Add Context as a mechanism to pass data between plugins.

pkg/scheduler/plugins/v1alpha1/context.go

+/*
+Copyright 2018 The Kubernetes 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 v1alpha1
+
+import (
+	"errors"
+	"sync"
+)
+
+const (
+	// NotFound is the not found error message.
+	NotFound = "not found"
+)
+
+// ContextData is a generic type for arbitrary data stored in PluginContext.
+type ContextData interface{}
+
+// ContextKey is the type of keys stored in PluginContext.
+type ContextKey string
+
+// PluginContext provides a mechanism for plugins to store and retrieve arbitrary data.
+// ContextData stored by one plugin can be read, altered, or deleted by another plugin.
+// PluginContext does not provide any data protection, as all plugins are assumed to be
+// trusted.
+type PluginContext struct {
+	Mx      sync.RWMutex
+	storage map[ContextKey]ContextData
+}
+
+// NewPluginContext initializes a new PluginContext and returns its pointer.
+func NewPluginContext() *PluginContext {
+	return &PluginContext{
+		storage: make(map[ContextKey]ContextData),
+	}
+}
+
+// Read retrieves data with the given "key" from PluginContext. If the key is not
+// present an error is returned.
+func (c *PluginContext) Read(key ContextKey) (ContextData, error) {
+	if v, ok := c.storage[key]; ok {
+		return v, nil
+	}
+	return nil, errors.New(NotFound)
+}
+
+// SyncRead is the thread safe version of Read(...).
+func (c *PluginContext) SyncRead(key ContextKey) (ContextData, error) {
+	c.Mx.RLock()
+	defer c.Mx.RUnlock()
+	return c.Read(key)
+}
+
+// Write stores the given "val" in PluginContext with the given "key".
+func (c *PluginContext) Write(key ContextKey, val ContextData) {
+	c.storage[key] = val
+}
+
+// SyncWrite is the thread safe version of Write(...).
+func (c *PluginContext) SyncWrite(key ContextKey, val ContextData) {
+	c.Mx.Lock()
+	defer c.Mx.Unlock()
+	c.Write(key, val)
+}
+
+// Delete deletes data with the given key from PluginContext.
+func (c *PluginContext) Delete(key ContextKey) {
+	delete(c.storage, key)
+}
+
+// SyncDelete is the thread safe version of Write(...).
+func (c *PluginContext) SyncDelete(key ContextKey) {
+	c.Mx.Lock()
+	defer c.Mx.Unlock()
+	c.Delete(key)
+}
+
+// Reset removes all the information in the PluginContext.
+func (c *PluginContext) Reset() {
+	c.storage = make(map[ContextKey]ContextData)
+}