les: implement request distributor, fix blocking issues (#3660)

* les: implement request distributor, fix blocking issues
* core: moved header validation before chain mutex lock

les/execqueue.go

@@ -0,0 +1,71 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package les
+
+import (
+	"sync/atomic"
+)
+
+// ExecQueue implements a queue that executes function calls in a single thread,
+// in the same order as they have been queued.
+type execQueue struct {
+	chn                 chan func()
+	cnt, stop, capacity int32
+}
+
+// NewExecQueue creates a new execution queue.
+func newExecQueue(capacity int32) *execQueue {
+	q := &execQueue{
+		chn:      make(chan func(), capacity),
+		capacity: capacity,
+	}
+	go q.loop()
+	return q
+}
+
+func (q *execQueue) loop() {
+	for f := range q.chn {
+		atomic.AddInt32(&q.cnt, -1)
+		if atomic.LoadInt32(&q.stop) != 0 {
+			return
+		}
+		f()
+	}
+}
+
+// CanQueue returns true if more  function calls can be added to the execution queue.
+func (q *execQueue) canQueue() bool {
+	return atomic.LoadInt32(&q.stop) == 0 && atomic.LoadInt32(&q.cnt) < q.capacity
+}
+
+// Queue adds a function call to the execution queue. Returns true if successful.
+func (q *execQueue) queue(f func()) bool {
+	if atomic.LoadInt32(&q.stop) != 0 {
+		return false
+	}
+	if atomic.AddInt32(&q.cnt, 1) > q.capacity {
+		atomic.AddInt32(&q.cnt, -1)
+		return false
+	}
+	q.chn <- f
+	return true
+}
+
+// Stop stops the exec queue.
+func (q *execQueue) quit() {
+	atomic.StoreInt32(&q.stop, 1)
+}