libc: fix WRONLY/RDONLY test in fmemopen

[emaste]

Summary

PR: 281953

[emaste]

Add test

[fuz]

Code should account for O_RDWR case.
The earlier test if (!(flags & O_RDWR) && buf == NULL) in the same file fmemopen.c is also dubious and should be adjusted similarly.
Test should also check a read/write mode like "r+" or "w+".

lib/libc/stdio/fmemopen.c

141

What happens if fmemopen() is called with mode "r+" or "w+", yielding (flags & O_ACCMODE) == O_RDWR?

[khng]

lib/libc/stdio/fmemopen.c

141

oops, that's a nice catch.

[emaste]

lib/libc/stdio/fmemopen.c

141

Then both tests are false and we use fmemopen_read and fmemopen_write?

[khng]

lib/libc/stdio/fmemopen.c

141

yep, that sounds correct.

[emaste]

r+ and w+ are implicitly tested in other test cases; I'm not sure we need to duplicate that (but could add a comment).

[emaste]

add a comment, fix another case reported by @fuz (that worked by accident, as it turns out)

[emaste]

update comment

[fuz]

LGTM

[jrtc27]

lib/libc/tests/stdio/fmemopen2_test.c

298

as above