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x/sys/unix: No clean way to call utimes(path, NULL) or friends #11830

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tv42 opened this issue Jul 23, 2015 · 6 comments
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x/sys/unix: No clean way to call utimes(path, NULL) or friends #11830

tv42 opened this issue Jul 23, 2015 · 6 comments
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@tv42
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tv42 commented Jul 23, 2015

The kernel differentiates between setting time to now vs setting it to some timestamp (that may be very very recent past). For example, FUSE sets a separate flag that signals this:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=17637cbaba592076c221dc045ca78422b4af6290

This is normally achieved by passing NULL as times to utimes(2) and friends.

The Go syscall wrappers all explicitly prevent this:

func Utimes(path string, tv []Timeval) (err error) {
    if len(tv) != 2 {
        return EINVAL
    }
    return utimes(path, (*[2]Timeval)(unsafe.Pointer(&tv[0])))
}

My use case: I wanted to write a FUSE unit test that behaves like touch, and actually causes that flag to be set. Measuring current time and passing it as argument is not good enough.
@ianlancetaylor ianlancetaylor added this to the Unreleased milestone Jul 23, 2015
@bradfitz
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Seems like an easy enough fix. Want to send a CL?

// Utimes blah blah. If tv is nil, blah. Otherwise tv must contain exactly 2 items and blah.
func Utimes(path string, tv []Timeval) (err error) {
    if tv == nil {
        return utimes(path, nil)
    }
    switch len(tv) != 2 {
        return EINVAL
    }
    return utimes(path, (*[2]Timeval)(unsafe.Pointer(&tv[0])))
}

@tv42
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tv42 commented Jul 23, 2015

Sorry, I'm not very thrilled about CLAs, and have not signed the Google CLA.

@ebfe
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ebfe commented Jul 23, 2015

As a workaround

const UTIME_NOW = (1<<30)-1
UtimesNano(path, []Timespec{{0, UTIME_NOW}, {0, UTIME_NOW}})

might work.

@gopherbot
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CL https://golang.org/cl/12648 mentions this issue.

@tv42
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tv42 commented Jul 24, 2015

This leaves a bunch of the sister calls, and platforms other than linux, still enforcing that logic.

$ git grep -B 1 'len\(.*\) != 2'|grep func|grep -v Pipe
unix/syscall_bsd.go-func Utimes(path string, tv []Timeval) (err error) {
unix/syscall_bsd.go-func Futimes(fd int, tv []Timeval) (err error) {
unix/syscall_linux.go-func UtimesNano(path string, ts []Timespec) (err error) {
unix/syscall_linux.go-func UtimesNanoAt(dirfd int, path string, ts []Timespec, flags int) (err error) {
unix/syscall_linux.go-func Futimesat(dirfd int, path string, tv []Timeval) (err error) {
unix/syscall_solaris.go-func UtimesNano(path string, ts []Timespec) (err error) {
windows/syscall_windows.go-func Utimes(path string, tv []Timeval) (err error) {
windows/syscall_windows.go-func UtimesNano(path string, ts []Timespec) (err error) {

@mdempsky mdempsky reopened this Jul 24, 2015
@gopherbot
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CL https://golang.org/cl/12690 mentions this issue.

@golang golang locked and limited conversation to collaborators Aug 5, 2016
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@bradfitz @mdempsky @tv42 @ebfe @ianlancetaylor @gopherbot