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authorPéter Szilágyi <peterke@gmail.com>2015-06-15 20:18:04 +0800
committerPéter Szilágyi <peterke@gmail.com>2015-06-15 20:18:04 +0800
commitcf7c44a7f6e5422148e0be98186d5570ce3e0ac5 (patch)
tree85fd96f2137dbe70f0b1e3fc822792528db7b4fb
parent9c03c374e32541c905adb9a3b8783cd721117030 (diff)
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eth/downloader: detailed comment for the race corner case
-rw-r--r--eth/downloader/downloader.go19
1 files changed, 15 insertions, 4 deletions
diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index 7f8ef12ee..306c4fd2d 100644
--- a/eth/downloader/downloader.go
+++ b/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()
- }
+ 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