Multiple Calls to zstd.(*Encoder).Close() Write Duplicate CRC Checksums #1016
Description
When invoking .Close() multiple times on a zstd.(*Encoder) instance, the CRC checksum is written to the underlying stream multiple times. This behavior prevents reliable use of defer zstdWr.Close() in conjunction with an explicit zstdWr.Close() mid-function, as it results in duplicate checksum entries.
Other io.WriteCloser implementations, such as Go's standard library gzip.(*Writer).Close(), prevent this issue by setting a closed flag internally, ensuring the Close() method is idempotent.
Steps to Reproduce:
package main
import (
"bytes"
"crypto/rand"
"github.com/klauspost/compress/zstd"
)
func main() {
buf := bytes.NewBuffer(nil)
encoder, _ := zstd.NewWriter(buf)
b := make([]byte, 131072)
rand.Read(b)
encoder.Write(b)
encoder.Close() // First close
encoder.Close() // Second close (shouldn't write checksum again)
b = buf.Bytes()
// Compare the last two 4-byte sequences in the buffer (which are the checksums)
c1 := b[len(b)-4:]
c2 := b[len(b)-8 : len(b)-4]
if string(c1) == string(c2) {
panic("Duplicate CRC checksums written")
}
}