os: files not correctly closed in MacOS X

[FrankStorbeck]

used go version 1.8.3
go env shows:
GOARCH="amd64"
GOBIN="/Users/franks/Documents/Development/sandbox/franks/go/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/franks/Documents/Development/sandbox/franks/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/y1/6h37hs7h8xj9vn001s7c4_h00000gn/T/go-build852796593=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

the following code shows that after closing a file and then rewriting to that file results in a file still
holding the tail of the first write.

Code:

package main

import (
	"bufio"
	"io"
	"log"
	"os"
	"os/exec"
)

func doWrite(aContent string) error {
	f, err := os.OpenFile("content.txt", os.O_WRONLY|os.O_CREATE, 0600)
	if err != nil {
		return err
	}
	defer f.Close()

	_, err = f.WriteString(aContent + "\n")
	return err
}

func doRead() (string, error) {
	f, err := os.Open("content.txt")
	if err != nil {
		return "", err
	}
	defer f.Close()

	buf := bufio.NewReader(f)
	content := ""
	doRead := true
	for doRead {
		line, err := buf.ReadString('\n')
		if err != nil {
			if err != io.EOF {
				return "", err
			}
			doRead = false
		}
		content = content + line
	}

	return content, nil
}

func main() {
	// show go version first:
	cmd := exec.Command("go", "version")
	version, err := cmd.CombinedOutput()
	if err != nil {
		log.Fatalf("%s", err)
	}
	log.Printf("%s", string(version))

	// first write a long string to "content.txt"
	if err = doWrite("abcdefghijklmnopqrstuvwxyz"); err != nil {
		log.Fatalf("doWrite error (1): %s", err)
	}

	// read and show the contents:
	content, err := doRead()
	if err != nil {
		log.Fatalf("doRead error (1): %s", err)
	}
	log.Printf("doRead content (1): %q", content)

	// now write a short string again to "content.txt"
	if err = doWrite("abc"); err != nil {
		log.Fatalf("doWrite error (2): %s", err)
	}

	// read and show the contents:
	content, err = doRead()
	if err != nil {
		log.Fatalf("doRead error (2): %s", err)
	}
	log.Printf("doRead content (2): %q", content)
}

the program generates:

2017/06/27 17:50:26 go version go1.8.3 darwin/amd64
2017/06/27 17:50:26 doRead content (1): "abcdefghijklmnopqrstuvwxyz\n"
2017/06/27 17:50:26 doRead content (2): "abc\nefghijklmnopqrstuvwxyz\n"

I Expected:

2017/06/27 17:50:26 go version go1.8.3 darwin/amd64
2017/06/27 17:50:26 doRead content (1): "abcdefghijklmnopqrstuvwxyz\n"
2017/06/27 17:50:26 doRead content (2): "abc\n"
"abc\n"

but the program shows

"abc\nefghijklmnopqrstuvwxyz\n"

[bradfitz]

Closing a file descriptor doesn't delete the file.

And os.O_WRONLY|os.O_CREATE doesn't mean to replace the file. See https://golang.org/pkg/os/#pkg-constants or http://pubs.opengroup.org/onlinepubs/009695399/functions/open.html for more details. (Go's os.OpenFile is a thin wrapper around the operating system's functionality)

To delete the file, use os.Remove.

For questions about Go, see https://golang.org/wiki/Questions.