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133 lines
2.9 KiB
133 lines
2.9 KiB
// Package bgreader provides a io.Reader that can optionally buffer reads in the background.
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package bgreader
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import (
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"io"
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"sync"
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"github.com/jackc/pgx/v5/internal/iobufpool"
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)
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const (
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bgReaderStatusStopped = iota
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bgReaderStatusRunning
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bgReaderStatusStopping
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)
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// BGReader is an io.Reader that can optionally buffer reads in the background. It is safe for concurrent use.
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type BGReader struct {
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r io.Reader
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cond *sync.Cond
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bgReaderStatus int32
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readResults []readResult
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}
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type readResult struct {
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buf *[]byte
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err error
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}
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// Start starts the backgrounder reader. If the background reader is already running this is a no-op. The background
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// reader will stop automatically when the underlying reader returns an error.
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func (r *BGReader) Start() {
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r.cond.L.Lock()
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defer r.cond.L.Unlock()
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switch r.bgReaderStatus {
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case bgReaderStatusStopped:
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r.bgReaderStatus = bgReaderStatusRunning
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go r.bgRead()
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case bgReaderStatusRunning:
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// no-op
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case bgReaderStatusStopping:
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r.bgReaderStatus = bgReaderStatusRunning
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}
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}
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// Stop tells the background reader to stop after the in progress Read returns. It is safe to call Stop when the
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// background reader is not running.
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func (r *BGReader) Stop() {
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r.cond.L.Lock()
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defer r.cond.L.Unlock()
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switch r.bgReaderStatus {
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case bgReaderStatusStopped:
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// no-op
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case bgReaderStatusRunning:
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r.bgReaderStatus = bgReaderStatusStopping
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case bgReaderStatusStopping:
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// no-op
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}
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}
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func (r *BGReader) bgRead() {
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keepReading := true
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for keepReading {
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buf := iobufpool.Get(8192)
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n, err := r.r.Read(*buf)
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*buf = (*buf)[:n]
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r.cond.L.Lock()
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r.readResults = append(r.readResults, readResult{buf: buf, err: err})
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if r.bgReaderStatus == bgReaderStatusStopping || err != nil {
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r.bgReaderStatus = bgReaderStatusStopped
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keepReading = false
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}
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r.cond.L.Unlock()
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r.cond.Broadcast()
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}
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}
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// Read implements the io.Reader interface.
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func (r *BGReader) Read(p []byte) (int, error) {
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r.cond.L.Lock()
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defer r.cond.L.Unlock()
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if len(r.readResults) > 0 {
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return r.readFromReadResults(p)
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}
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// There are no unread background read results and the background reader is stopped.
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if r.bgReaderStatus == bgReaderStatusStopped {
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return r.r.Read(p)
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}
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// Wait for results from the background reader
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for len(r.readResults) == 0 {
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r.cond.Wait()
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}
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return r.readFromReadResults(p)
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}
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// readBackgroundResults reads a result previously read by the background reader. r.cond.L must be held.
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func (r *BGReader) readFromReadResults(p []byte) (int, error) {
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buf := r.readResults[0].buf
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var err error
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n := copy(p, *buf)
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if n == len(*buf) {
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err = r.readResults[0].err
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iobufpool.Put(buf)
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if len(r.readResults) == 1 {
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r.readResults = nil
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} else {
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r.readResults = r.readResults[1:]
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}
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} else {
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*buf = (*buf)[n:]
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r.readResults[0].buf = buf
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}
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return n, err
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}
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func New(r io.Reader) *BGReader {
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return &BGReader{
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r: r,
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cond: &sync.Cond{
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L: &sync.Mutex{},
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},
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}
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}
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