Musl libc strftime for format strings ending in %

In glibc if the format string ends in %, strftime and wcsftime will turn it into a %. In musl libc, if the format string ends in %, both wcsftime and strftime return 0 with errno 0 and leave random stuff in the buffer. There's no way for us to distinguish between this and "out of space" without looking at the format string. The following 7 character patch would fix musl to behave the same as glibc:

--- a/src/time/strftime.c
+++ b/src/time/strftime.c
@@ -225,7 +225,7 @@ size_t __strftime_l(char *restrict s, size_t n, const char
*restrict f, const st
 			s[l] = 0;
 			return l;
		}
-		if (*f != '%') {
+		if (*f != '%' || !f[1]) {
 			s[l++] = *f;
 			continue;
 		}

but it sounds like musl may be against applying this patch because it is their position that the behavior is undefined? See this thread:
https://www.openwall.com/lists/musl/2022/12/19/3

So in this case, we repeatedly get format_string returning 0 and end up here:
https://github.com/python/cpython/blob/main/Modules/timemodule.c?plain=1#L844
Then if HAVE_WCSFTIME we call PyUnicode_FromWideChar(*outbuf, 0) which notices we're making an empty string and returns here:
https://github.com/python/cpython/blob/main/Objects/unicodeobject.c?plain=1#L2004-L2005
On the other hand, if !HAVE_WCSFTIME we call PyUnicode_DecodeLocaleAndSize which checks if str[len] != '\0' and raises ValueError("Embedded null byte") here:
https://github.com/python/cpython/blob/main/Objects/unicodeobject.c?plain=1#L4004

So what to do? Well for one thing, it seems to me that we can tell whether format_time is trying to return an empty string by checking if the string ends in a null byte as it should e.g., *outbuf[buflen] == 0. This can allow us to be a bit more conservative in time_strftime1.

Linked PRs

• gh-127528

Modules/timemodule.c、特に time_strftime1 とその format_time の処理から始め、次に musl の src/time/strftime.c について説明されている動作と比較します。time.strftime を通じて末尾に % があるケースを再現し、リンクされている PR gh-127528 を確認します。プラットフォーム依存の結果が一貫して処理され、空でないバッファを空文字列として扱わないことが完了の条件です。