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Re: more bogus meson warnings

From: Paolo Bonzini
Subject: Re: more bogus meson warnings
Date: Wed, 3 Nov 2021 14:37:58 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0

On 11/3/21 13:19, Peter Maydell wrote:
This is not my experience. I find that:
  * test in configure: configure doesn't enable the thing
  * test in meson: meson produces a WARNING, but goes ahead anyway,
    and then the final link fails

So, in general, you _already_ have an improvement over what was there before.

Well, in practice what happened was that before the recent changes
configure correctly didn't put dynamic libraries into the link line,
and so my configure options resulted in a successful build.

That depends on the individual test.  There are three different cases:

- pkg-config without compile test: in that case you were already disabling everything. You didn't see the improvement (WARN at configure/meson time vs. failure at build time) just because you already had the link failure years ago

- pkg-config with compile test: this was the case for a handful of libraries (spice, epoxy, virglrenderer, gnutls/nettle). Here indeed it's not an improvement in your experience. On the other hand, I don't see any reason to do this for 3-4 libraries out of the dozens that we test.

- library + compile test: the warning is likely not bogus

The warning is emitted in the case where the files are present but the
test program fails. Of course, Meson doesn't have a crystal ball for
the cc.links test when it says that the static lzo2 library is there
(apparently it is).

They're two tests under the hood, but they both need to pass
for us to be able to use the feature. If they don't both pass,
then meson should just quietly say "OK, we don't have this thing"
(ie it could print a "Something or other: NO" line, but it should
not be printing a "WARNING" line).

Of course it's possible to downgrade the warning line, but I'd rather first be sure that the warning is bogus. If you _do_ have a header and static library, but somehow it cannot be used to link the test program, it would be correct to warn. Unlike the configure script, Meson does have code to distinguish static vs shared libraries, so the compile test should be unnecessary; I would like to understand what causes it to fail, so that your system says "Library snappy found: YES" (and warns), whereas mine says "no". I can check that using config-host.mak and meson-logs/meson-log.txt.


I just want meson to follow the convention that we have had for
years, which is:
  * if I say --enable-foo, then failing to find foo should be an
  * if I say --disable-foo, then don't probe for foo at all
  * if I say nothing, then probe for the various things we need to
    enable the foo feature, and if they're not presentor not usable
    for some reason then just quietly don't enable the foo option

I specifically do *not* want meson to print anything saying
"WARNING" for case 3, because this should be a fairly normal
state of affairs.

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