APFS in Mac OS X 10.13 supports nanosecond timestamps. os.Stat returns the full resolution timestamps, but os.Chtimes is unable to set them, since it doesn't call the right system call. This means it is impossible to "round trip" a file and preserve these details.

I have a change that implements this, by calling setattrlist in syscall_darwin.go. This works on at least Mac OS X 10.11.6 (it drops the resolution), and on Mac OS X 10.13 on APFS, which I will submit shortly.

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

go version go1.9.2 darwin/amd64
go version devel +018642d Wed Nov 1 14:35:30 2017 +0000 darwin/amd64

What did you do?

The following code on Mac OS X creates a temporary file, displays its ModTime, then sets it to a time in the past, and prints the new ModTime. Ideally, the full nanosecond resolution should be displayed and preserved in all cases.

package main

import (
	"fmt"
	"os"
	"runtime"
	"time"
)

func main() {
	fmt.Println("go version:", runtime.Version())

	// create a new file
	const filePath = "example"
	f, err := os.OpenFile(filePath, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0600)
	if err != nil {
		panic(err)
	}
	f.Close()

	stat, err := os.Stat(filePath)
	if err != nil {
		panic(err)
	}
	fmt.Println("mtime after creation:", stat.ModTime())

	// change the time with nanosecond precision
	t := time.Unix(1509480275, 123456789)
	fmt.Println("setting mtime to:", t)
	err = os.Chtimes(filePath, t, t)
	if err != nil {
		panic(err)
	}
	stat, err = os.Stat(filePath)
	if err != nil {
		panic(err)
	}
	fmt.Println("mtime after set:", stat.ModTime())
}

Output on Mac OS X 10.13

When the file is first created, it has nanosecond precision. After the call to Chtimes, the mtime is truncated to microseconds, since Darwin calls utimes which takes a struct timeval with microsecond precision:

go version: go1.9.2
mtime after creation: 2017-11-01 12:53:21.849659583 -0400 EDT
setting mtime to: 2017-10-31 16:04:35.123456789 -0400 EDT
mtime after set: 2017-10-31 16:04:35.123457 -0400 EDT

The fixed version calls setattrlist which takes a struct timespec with nanosecond resolution:

go version: devel +018642d Wed Nov 1 14:35:30 2017 +0000
mtime after creation: 2017-11-01 12:53:27.388937366 -0400 EDT
setting mtime to: 2017-10-31 16:04:35.123456789 -0400 EDT
mtime after set: 2017-10-31 16:04:35.123456789 -0400 EDT

Output on Mac OS X 10.11

With or without the case is the same: HFS only has seconds resolution, so the timestamps are truncated.

go version: go1.9.2
mtime after creation: 2017-11-01 12:03:12 -0400 EDT
setting mtime to: 2017-10-31 16:04:35.123456789 -0400 EDT
mtime after set: 2017-10-31 16:04:35 -0400 EDT

go version: devel +018642d Wed Nov 1 14:35:30 2017 +0000
mtime after creation: 2017-11-01 12:07:38 -0400 EDT
setting mtime to: 2017-10-31 16:04:35.123456789 -0400 EDT
mtime after set: 2017-10-31 16:04:35 -0400 EDT