eth/downloader: detailed comment for the race corner case

This commit is contained in:
Péter Szilágyi 2015-06-15 15:18:04 +03:00
parent 9c03c374e3
commit cf7c44a7f6

View File

@ -522,9 +522,7 @@ out:
peer.Promote() peer.Promote()
peer.SetIdle() peer.SetIdle()
glog.V(logger.Detail).Infof("%s: delivered %d blocks", peer, len(blockPack.blocks)) 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: case errInvalidChain:
// The hash chain is invalid (blocks are not ordered properly), abort // 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. // 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) { func (d *Downloader) process() (err error) {
// Make sure only one goroutine is ever allowed to process blocks at once // Make sure only one goroutine is ever allowed to process blocks at once
if !atomic.CompareAndSwapInt32(&d.processing, 0, 1) { 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 // Try to inset the blocks, drop the originating peer if there's an error
index, err := d.insertChain(raw) index, err := d.insertChain(raw)
if err != nil { 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.dropPeer(blocks[index].OriginPeer)
d.Cancel() d.Cancel()
return errCancelChainImport return errCancelChainImport