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