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keys.go
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keys.go
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// Copyright (C) 2017 Google Inc.
//
// 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 keys
import "context"
// keyType is hidden type so nobody can use the key list directly
type keySetType int
// keySet is the hidden key used to store the key list on the context.
const keySet = keySetType(0)
// Get returns the list of potential keys to use.
func Get(ctx context.Context) []interface{} {
seen := map[interface{}]bool{}
result := make([]interface{}, 0, 10)
for link, _ := ctx.Value(keySet).(*Link); link != nil; link = link.Next {
if !seen[link.Value] {
seen[link.Value] = true
result = append(result, link.Value)
}
}
return result
}
// WithValue registers the key as well as adding the value to the context.
func WithValue(ctx context.Context, key interface{}, value interface{}) context.Context {
old, _ := ctx.Value(keySet).(*Link)
ctx = context.WithValue(ctx, key, value)
return context.WithValue(ctx, keySet, &Link{Value: key, Next: old})
}
// Clone copies values from one context to another.
// This is used to produce associated but detached contexts.
func Clone(ctx context.Context, from context.Context) context.Context {
for _, key := range Get(from) {
ctx = WithValue(ctx, key, from.Value(key))
}
return ctx
}