == 0 {
 					go d.process()
-					}
 
 				case errInvalidChain:
 					// The hash chain is invalid (blocks are not ordered properly), abort
@@ -701,6 +699,19 @@ func (d *Downloader) banBlocks(peerId string, head common.Hash) error {
 }
 
 // process takes blocks from the queue and tries to import them into the chain.
+//
+// The algorithmic flow is as follows:
+//  - The `processing` flag is swapped to 1 to ensure singleton access
+//  - The current `cancel` channel is retrieved to detect sync abortions
+//  - Blocks are iteratively taken from the cache and inserted into the chain
+//  - When the cache becomes empty, insertion stops
+//  - The `processing` flag is swapped back to 0
+//  - A post-exit check is made whether new blocks became available
+//     - This step is important: it handles a potential race condition between
+//       checking for no more work, and releasing the processing "mutex". In
+//       between these state changes, a block may have arrived, but a processing
+//       attempt denied, so we need to re-enter to ensure the block isn't left
+//       to idle in the cache.
 func (d *Downloader) process() (err error) {
 	// Make sure only one goroutine is ever allowed to process blocks at once
 	if !atomic.CompareAndSwapInt32(&d.processing, 0, 1) {
@@ -763,7 +774,7 @@ func (d *Downloader) process() (err error) {
 			// Try to inset the blocks, drop the originating peer if there's an error
 			index, err := d.insertChain(raw)
 			if err != nil {
-				glog.V(logger.Debug).Infoln("Block #%d import failed:", raw[index].NumberU64(), err)
+				glog.V(logger.Debug).Infof("Block #%d import failed: %v", raw[index].NumberU64(), err)
 				d.dropPeer(blocks[index].OriginPeer)
 				d.Cancel()
 				return errCancelChainImport