116 lines
3.3 KiB
Go
116 lines
3.3 KiB
Go
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// Copyright 2016 Google Inc. All Rights Reserved.
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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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// http://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 pubsub
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import (
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"sync"
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"golang.org/x/net/context"
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)
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// puller fetches messages from the server in a batch.
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type puller struct {
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ctx context.Context
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cancel context.CancelFunc
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// keepAlive takes ownership of the lifetime of the message identified
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// by ackID, ensuring that its ack deadline does not expire. It should
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// be called each time a new message is fetched from the server, even
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// if it is not yet returned from Next.
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keepAlive func(ackID string)
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// abandon should be called for each message which has previously been
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// passed to keepAlive, but will never be returned by Next.
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abandon func(ackID string)
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// fetch fetches a batch of messages from the server.
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fetch func() ([]*Message, error)
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mu sync.Mutex
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buf []*Message
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}
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// newPuller constructs a new puller.
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// batchSize is the maximum number of messages to fetch at once.
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// No more than batchSize messages will be outstanding at any time.
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func newPuller(s service, subName string, ctx context.Context, batchSize int32, keepAlive, abandon func(ackID string)) *puller {
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ctx, cancel := context.WithCancel(ctx)
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return &puller{
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cancel: cancel,
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keepAlive: keepAlive,
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abandon: abandon,
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ctx: ctx,
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fetch: func() ([]*Message, error) { return s.fetchMessages(ctx, subName, batchSize) },
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}
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}
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const maxPullAttempts = 2
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// Next returns the next message from the server, fetching a new batch if necessary.
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// keepAlive is called with the ackIDs of newly fetched messages.
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// If p.Ctx has already been cancelled before Next is called, no new messages
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// will be fetched.
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func (p *puller) Next() (*Message, error) {
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p.mu.Lock()
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defer p.mu.Unlock()
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// If ctx has been cancelled, return straight away (even if there are buffered messages available).
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select {
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case <-p.ctx.Done():
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return nil, p.ctx.Err()
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default:
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}
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for len(p.buf) == 0 {
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var buf []*Message
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var err error
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for i := 0; i < maxPullAttempts; i++ {
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// Once Stop has completed, all future calls to Next will immediately fail at this point.
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buf, err = p.fetch()
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if err == nil || err == context.Canceled || err == context.DeadlineExceeded {
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break
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}
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}
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if err != nil {
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return nil, err
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}
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for _, m := range buf {
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p.keepAlive(m.ackID)
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}
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p.buf = buf
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}
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m := p.buf[0]
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p.buf = p.buf[1:]
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return m, nil
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}
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// Stop aborts any pending calls to Next, and prevents any future ones from succeeding.
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// Stop also abandons any messages that have been pre-fetched.
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// Once Stop completes, no calls to Next will succeed.
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func (p *puller) Stop() {
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// Next may be executing in another goroutine. Cancel it, and then wait until it terminates.
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p.cancel()
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p.mu.Lock()
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defer p.mu.Unlock()
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for _, m := range p.buf {
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p.abandon(m.ackID)
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}
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p.buf = nil
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}
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