[musl] getopt_long() can corrupt argv when an argument for a short option is missing

[Alexey Izbyshev <izbyshev@ispras.ru>]

POSIX requires getopt() to set optind to argc + 1 in case of a missing 
argument[1], and musl follows it. This bites getopt_long() (which reuses 
getopt()) in two ways:

* getopt_long() moves argv[optind - 1] (NULL) when permuting argv to 
make all options precede other arguments, essentially corrupting argv.

* even when permuting is not required, getopt_long() is both 
incompatible with glibc (which doesn't increment optind past NULL) and 
inconsistent with itself (for a long option with a missing argument, 
musl doesn't increment optind past NULL too).

Example of the wrong NULL shifting:

#include <getopt.h>
#include <stdio.h>

int main(int argc, char *argv[]) {
     for (int i = 0; i < 2; i++) {
         int r = getopt_long(argc, argv, "o:", NULL, NULL);
         printf("r: %d\n", r);
         printf("optind: %d\n", optind);
         for (int i = 0; i <= argc; i++)
             printf("%d: '%s'\n", i, argv[i]);
     }
}

With glibc:
$ ./a.out arg -o
./a.out: option requires an argument -- 'o'
r: 63
optind: 3
0: './a.out'
1: 'arg'
2: '-o'
3: '(null)'
r: -1
optind: 2
0: './a.out'
1: '-o'
2: 'arg'
3: '(null)'

(Note that glibc permutes argv *before* parsing then next option, and 
even before comparing optind and argc, so argv is still permuted on the 
second invocation.)

With musl:
$ ./a.out arg -o
./a.out: option requires an argument: o
r: 63
optind: 3
0: './a.out'
1: '-o'
2: '(null)'
3: 'arg'
r: -1
optind: 3
0: './a.out'
1: '-o'
2: '(null)'
3: 'arg'

Maybe we could just skip permuting and adjust optind if we detected a 
missing argument?

         resumed = optind;
         ret = __getopt_long_core(argc, argv, optstring, longopts, idx, 
longonly);
+       if (optind > argc)
+               return optind--, ret;
         if (resumed > skipped) {

On a subsequent invocation we won't permute, unlike glibc, but maybe 
this is a good thing, given that such permutation makes it look like 
there is no missing argument, essentially changing the command 
semantics.

Alexey

[1] 
https://pubs.opengroup.org/onlinepubs/9699919799/functions/getopt.html