lib/model, lib/protocol: Handle request concurrency in model (#5216)

lib/model/bytesemaphore.go

@@ -0,0 +1,50 @@
+// Copyright (C) 2018 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package model
+
+import "sync"
+
+type byteSemaphore struct {
+	max       int
+	available int
+	mut       sync.Mutex
+	cond      *sync.Cond
+}
+
+func newByteSemaphore(max int) *byteSemaphore {
+	s := byteSemaphore{
+		max:       max,
+		available: max,
+	}
+	s.cond = sync.NewCond(&s.mut)
+	return &s
+}
+
+func (s *byteSemaphore) take(bytes int) {
+	if bytes > s.max {
+		bytes = s.max
+	}
+	s.mut.Lock()
+	for bytes > s.available {
+		s.cond.Wait()
+	}
+	s.available -= bytes
+	s.mut.Unlock()
+}
+
+func (s *byteSemaphore) give(bytes int) {
+	if bytes > s.max {
+		bytes = s.max
+	}
+	s.mut.Lock()
+	if s.available+bytes > s.max {
+		panic("bug: can never give more than max")
+	}
+	s.available += bytes
+	s.cond.Broadcast()
+	s.mut.Unlock()
+}