Encoder.ReadFrom() (added in v4) is buggy

[lizthegrey]

I appear to have truncated output (corresponding to partial input), and err.EOF when io.CopyN() followed by some followup Write() on v4, whereas v3 works fine and doesn't operate on only partial input.

when I buffer the output (which makes a copy for safety) in a bytes.Buffer() first before then using io.Copy() to send it all to lz4 at the same time, the output is not corrupted, because the copy is done with a single write and thus the Close() I manually trigger after is idempotent.

3d212bc (and all subsequent commits) fail, bb3370a passes my (non-minimal) testcase

[lizthegrey]

Verified my fixed branch works with go mod edit -replace

[lizthegrey]

Repro code:

package main

import (
	"bytes"
	"encoding/hex"
	"fmt"
	"io"
	"os"
	"testing"

	lzv3 "github.com/pierrec/lz4/v3"
	lzv4 "github.com/pierrec/lz4/v4"
)

type Generator func(io.WriteCloser) io.WriteCloser

func TestFoo(t *testing.T) {
	for i, gen := range []Generator{initV3, initV4} {
		fmt.Printf("\n===Generator %d, piecemeal===\n", i)
		dumper := hex.Dumper(os.Stdout)
		coder := gen(dumper)
		for _, v := range inStream {
			n, err := io.CopyN(coder, bytes.NewReader(v), int64(len(v)))
			if n != int64(len(v)) || err != nil {
				fmt.Printf("Failed to write %v\n", err)
			}
		}
		coder.Close()
		dumper.Close()

		fmt.Printf("\n===Generator %d, clean buffered===\n", i)
		var allAtOnce bytes.Buffer
		for _, v := range inStream {
			n, err := io.CopyN(&allAtOnce, bytes.NewReader(v), int64(len(v)))
			if n != int64(len(v)) || err != nil {
				fmt.Printf("Failed to write: %v\n", err)
				break
			}
		}

		dumper2 := hex.Dumper(os.Stdout)
		coder2 := gen(dumper2)
		io.Copy(coder2, bytes.NewReader(allAtOnce.Bytes()))
		coder2.Close()
		dumper2.Close()
	}
	fmt.Println()
}

func initV3(out io.WriteCloser) io.WriteCloser {
	lz4Writer := lzv3.NewWriter(out)
	return lz4Writer
}

func initV4(out io.WriteCloser) io.WriteCloser {
	lz4Writer := lzv4.NewWriter(out)
	return lz4Writer
}

var inStream = [][]byte{
	{5, 0, 4, 0, 0, 0, 116, 104, 105, 115, 2, 0, 0, 0, 105, 115, 1, 0, 0, 0, 97, 4, 0, 0, 0, 116, 101, 115, 116, 8, 0, 0, 0, 112, 114, 111, 98, 97, 98, 108, 121},
	{1, 0, 0, 0, 13, 0, 0, 0},
	{255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 15},
}