reverse_proxy: WIP refactor and support for FastCGI

usagepool.go

@@ -0,0 +1,86 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package caddy
+
+import (
+	"fmt"
+	"sync"
+	"sync/atomic"
+)
+
+// UsagePool is a thread-safe map that pools values
+// based on usage; a LoadOrStore operation increments
+// the usage, and a Delete decrements from the usage.
+// If the usage count reaches 0, the value will be
+// removed from the map. There is no way to overwrite
+// existing keys in the pool without first deleting
+// it as many times as it was stored. Deleting too
+// many times will panic.
+//
+// An empty UsagePool is NOT safe to use; always call
+// NewUsagePool() to make a new value.
+type UsagePool struct {
+	pool *sync.Map
+}
+
+// NewUsagePool returns a new usage pool.
+func NewUsagePool() *UsagePool {
+	return &UsagePool{pool: new(sync.Map)}
+}
+
+// Delete decrements the usage count for key and removes the
+// value from the underlying map if the usage is 0. It returns
+// true if the usage count reached 0 and the value was deleted.
+// It panics if the usage count drops below 0; always call
+// Delete precisely as many times as LoadOrStore.
+func (up *UsagePool) Delete(key interface{}) (deleted bool) {
+	usageVal, ok := up.pool.Load(key)
+	if !ok {
+		return false
+	}
+	upv := usageVal.(*usagePoolVal)
+	newUsage := atomic.AddInt32(&upv.usage, -1)
+	if newUsage == 0 {
+		up.pool.Delete(key)
+		return true
+	} else if newUsage < 0 {
+		panic(fmt.Sprintf("deleted more than stored: %#v (usage: %d)",
+			upv.value, upv.usage))
+	}
+	return false
+}
+
+// LoadOrStore puts val in the pool and returns false if key does
+// not already exist; otherwise if the key exists, it loads the
+// existing value, increments the usage for that value, and returns
+// the value along with true.
+func (up *UsagePool) LoadOrStore(key, val interface{}) (actual interface{}, loaded bool) {
+	usageVal := &usagePoolVal{
+		usage: 1,
+		value: val,
+	}
+	actual, loaded = up.pool.LoadOrStore(key, usageVal)
+	if loaded {
+		upv := actual.(*usagePoolVal)
+		actual = upv.value
+		atomic.AddInt32(&upv.usage, 1)
+	}
+	return
+}
+
+type usagePoolVal struct {
+	usage int32 // accessed atomically; must be 64-bit aligned for 32-bit systems
+	value interface{}
+}