os: fix os.File's tell/0 method for windows (fix #24217) (#24218)

Author: kbkpbot

vlib/builtin/cfns.c.v

@@ -148,7 +148,7 @@ fn C.rename(old_filename &char, new_filename &char) int
 
 fn C.fgets(str &char, n int, stream &C.FILE) int
 
-fn C._telli64(handle int) isize
+fn C.fgetpos(&C.FILE, voidptr) int
 
 @[trusted]
 fn C.sigemptyset() int

vlib/os/file.c.v

@@ -861,13 +861,15 @@ pub fn (f &File) tell() !i64 {
 	if !f.is_opened {
 		return error_file_not_opened()
 	}
-	mut pos := isize(0)
+
+	mut pos := i64(0)
+	mut ret := 0
 	$if windows {
-		pos = C._telli64(f.fd)
+		ret = C.fgetpos(f.cfile, &pos)
 	} $else {
-		pos = C.ftell(f.cfile)
+		pos = i64(C.ftell(f.cfile))
 	}
-	if pos == -1 {
+	if ret == -1 || pos == -1 {
 		return error(posix_get_error_msg(C.errno))
 	}
 	return pos

vlib/os/file_test.v

@@ -357,6 +357,12 @@ fn test_tell() {
 		mut f := os.open_file(tfile, 'r')!
 		f.seek(-5, .end)!
 		pos := f.tell()!
+		f.seek(0, .start)!
+		c1 := f.tell()!
+		_ := f.read_bytes(8)
+		c2 := f.tell()!
+		assert c1 == 0
+		assert c2 == 8
 		f.close()
 		// dump(pos)
 		assert pos == size - 5