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