[musl] errno on writing to read-only files

["(GalaxyMaster)" <galaxy@openwall.com.au>]

Hello,

I am observing the following on musl and I am not sure that this is the way it
should be:
===
galaxy@archlinux:~/musl-tests $ cat fput-to-readonly.c 
#include <stdio.h>
#include <errno.h>

int main() {
	FILE *f;
	int i = 0;
	f = fopen("fput-to-readonly.c", "r");
	errno = 0;
	i = fputs("should not be written", f);
	printf("i = %d (should be negative [EOF = %d])\n", i, EOF);
	printf("errno = %d\n", errno);
	return 0;
}
galaxy@archlinux:~/musl-tests $ gcc -o fput-to-readonly fput-to-readonly.c 
galaxy@archlinux:~/musl-tests $ ./fput-to-readonly 
i = -1 (should be negative [EOF = -1])
errno = 0
galaxy@archlinux:~/musl-tests $
===

Logically, I would expect the errno variable to be set to something since there
was clearly an error and the data has not been written to the destination.

Glibc returns EBADF (9) in this case:
===
[galaxy@archlinux musl-tests]$ ./fput-to-readonly 
i = -1 (should be negative [EOF = -1])
errno = 9
[galaxy@archlinux musl-tests]$
===

Should not we do the same?  It kind of makes sense since the descriptor we are
asked to write to is read-only.  I think it would be just one line added to
src/stdio/__towrite.c, something like:
===
--- musl-b76f37fd5625d038141b52184956fb4b7838e9a5.orig/src/stdio/__towrite.c	2021-09-24 00:09:22.000000000 +0000
+++ musl-b76f37fd5625d038141b52184956fb4b7838e9a5/src/stdio/__towrite.c	2021-10-13 01:16:04.713069382 +0000
@@ -5,6 +5,7 @@ int __towrite(FILE *f)
 	f->mode |= f->mode-1;
 	if (f->flags & F_NOWR) {
 		f->flags |= F_ERR;
+		errno = EBADF;
 		return EOF;
 	}
 	/* Clear read buffer (easier than summoning nasal demons) */
===

-- 
(GM)