Re: [PATCH] ci: update 'static-analysis' to Ubuntu 22.04

[Jeff King <peff@peff.net>]

On Thu, Aug 25, 2022 at 12:47:53PM +0200, Ævar Arnfjörð Bjarmason wrote:

> What's happening here is that coccinelle can no longer properly parse
> the file after the UNUSED() macros were applied to refs.c.

Ugh. Thanks for finding this.

I'm a little disappointed in coccinelle here. I mean, I get that it
probably can't just use the output of "gcc -E"; it wants to see and
transform the code as written (and maybe even translate macros). But
if it doesn't understand the macros enough to see inside them, there are
a lot of things it is going to get confused by.

A simple example like this shows that it doesn't really see inside them:

  $ cat foo.c
  #define foo(x) bar(x)
  static void fun(int x) { foo(x); }

  $ cat foo.cocci
  expression E;
  @@
  - bar(E);
  + changed(E);

I wonder what it makes of code-generation macros like
for_each_string_list_item(). Or all of commit-slab.

Likewise, the manner in which it fails is unexpected. I could see it
getting confused about the line in question, but why would that cause it
to fail to see the of a strbuf in another function entirely?

I realize that pseudo-parsing C is an incredibly hard problem. But I
also wonder how robust the tool actually is in practice. We've had a lot
of mystery "now why didn't it catch this case" incidents over the years.
Just running the tip of master with --verbose-parsing yields quite a few
"bad" and "parse error" entries in the log. I wonder how much that
contributes.

> We should probably coerce coccinelle into stopping in general if it's
> encountering "BAD:!!!!!" parse errors behind the scenes, as it clearly
> results in broken logic, but offhand (and from briefly browsing the
> manpage) I don't know a way to do that.

You could detect the BAD entries from the log and complain before
returning the exit code to make. But since there are several instances
already, I suspect it's a losing battle. We need some way to actually
_fix_ them. And in the meantime, it does seem to _mostly_ work, as it
clearly has some kind of resync logic.

> But the fix here isn't to delete unused.cocci, but to hold off on the
> UNUSEwork D() patches until we figure out how to make coccinelle jive with
> them.

Yeah, my general skepticism and disappointment above notwithstanding,
this seems like the best path forward from here. I tried a few other
tricks (like --macro-file and --iso-file), but if its parser chokes, I
don't think there's much we can do about it. Even if we wrote a patch to
coccinelle itself (and I have no interest in doing that myself), it
would take a while to become available.

-Peff