os: OpenFile always truncates (ignores flags)

[aytekinar]

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

$ go version
go version go1.18.3 linux/amd64

Does this issue reproduce with the latest release?

Yes.

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

My host OS is Windows 11 Enterprise.

I am using Go inside a (Docker-based) development container (based on the image golang:1.18-bullseye) under Visual Studio Code.

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18.3"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/workspace/go.mod"
GOWORK=""
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-build2210480654=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I have used the following Go code:

// main.go
package main

import (
	"log"
	"os"
)

func overwriteFileWithGarbage(garbage string) (n int, err error) {
	file, err := os.OpenFile("/tmp/some-file.txt", os.O_CREATE|os.O_WRONLY, 0666)
	if err != nil {
		return
	}
	defer func() {
		if fileErr := file.Close(); err == nil {
			err = fileErr
		}
	}()
	n, err = file.WriteString(garbage)
	return
}

func main() {
	if n, err := overwriteFileWithGarbage("garbage"); err != nil {
		log.Fatalf("error occurred: %v\n", err)
	} else {
		log.Printf("%d bytes written\n", n)
	}
}

and run the following to reproduce the issue:

$ echo "line-1" >> /tmp/some-file.txt
$ echo "line-2" >> /tmp/some-file.txt
$ echo "line-3" >> /tmp/some-file.txt
$ cat /tmp/some-file.txt
line-1
line-2
line-3
$ go run main.go
2022/06/28 21:56:02 7 bytes written

What did you expect to see?

$ cat /tmp/some-file.txt 
garbage

What did you see instead?

$ cat /tmp/some-file.txt 
garbageline-2
line-3

[seankhliao]

looks normal, did your want O_TRUNC?

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions

[aytekinar]

Apologies for the noise! I think I should take a break, now.

Somehow, I have been reading/inferring O_TRUNC as being O_APPEND. Hence, leaving O_TRUNC outside, I was thinking/hoping to get the file created from scratch every time 🤦🏻 Totally my bad!