Fix loss of precision for setting timestamps so roundtrip happens properly

chtimes.go

@@ -0,0 +1,14 @@
+// Default implementation of Chtimes
+
+// +build !linux
+
+package main
+
+import (
+	"os"
+	"time"
+)
+
+func Chtimes(name string, atime time.Time, mtime time.Time) error {
+	return os.Chtimes(name, atime, mtime)
+}

chtimes_linux.go

@@ -0,0 +1,78 @@
+// An implementation of Chtimes which preserves nanosecond precision under linux
+//
+// Should go in standard library, but here for now
+
+// +build linux
+
+package main
+
+import (
+	"os"
+	"syscall"
+	"time"
+	"unsafe"
+)
+
+// COPIED from syscall
+// byteSliceFromString returns a NUL-terminated slice of bytes
+// containing the text of s. If s contains a NUL byte at any
+// location, it returns (nil, EINVAL).
+func byteSliceFromString(s string) ([]byte, error) {
+	for i := 0; i < len(s); i++ {
+		if s[i] == 0 {
+			return nil, syscall.EINVAL
+		}
+	}
+	a := make([]byte, len(s)+1)
+	copy(a, s)
+	return a, nil
+}
+
+// COPIED from syscall
+// bytePtrFromString returns a pointer to a NUL-terminated array of
+// bytes containing the text of s. If s contains a NUL byte at any
+// location, it returns (nil, EINVAL).
+func bytePtrFromString(s string) (*byte, error) {
+	a, err := byteSliceFromString(s)
+	if err != nil {
+		return nil, err
+	}
+	return &a[0], nil
+}
+
+// COPIED from syscall auto generated code modified from utimes
+func utimensat(dirfd int, path string, times *[2]syscall.Timespec) (err error) {
+	var _p0 *byte
+	_p0, err = bytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := syscall.Syscall(syscall.SYS_UTIMENSAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(times)))
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// FIXME needs defining properly!
+const AT_FDCWD = -100
+
+// COPIED from syscall and modified
+//sys	utimes(path string, times *[2]Timeval) (err error)
+func Utimensat(dirfd int, path string, ts []syscall.Timespec) (err error) {
+	if len(ts) != 2 {
+		return syscall.EINVAL
+	}
+	return utimensat(dirfd, path, (*[2]syscall.Timespec)(unsafe.Pointer(&ts[0])))
+}
+
+// COPIED from syscall and modified
+func Chtimes(name string, atime time.Time, mtime time.Time) error {
+	var utimes [2]syscall.Timespec
+	utimes[0] = syscall.NsecToTimespec(atime.UnixNano())
+	utimes[1] = syscall.NsecToTimespec(mtime.UnixNano())
+	if e := Utimensat(AT_FDCWD, name, utimes[0:]); e != nil {
+		return &os.PathError{"chtimes", name, e}
+	}
+	return nil
+}

notes.txt

@@ -48,40 +48,8 @@ Need to make directory objects otherwise can't upload an empty directory
 
 Make a fs.Errorf and count errors and log them at a different level
 
-Seem to be able to read stats to ns precision (using struct stat
+Submit chtimes_linux.go to standard library
-defined in asm-generic/stat.h)
+  * Can also fix Futime which isn't using the syscall under linux
+  * Also for all the *bsd so likely they could be fixed too
 
-https://www.kernel.org/doc/man-pages/online/pages/man2/stat.2.html
-
- but only write them to us precision.
-This is confirmed by looking at the source for Chtimes
-
-func Chtimes(name string, atime time.Time, mtime time.Time) error {
-	var utimes [2]syscall.Timeval
-	atime_ns := atime.Unix()*1e9 + int64(atime.Nanosecond())
-	mtime_ns := mtime.Unix()*1e9 + int64(mtime.Nanosecond())
-	utimes[0] = syscall.NsecToTimeval(atime_ns)
-	utimes[1] = syscall.NsecToTimeval(mtime_ns)
-	if e := syscall.Utimes(name, utimes[0:]); e != nil {
-		return &PathError{"chtimes", name, e}
-	}
-	return nil
-}
-
-And 
-
-func NsecToTimeval(nsec int64) (tv Timeval) {
-	nsec += 999 // round up to microsecond
-	tv.Sec = nsec / 1e9
-	tv.Usec = nsec % 1e9 / 1e3
-	return
-}
-
-Seems likely Go should be using
-  utimensat()  and  futimens()
-Instead of utimes() - the syscall is defined already
-SYS_UTIMENSAT
-Also for all the *bsd so likely they could be fixed too
-
-Can also fix Futime which isn't using the syscall under linux
 

swiftsync.go

@@ -77,7 +77,7 @@ func (fs *FsObject) md5sum() (string, error) {
 
 // Sets the modification time of the local fs object
 func (fs *FsObject) SetModTime(modTime time.Time) {
-	err := os.Chtimes(fs.path, modTime, modTime)
+	err := Chtimes(fs.path, modTime, modTime)
 	if err != nil {
 		fs.Debugf("Failed to set mtime on file: %s", err)
 	}