Improve strftime error test

strftime/strftime.go

@@ -14,27 +14,33 @@ import (
 	"unsafe"
 )
 
+// Note: The use of errno below is not thread-safe.
+//
+// Even if we added a mutex, it would not be thread-safe
+// relative to other C stdlib calls that don't use that mutex.
+//
+// OTOH, I can't find a format string that C strftime thinks is illegal
+// to test this with, so maybe this is a non-issue
+
 // Strftime wraps the C Strftime function
 func Strftime(format string, t time.Time) (string, error) {
-	_, off := t.Zone()
-	tz := t.Location()
 	var (
-		secs    = t.Sub(time.Date(1970, 1, 1, 0, 0, 0, 0, tz)).Seconds() - float64(off)
-		tt      = C.time_t(secs)
-		tm      = C.struct_tm{}
-		cFormat = C.CString(format)
-		cOut    [256]C.char
+		_, offset = t.Zone()
+		tz        = t.Location()
+		secs      = t.Sub(time.Date(1970, 1, 1, 0, 0, 0, 0, tz)).Seconds() - float64(offset)
+		tt        = C.time_t(secs)
+		tm        = C.struct_tm{}
+		cFormat   = C.CString(format)
+		cOut      [256]C.char
 	)
 	defer C.free(unsafe.Pointer(cFormat)) // nolint: gas
 	C.localtime_r(&tt, &tm)
 	size := C.strftime(&cOut[0], C.size_t(len(cOut)), cFormat, &tm)
-	// This is not thread-safe. Even if we add a mutex, it is not thread-safe
-	// relative to other C stdlib calls that don't use it.
-	// OTOH, I can't find a format string that C strftime thinks is illegal,
-	// so maybe this is a non-issue
 	if size == 0 {
-		errno := C.read_errno()
-		return "", syscall.Errno(errno)
+		// If size == 0 there *might* be an error.
+		if errno := C.read_errno(); errno != 0 {
+			return "", syscall.Errno(errno)
+		}
 	}
 	return C.GoString(&cOut[0]), nil
 }

strftime/strftime_test.go

@@ -42,6 +42,7 @@ func TestStrftime(t *testing.T) {
 		{"%Y/%-m/%-d", "2006/1/2"},
 		{"%a, %b %d, %Y %z", "Mon, Jan 02, 2006 -0500"},
 		{"%a, %b %d, %Y %Z", "Mon, Jan 02, 2006 EST"},
+		{"", ""},
 	}
 	for _, test := range tests {
 		s, err := Strftime(test.format, rt)