166 lines
5 KiB
Go
166 lines
5 KiB
Go
// Copyright 2018 The Go Cloud Development Kit Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Package constantvar provides a runtimevar implementation with Variables
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// that never change. Use New, NewBytes, or NewError to construct a
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// *runtimevar.Variable.
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//
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// # URLs
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//
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// For runtimevar.OpenVariable, constantvar registers for the scheme "constant".
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// For more details on the URL format, see URLOpener.
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// See https://gocloud.dev/concepts/urls/ for background information.
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//
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// # As
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//
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// constantvar does not support any types for As.
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package constantvar // import "gocloud.dev/runtimevar/constantvar"
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import (
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"context"
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"errors"
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"fmt"
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"net/url"
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"time"
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"gocloud.dev/gcerrors"
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"gocloud.dev/runtimevar"
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"gocloud.dev/runtimevar/driver"
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)
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func init() {
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runtimevar.DefaultURLMux().RegisterVariable(Scheme, &URLOpener{})
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}
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// Scheme is the URL scheme constantvar registers its URLOpener under on blob.DefaultMux.
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const Scheme = "constant"
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// URLOpener opens constantvar URLs like "constant://?val=foo&decoder=string".
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//
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// The host and path are ignored.
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//
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// The following URL parameters are supported:
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// - val: The value to use for the constant Variable. The bytes from val
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// are passed to NewBytes.
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// - err: The error to use for the constant Variable. A new error is created
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// using errors.New and passed to NewError.
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// - decoder: The decoder to use. Defaults to runtimevar.BytesDecoder.
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// See runtimevar.DecoderByName for supported values.
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//
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// If both "err" and "val" are provided, "val" is ignored.
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type URLOpener struct {
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// Decoder specifies the decoder to use if one is not specified in the URL.
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// Defaults to runtimevar.BytesDecoder.
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Decoder *runtimevar.Decoder
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}
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// OpenVariableURL opens the variable at the URL's path. See the package doc
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// for more details.
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func (o *URLOpener) OpenVariableURL(ctx context.Context, u *url.URL) (*runtimevar.Variable, error) {
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q := u.Query()
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val := q.Get("val")
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q.Del("val")
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errVal := q.Get("err")
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q.Del("err")
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decoderName := q.Get("decoder")
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q.Del("decoder")
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decoder, err := runtimevar.DecoderByName(ctx, decoderName, o.Decoder)
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if err != nil {
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return nil, fmt.Errorf("open variable %v: invalid decoder: %v", u, err)
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}
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for param := range q {
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return nil, fmt.Errorf("open variable %v: invalid query parameter %q", u, param)
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}
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if errVal != "" {
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return NewError(errors.New(errVal)), nil
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}
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return NewBytes([]byte(val), decoder), nil
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}
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var errNotExist = errors.New("variable does not exist")
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// New constructs a *runtimevar.Variable holding value.
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func New(value interface{}) *runtimevar.Variable {
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return runtimevar.New(&watcher{value: value, t: time.Now()})
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}
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// NewBytes uses decoder to decode b. If the decode succeeds, it constructs
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// a *runtimevar.Variable holding the decoded value. If the decode fails, it
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// constructs a runtimevar.Variable that always fails with the error.
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func NewBytes(b []byte, decoder *runtimevar.Decoder) *runtimevar.Variable {
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value, err := decoder.Decode(context.Background(), b)
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if err != nil {
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return NewError(err)
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}
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return New(value)
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}
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// NewError constructs a *runtimevar.Variable that always fails. Runtimevar
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// wraps errors returned by drivers, so the error returned
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// by runtimevar will not equal err.
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func NewError(err error) *runtimevar.Variable {
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return runtimevar.New(&watcher{err: err})
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}
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// watcher implements driver.Watcher and driver.State.
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type watcher struct {
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value interface{}
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err error
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t time.Time
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}
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// Value implements driver.State.Value.
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func (w *watcher) Value() (interface{}, error) {
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return w.value, w.err
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}
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// UpdateTime implements driver.State.UpdateTime.
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func (w *watcher) UpdateTime() time.Time {
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return w.t
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}
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// As implements driver.State.As.
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func (w *watcher) As(i interface{}) bool {
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return false
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}
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// WatchVariable implements driver.WatchVariable.
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func (w *watcher) WatchVariable(ctx context.Context, prev driver.State) (driver.State, time.Duration) {
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// The first time this is called, return the constant value.
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if prev == nil {
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return w, 0
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}
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// On subsequent calls, block forever as the value will never change.
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<-ctx.Done()
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w.err = ctx.Err()
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return w, 0
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}
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// Close implements driver.Close.
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func (*watcher) Close() error { return nil }
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// ErrorAs implements driver.ErrorAs.
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func (*watcher) ErrorAs(err error, i interface{}) bool { return false }
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// ErrorCode implements driver.ErrorCode
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func (*watcher) ErrorCode(err error) gcerrors.ErrorCode {
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if err == errNotExist {
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return gcerrors.NotFound
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}
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return gcerrors.Unknown
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}
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