mirror of
https://github.com/restic/restic.git
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259 lines
8.9 KiB
Go
259 lines
8.9 KiB
Go
// 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 bundler supports bundling (batching) of items. Bundling amortizes an
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// action with fixed costs over multiple items. For example, if an API provides
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// an RPC that accepts a list of items as input, but clients would prefer
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// adding items one at a time, then a Bundler can accept individual items from
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// the client and bundle many of them into a single RPC.
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//
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// This package is experimental and subject to change without notice.
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package bundler
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import (
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"errors"
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"reflect"
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"sync"
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"time"
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"golang.org/x/net/context"
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"golang.org/x/sync/semaphore"
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)
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const (
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DefaultDelayThreshold = time.Second
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DefaultBundleCountThreshold = 10
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DefaultBundleByteThreshold = 1e6 // 1M
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DefaultBufferedByteLimit = 1e9 // 1G
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)
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var (
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// ErrOverflow indicates that Bundler's stored bytes exceeds its BufferedByteLimit.
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ErrOverflow = errors.New("bundler reached buffered byte limit")
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// ErrOversizedItem indicates that an item's size exceeds the maximum bundle size.
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ErrOversizedItem = errors.New("item size exceeds bundle byte limit")
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)
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// A Bundler collects items added to it into a bundle until the bundle
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// exceeds a given size, then calls a user-provided function to handle the bundle.
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type Bundler struct {
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// Starting from the time that the first message is added to a bundle, once
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// this delay has passed, handle the bundle. The default is DefaultDelayThreshold.
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DelayThreshold time.Duration
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// Once a bundle has this many items, handle the bundle. Since only one
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// item at a time is added to a bundle, no bundle will exceed this
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// threshold, so it also serves as a limit. The default is
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// DefaultBundleCountThreshold.
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BundleCountThreshold int
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// Once the number of bytes in current bundle reaches this threshold, handle
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// the bundle. The default is DefaultBundleByteThreshold. This triggers handling,
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// but does not cap the total size of a bundle.
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BundleByteThreshold int
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// The maximum size of a bundle, in bytes. Zero means unlimited.
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BundleByteLimit int
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// The maximum number of bytes that the Bundler will keep in memory before
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// returning ErrOverflow. The default is DefaultBufferedByteLimit.
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BufferedByteLimit int
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handler func(interface{}) // called to handle a bundle
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itemSliceZero reflect.Value // nil (zero value) for slice of items
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flushTimer *time.Timer // implements DelayThreshold
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mu sync.Mutex
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sem *semaphore.Weighted // enforces BufferedByteLimit
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semOnce sync.Once
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curBundle bundle // incoming items added to this bundle
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handlingc <-chan struct{} // set to non-nil while a handler is running; closed when it returns
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}
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type bundle struct {
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items reflect.Value // slice of item type
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size int // size in bytes of all items
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}
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// NewBundler creates a new Bundler.
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//
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// itemExample is a value of the type that will be bundled. For example, if you
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// want to create bundles of *Entry, you could pass &Entry{} for itemExample.
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//
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// handler is a function that will be called on each bundle. If itemExample is
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// of type T, the argument to handler is of type []T. handler is always called
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// sequentially for each bundle, and never in parallel.
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//
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// Configure the Bundler by setting its thresholds and limits before calling
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// any of its methods.
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func NewBundler(itemExample interface{}, handler func(interface{})) *Bundler {
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b := &Bundler{
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DelayThreshold: DefaultDelayThreshold,
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BundleCountThreshold: DefaultBundleCountThreshold,
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BundleByteThreshold: DefaultBundleByteThreshold,
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BufferedByteLimit: DefaultBufferedByteLimit,
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handler: handler,
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itemSliceZero: reflect.Zero(reflect.SliceOf(reflect.TypeOf(itemExample))),
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}
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b.curBundle.items = b.itemSliceZero
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return b
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}
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func (b *Bundler) sema() *semaphore.Weighted {
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// Create the semaphore lazily, because the user may set BufferedByteLimit
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// after NewBundler.
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b.semOnce.Do(func() {
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b.sem = semaphore.NewWeighted(int64(b.BufferedByteLimit))
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})
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return b.sem
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}
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// Add adds item to the current bundle. It marks the bundle for handling and
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// starts a new one if any of the thresholds or limits are exceeded.
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//
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// If the item's size exceeds the maximum bundle size (Bundler.BundleByteLimit), then
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// the item can never be handled. Add returns ErrOversizedItem in this case.
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//
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// If adding the item would exceed the maximum memory allowed
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// (Bundler.BufferedByteLimit) or an AddWait call is blocked waiting for
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// memory, Add returns ErrOverflow.
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//
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// Add never blocks.
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func (b *Bundler) Add(item interface{}, size int) error {
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// If this item exceeds the maximum size of a bundle,
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// we can never send it.
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if b.BundleByteLimit > 0 && size > b.BundleByteLimit {
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return ErrOversizedItem
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}
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// If adding this item would exceed our allotted memory
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// footprint, we can't accept it.
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// (TryAcquire also returns false if anything is waiting on the semaphore,
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// so calls to Add and AddWait shouldn't be mixed.)
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if !b.sema().TryAcquire(int64(size)) {
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return ErrOverflow
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}
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b.add(item, size)
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return nil
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}
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// add adds item to the current bundle. It marks the bundle for handling and
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// starts a new one if any of the thresholds or limits are exceeded.
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func (b *Bundler) add(item interface{}, size int) {
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b.mu.Lock()
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defer b.mu.Unlock()
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// If adding this item to the current bundle would cause it to exceed the
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// maximum bundle size, close the current bundle and start a new one.
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if b.BundleByteLimit > 0 && b.curBundle.size+size > b.BundleByteLimit {
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b.startFlushLocked()
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}
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// Add the item.
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b.curBundle.items = reflect.Append(b.curBundle.items, reflect.ValueOf(item))
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b.curBundle.size += size
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// Start a timer to flush the item if one isn't already running.
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// startFlushLocked clears the timer and closes the bundle at the same time,
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// so we only allocate a new timer for the first item in each bundle.
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// (We could try to call Reset on the timer instead, but that would add a lot
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// of complexity to the code just to save one small allocation.)
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if b.flushTimer == nil {
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b.flushTimer = time.AfterFunc(b.DelayThreshold, b.Flush)
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}
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// If the current bundle equals the count threshold, close it.
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if b.curBundle.items.Len() == b.BundleCountThreshold {
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b.startFlushLocked()
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}
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// If the current bundle equals or exceeds the byte threshold, close it.
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if b.curBundle.size >= b.BundleByteThreshold {
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b.startFlushLocked()
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}
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}
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// AddWait adds item to the current bundle. It marks the bundle for handling and
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// starts a new one if any of the thresholds or limits are exceeded.
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//
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// If the item's size exceeds the maximum bundle size (Bundler.BundleByteLimit), then
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// the item can never be handled. AddWait returns ErrOversizedItem in this case.
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//
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// If adding the item would exceed the maximum memory allowed (Bundler.BufferedByteLimit),
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// AddWait blocks until space is available or ctx is done.
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//
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// Calls to Add and AddWait should not be mixed on the same Bundler.
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func (b *Bundler) AddWait(ctx context.Context, item interface{}, size int) error {
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// If this item exceeds the maximum size of a bundle,
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// we can never send it.
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if b.BundleByteLimit > 0 && size > b.BundleByteLimit {
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return ErrOversizedItem
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}
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// If adding this item would exceed our allotted memory footprint, block
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// until space is available. The semaphore is FIFO, so there will be no
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// starvation.
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if err := b.sema().Acquire(ctx, int64(size)); err != nil {
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return err
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}
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// Here, we've reserved space for item. Other goroutines can call AddWait
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// and even acquire space, but no one can take away our reservation
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// (assuming sem.Release is used correctly). So there is no race condition
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// resulting from locking the mutex after sem.Acquire returns.
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b.add(item, size)
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return nil
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}
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// Flush invokes the handler for all remaining items in the Bundler and waits
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// for it to return.
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func (b *Bundler) Flush() {
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b.mu.Lock()
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b.startFlushLocked()
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done := b.handlingc
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b.mu.Unlock()
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if done != nil {
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<-done
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}
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}
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func (b *Bundler) startFlushLocked() {
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if b.flushTimer != nil {
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b.flushTimer.Stop()
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b.flushTimer = nil
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}
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if b.curBundle.items.Len() == 0 {
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return
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}
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bun := b.curBundle
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b.curBundle = bundle{items: b.itemSliceZero}
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done := make(chan struct{})
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var running <-chan struct{}
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running, b.handlingc = b.handlingc, done
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go func() {
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defer func() {
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b.sem.Release(int64(bun.size))
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close(done)
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}()
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if running != nil {
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// Wait for our turn to call the handler.
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<-running
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}
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b.handler(bun.items.Interface())
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}()
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}
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