configure: clarify --enable-debug option

[jay]

• Change --enable-debug help text to say it is for developing curl.

• Warn when --enable-debug is used.

This change copies the help text and warning from cmake ENABLE_DEBUG. Also, it shortens the "for debugging curl itself" to just say "for debugging curl".

Now it looks like this:

  --enable-debug          Enable curl debug features (for developing curl)
  --disable-debug         Disable curl debug features

Ref: #22481 (comment)

Reported-by: Viktor Szakats

Closes #xxxx

[vszakats]

Thanks.

A slightly related issue is that with autotools, this option also serves
to enable compiler debug options (aka -g, via $flags_dbg_yes).
There is no separate option to enable only the latter. One may pass them
manually, but if someone is looking at --help, wanting to do this, there
is no way (and also easy to think --enable-debug is the answer). IMO
there are valid cases to build libcurl with debug info and symbols, but
without the development parts.

[bagder]

wanting to do this, there is no way

Traditionally, configure options are not used for simply adding CFLAGS options. You can just CFLAGS=g ./configure.

[MichalPetryka]

I wonder if it'd be better to rename this to --enable-testing or sth to not conflate it with Debug compiler configuration.

[icing]

I wonder if it'd be better to rename this to --enable-testing or sth to not conflate it with Debug compiler configuration.

That probably would break several deployed build pipelines in the world.

[MichalPetryka]

I wonder if it'd be better to rename this to --enable-testing or sth to not conflate it with Debug compiler configuration.

That probably would break several deployed build pipelines in the world.

For now both options could work with the debug being effectively a deprecated name that wouldn't be in the docs.

[vszakats]

I wonder if it'd be better to rename this to --enable-testing or sth to not conflate it with Debug compiler configuration.

That probably would break several deployed build pipelines in the world.

For now both options could work with the debug being effectively a deprecated name that wouldn't be in the docs.

Agreed, renaming an option is certainly possible, if there is a strong reason to do
it, since it also comes with disturbance. I tend to agree this is a strong one, but
that's only me. 'testing' may work, though in CMake BUILD_TESTING is already
used and means something else, so I'd probably seek more options. (It's a
CMake keyword, not curl-specific:
https://cmake.org/cmake/help/latest/variable/BUILD_TESTING.html)

[bagder]

I think enable testing is a downgrade. It's been called debugging since the dawn of time. Why is that wrong now?

[bagder]

not conflate it with Debug compiler configuration.

The compilers have no debug configuration do they? They have -g, the debug symbols option. Is that really so commonly confused?

[vszakats]

The compilers have no debug configuration do they? They have -g, the debug symbols option. Is that really so commonly confused?

Not the OP, but I've been personally confused by the semantics of --enable-debug
several times. I don't know how frequent it is in general. But I'm also not using autotools
by default, which mean it's more of an outsider's opinion. Compilers have no debug
configuration, but in general, configuring for debug may mean more than just -g,
so other build tools or build frameworks often offer 'option packs' that cover common
combinations. (and abstract them to work more or less the same for all supported
compilers.) This is not a thing in autotools. But it's also one of the two things done
by curl's --enable-debug, DEBUGBUILD being the other. So if someone wants
a "just debug" build that matches compiler options set by --enable-debug, it's a
dive into the m4 sources to figure out what those are for a given compiler. This is
friction and not practical (esp in CI), which means some build combinations are not
tested in CI with autotools. This is fine, because CMake can fill in, and vice versa.
All in all, having slightly different semantics in build tools is useful, and expected.
That said, having unambiguous names always help, too, I find.

[bagder]

I don't know how frequent it is in general. But I'm also not using autotools by default

--enable-debug is not really "an autotools thing" and I have no idea if other projects have such an option in their autotools scripts. It is an option I felt I needed to enable things in the build for debugging purposes so I added it (back in February 2001). The name, the function, what it does started out what I think are useful when debugging curl. It has been tweaked and extended ever since.

[bagder]

some build combinations are not tested in CI with autotools

We have literally billions of build combinations with both setups. Most build combinations are not tested. With either configure method.

[vszakats]

some build combinations are not tested in CI with autotools

We have literally billions of build combinations with both setups. Most build combinations are not tested. With either configure method.

Right, but what we consider "important" ones, are. I find great value in
mixing and matching options. The ability to tweak one toggle at a time
helps a great deal to narrow down root causes. When they are bundled
up (sometimes by accident), difficulty can escalate.

[jay]

I think a name change is unwarranted. The option needed clarification and this PR addresses that so I'm going to land it.