What version of Go are you using (go version)?

$ go version
go version go1.15 linux/amd64

Does this issue reproduce with the latest release?

I'm able to repro this in since go1.15, including the latest go1.15.2.

What operating system and processor architecture are you using (go env)?

$ go env
GO111MODULE="off"

GOARCH="amd64"

GOBIN="/usr/local/google/home/yekuang/infra/go/bin"

GOCACHE="/usr/local/google/home/yekuang/infra/go/.cache"

GOENV="/usr/local/google/home/yekuang/.config/go/env"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="linux"

GOINSECURE=""

GOMODCACHE="/usr/local/google/home/yekuang/infra/go/.vendor/pkg/mod"

GONOPROXY=""

GONOSUMDB=""

GOOS="linux"

GOPATH="/usr/local/google/home/yekuang/infra/go/.vendor:/usr/local/google/home/yekuang/infra/go"

GOPRIVATE=""

GOPROXY="off"

GOROOT="/usr/local/google/home/yekuang/golang/go"

GOSUMDB="sum.golang.org"

GOTMPDIR=""

GOTOOLDIR="/usr/local/google/home/yekuang/golang/go/pkg/tool/linux_amd64"

GCCGO="gccgo"

AR="ar"

CC="gcc"

CXX="g++"

CGO_ENABLED="1"

GOMOD=""

CGO_CFLAGS="-g -O2"

CGO_CPPFLAGS=""

CGO_CXXFLAGS="-g -O2"

CGO_FFLAGS="-g -O2"

CGO_LDFLAGS="-g -O2"

PKG_CONFIG="pkg-config"

GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build699870689=/tmp/go-build -gno-record-gcc-switches"

What did you do?

We have compressed the same data using zlib.NewWriterLevel(writer, zlib.BestSpeed), and found that the output between go1.15 and go1.14.2 were different. This difference has eventually led to a data corruption when we uploaded the compressed file to GCS.

Here's the minimal reproducible example I have:

package main
import (
	"bufio"
	"compress/zlib"
	"flag"
	"fmt"
	"io"
	"os"
	"path/filepath"
)
func main() {
	var filename string
	flag.StringVar(&filename, "f", "", "filename")
	flag.Parse()
	fi, err := os.Open(filename)
	if err != nil {
		panic(err)
	}
	defer fi.Close()
	outname := filepath.Base(filename) + "-gzip"
	fo, err := os.Create(outname)
	if err != nil {
		panic(err)
	}
	defer fo.Close()
	fmt.Printf("%s -> %s\n", filename, outname)
	const outBufSize = 1024 * 1024
	foWr := bufio.NewWriterSize(fo, outBufSize)
	compressor, err := zlib.NewWriterLevel(foWr, zlib.BestSpeed)
	buf := make([]byte, outBufSize*3)
	if _, err := io.CopyBuffer(compressor, fi, buf); err != nil {
		compressor.Close()
		panic(err)
	}
	// compressor needs to be closed first to flush the rest of the data
	// into the bufio.Writer
	if err := compressor.Close(); err != nil {
		panic(err)
	}
	if err := foWr.Flush(); err != nil {
		panic(err)
	}
}

The input data was too big to be shared (2.5G). But I can share the data internally (FYI, my LDAP is yekuang@).

What did you expect to see?

No difference in the compressed data between go1.14.2 and go.1.15.

What did you see instead?

Compressed data were different.

Size of the compressed data:

• go1.14.2: 728571269
• go1.15: 728570333

I also did a cmp, and they started to differ at byte 363266597.

Let me know if you need more information, thanks!