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Gold readiness obstacle #4: libtool (part 1)

In my current series of posts about gold this time I’m presenting you with a two-parter that shows how GNU libtool is causing further problem with this new link editor. The reason why I split this into two part is because it hits two different issues with it: one is a “minor inconvenience” due to its design, and the other is a known bug due to it.

You might remember me distinguishing into two schools of build configuration systems one relying on tests being compiled and executed, and the other relying on knowing intimate details about the working of the tools to be used with it. Autotools for the vast part are designed to fall squarely into the former category, with a number of advantages, but also of disadvantages, the most obvious of which is the slowness of the ./configure process.

This is true of moth autoconf and autoconf macro; on the other hand, libtool works vastly as a repository of knowledge, knowing rules about various operating systems, link editors and compilers. It shouldn’t surprise anybody, given that it already spends way more time than one would like, to discover how to build shared libraries; if it was a pure guessing game, it would probably make it unbearable to use.

Unfortunately this makes libtool vulnerable to the main issue of knowledge repository systems, with a bad twist: not only it becomes outdated as operating systems, link editors and compilers are released, but due to the autotools philosophy of not requiring the tools themselves at build time, simply updating the system copy of libtool is not enough. For details about this statement see my previous post on libtool from which I still haven’t had time to distil documentation for my guide.

What this boils down to is simple yet scary: even though libtool properly implemented support for gold starting from version 2.2.7 – which actually only means that it now knows that gold supports anonymous versions in linker scripts – any package whose autotools had been built with older versions wouldn’t support gold out of the box. Luckily for us, this isn’t an excessively invasive issue: anonymous versioning, as far as I can tell, is only used when using libtool to export and hide symbols which for good or bad is not used that often.

To make sure that projects know about gold support for the feature, you’re left with two solutions. The former is the obvious one of rebuilding autotools; while this is often necessary for other reasons, it isn’t that good an idea, because it wastes time. I have other notes about the rebuilding of autotools but I’ll skip over them for now. In Gentoo we have already a method to take care of this, consisting of the elibtoolize function. This function applies a number of patches over an already-generated autotools build system, to fix common issues (mostly due to libtool) without having to rebuild autotools altogether.

My first encounter with this interface was due to my early Gentoo/FreeBSD work. One of the things libtool “knows” about, is that FreeBSD does not like having two-part sonames which made many minor version bump, which should have kept the same, or a compatible, ABI into a link-breaking update. Since we didn’t look for binary compatibility with the original FreeBSD I went out of my way to patch libtool so that it would use the Linux naming scheme instead. Given that I already knew how to set it up, it wasn’t that difficult to add one extra patch when gold is used as the link editor of choice.

Unfortunately, having worked with that before also means I know the bad side of elibtoolize: too many packages do not use it at all. This is partly because developers do not know about the need for it (it applies, among others, the --as-needed compatibility patch), and partly because they think that it causes autotools to be rebuilt, rather than preventing the requirement for it.

Waiting for other obstacles to be solved, this one is probably going to be bothering us for months, if not years, to come. I don’t expect a resolution anytime soon, given that the idea of having a comprehensive, versioned system for autopatching packages, which was requested/proposed many times, never went into fruition. Maybe I’ll try pushing for it for the next GSoC (I note that I only partecipate into even years editions).

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