unix: add function for converting time.Time to Timespec

Add function TimeToTimespec for converting time.Time to Timespec.

Fixes golang/go#23078.

Change-Id: I685a4871d49131191f330b56b15bfd4f3131a9a5
Reviewed-on: https://go-review.googlesource.com/82919
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/timestruct.go

@@ -6,6 +6,8 @@
 
 package unix
 
+import "time"
+
 // TimespecToNsec converts a Timespec value into a number of
 // nanoseconds since the Unix epoch.
 func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }
@@ -22,6 +24,24 @@ func NsecToTimespec(nsec int64) Timespec {
 	return setTimespec(sec, nsec)
 }
 
+// TimeToTimespec converts t into a Timespec.
+// On some 32-bit systems the range of valid Timespec values are smaller
+// than that of time.Time values.  So if t is out of the valid range of
+// Timespec, it returns a zero Timespec and ERANGE.
+func TimeToTimespec(t time.Time) (Timespec, error) {
+	sec := t.Unix()
+	nsec := int64(t.Nanosecond())
+	ts := setTimespec(sec, nsec)
+
+	// Currently all targets have either int32 or int64 for Timespec.Sec.
+	// If there were a new target with floating point type for it, we have
+	// to consider the rounding error.
+	if int64(ts.Sec) != sec {
+		return Timespec{}, ERANGE
+	}
+	return ts, nil
+}
+
 // TimevalToNsec converts a Timeval value into a number of nanoseconds
 // since the Unix epoch.
 func TimevalToNsec(tv Timeval) int64 { return int64(tv.Sec)*1e9 + int64(tv.Usec)*1e3 }