io.Copy(pgzip.Reader) returns io.EOF if stream already complete due to WriteTo implementation
cyphar opened this issue · comments
It turns out that if you have a pgzip.Reader
which has read to the end of the
stream, if you call io.Copy
on that stream you get io.EOF
-- which should
never happen and causes spurious errors on callers that check error values from
io.Copy
. I hit this when working on opencontainers/umoci#360.
This happens because (as an optimisation) io.Copy
will use the WriteTo
method of the reader (or ReadFrom
method of the writer) if they support that
method. And in this mode, io.Copy
simply returns whatever error the reader or
writer give it -- meaning it doesn't hide io.EOF
s returned from those
methods. In the normal Read
+Write
mode, io.Copy
does hide the error.
It seems as though this is at some level a stdlib bug, because this requirement
of io.WriterTo
and io.ReaderTo
implementations (don't return io.EOF
because io.Copy
can't handle it) is not spelled out anywhere in the
documentation. So either io.Copy
should handle this, or this requirment
should be documented. So I will open a parallel issue on the Go tracker for
this problem.
But for now, it seems that the WriteTo
implementation should avoid returning
io.EOF
. If the reader reaches an io.EOF
before it is expected, the error
should instead be io.ErrUnexpectedEOF
.
Here's a simple reproducer -- note that in this case, Read
is called until the end of the stream is reached and then WriteTo
is called. https://play.golang.org/p/touWFY2ATjv
package main
import (
"fmt"
"bytes"
"io"
"io/ioutil"
"github.com/klauspost/pgzip"
)
// echo hello | gzip -c | xxd -i
var gzipData = []byte{
0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03, 0xcb, 0x48,
0xcd, 0xc9, 0xc9, 0xe7, 0x02, 0x00, 0x20, 0x30, 0x3a, 0x36, 0x06, 0x00,
0x00, 0x00,
}
func main() {
buf := bytes.NewBuffer(gzipData)
rdr, err := pgzip.NewReader(buf)
if err != nil {
panic(err)
}
b, err := ioutil.ReadAll(rdr)
if err != nil {
panic(err)
}
fmt.Printf("read %q from stream\n", string(b))
n, err := io.Copy(ioutil.Discard, rdr)
fmt.Printf("io.Copy at end of stream: n=%v, err=%v\n", n, err)
}