Created
December 9, 2014 21:51
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CancellableCopy wants to stop an in-progress io.Copy, but suffers from a race
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package main | |
import ( | |
"bytes" | |
"errors" | |
"io" | |
"io/ioutil" | |
"os" | |
) | |
func CancellableCopy(r io.ReadCloser, cancelChan chan struct{}) error { | |
defer r.Close() | |
completeChan := make(chan error) | |
go func() { | |
completeChan <- thirdPartyConsumer(r) | |
}() | |
select { | |
case err := <-completeChan: | |
return err | |
case <-cancelChan: | |
return errors.New("canceled") | |
} | |
} | |
func main() { | |
filename := createTmpFile() | |
file, err := os.Open(filename) | |
panicOnError(err) | |
defer os.Remove(file.Name()) | |
cancel := make(chan struct{}) | |
close(cancel) | |
err = CancellableCopy(file, cancel) | |
} | |
func createTmpFile() string { | |
tmpFile, err := ioutil.TempFile("", "some-tmp-file") | |
panicOnError(err) | |
_, err = tmpFile.WriteString("this is a line of text\n") | |
panicOnError(err) | |
err = tmpFile.Close() | |
panicOnError(err) | |
return tmpFile.Name() | |
} | |
func thirdPartyConsumer(r io.Reader) error { | |
writer := new(bytes.Buffer) | |
_, err := io.Copy(writer, r) | |
return err | |
} | |
func panicOnError(err error) { | |
if err != nil { | |
panic(err.Error()) | |
} | |
} |
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here's the race: