core: prefetch next block state concurrently

core/blockchain_insert.go

@@ -80,10 +80,13 @@ func (st *insertStats) report(chain []*types.Block, index int, dirty common.Stor
 
 // insertIterator is a helper to assist during chain import.
 type insertIterator struct {
-	chain     types.Blocks
-	results   <-chan error
-	index     int
-	validator Validator
+	chain types.Blocks // Chain of blocks being iterated over
+
+	results <-chan error // Verification result sink from the consensus engine
+	errors  []error      // Header verification errors for the blocks
+
+	index     int       // Current offset of the iterator
+	validator Validator // Validator to run if verification succeeds
 }
 
 // newInsertIterator creates a new iterator based on the given blocks, which are
@@ -92,6 +95,7 @@ func newInsertIterator(chain types.Blocks, results <-chan error, validator Valid
 	return &insertIterator{
 		chain:     chain,
 		results:   results,
+		errors:    make([]error, 0, len(chain)),
 		index:     -1,
 		validator: validator,
 	}
@@ -100,17 +104,44 @@ func newInsertIterator(chain types.Blocks, results <-chan error, validator Valid
 // next returns the next block in the iterator, along with any potential validation
 // error for that block. When the end is reached, it will return (nil, nil).
 func (it *insertIterator) next() (*types.Block, error) {
+	// If we reached the end of the chain, abort
 	if it.index+1 >= len(it.chain) {
 		it.index = len(it.chain)
 		return nil, nil
 	}
+	// Advance the iterator and wait for verification result if not yet done
 	it.index++
-	if err := <-it.results; err != nil {
-		return it.chain[it.index], err
+	if len(it.errors) <= it.index {
+		it.errors = append(it.errors, <-it.results)
 	}
+	if it.errors[it.index] != nil {
+		return it.chain[it.index], it.errors[it.index]
+	}
+	// Block header valid, run body validation and return
 	return it.chain[it.index], it.validator.ValidateBody(it.chain[it.index])
 }
 
+// peek returns the next block in the iterator, along with any potential validation
+// error for that block, but does **not** advance the iterator.
+//
+// Both header and body validation errors (nil too) is cached into the iterator
+// to avoid duplicating work on the following next() call.
+func (it *insertIterator) peek() (*types.Block, error) {
+	// If we reached the end of the chain, abort
+	if it.index+1 >= len(it.chain) {
+		return nil, nil
+	}
+	// Wait for verification result if not yet done
+	if len(it.errors) <= it.index+1 {
+		it.errors = append(it.errors, <-it.results)
+	}
+	if it.errors[it.index+1] != nil {
+		return it.chain[it.index+1], it.errors[it.index+1]
+	}
+	// Block header valid, ignore body validation since we don't have a parent anyway
+	return it.chain[it.index+1], nil
+}
+
 // previous returns the previous header that was being processed, or nil.
 func (it *insertIterator) previous() *types.Header {
 	if it.index < 1 {