Subject: Re: Bug in unused.cocci?

From: Taylor Blau <[email-masked]>
Date: Thu, 11 May 2023 10:55:20 -0400

On Thu, May 11, 2023 at 10:16:21AM +0100, Phillip Wood wrote:
> I think this is due to a bug in unused.cocci. I'm not sure what is going
> wrong and admittedly we're unlikely to see code where an strbuf is
> initialized and then used it without calling any of the strbuf_* functions
> within our main codebase but it would be nice if the rule could handle this.

I don't think that this is a bug in unused.cocci, but rather a bug in
spatch not being able to read t/unit-tests/test-lib.h.

    $ spatch --verbose-parsing --debug --all-includes \
        --sp-file contrib/coccinelle/unused.old.cocci \
        t/unit-tests/t-strbuf.c | grep '^bad'
    init_defs_builtins: /usr/lib/coccinelle/standard.h
    -----------------------------------------------------------------------
    processing semantic patch file: contrib/coccinelle/unused.old.cocci
    with isos from: /usr/lib/coccinelle/standard.iso
    -----------------------------------------------------------------------

    HANDLING: t/unit-tests/t-strbuf.c
    parse error
     = error in t/unit-tests/test-lib.h; set verbose_parsing for more info
    badcount: 3
    bad: int test_done(void);
    bad:
    bad: /* Skip the current test. */
    BAD:!!!!! __attribute__((format (printf, 1, 2)))

>From my understanding, spatch happily ignores macros that it doesn't
understand (like check_uint() and check_char()), so to it this code
looks like:

    struct strbuf buf;

    strbuf_init(&buf, 1024);
    strbuf_release(&buf);

which it marks as unused and applies the patch. Strangely, if you force
it to pre-process with the appropriate macro file by passing it
explicitly, it works as expected:

    $ spatch --macro-file t/unit-tests/test-lib.h \
        --sp-file contrib/coccinelle/unused.old.cocci \
        t/unit-tests/t-strbuf.c
    init_defs_builtins: /usr/lib/coccinelle/standard.h
    init_defs: t/unit-tests/test-lib.h
    HANDLING: t/unit-tests/t-strbuf.c

I am puzzled by spatch's behavior here.

Thanks,
Taylor