Re: Dynamic loading progress

[Daniel Colascione]

On 02/13/2015 07:49 AM, Eli Zaretskii wrote:
>> Date: Fri, 13 Feb 2015 07:06:17 -0800
>> From: Daniel Colascione <address@hidden>
>> CC: address@hidden, address@hidden
>>
>> This way, instead of loaded modules needing to track Emacs' historically
>> unstable internal data structures, elisp needs to track module ABIs,
> 
> How's that better?  That's the same problem in the opposite direction,
> only much worse, since the modules out there are not under our control
> at all.

The risk isn't symmetric:

1) Shared libraries traditionally expose stable ABIs; when ABIs change
incompatibly, libraries change sonames. Shared libraries need stable
ABIs whether or not Emacs loads them: they're linked into other programs
and aren't necessarily rebuilt at the same time as their dependencies.
The Emacs core has never had a stable ABI, and creating one would
constrain our optimization opportunities.

(That is, libpng.so's ABI ought to change a lot less often than
/usr/bin/emacs's: the former promises to provide a stable ABI and the
latter never has.)

2) If a shared library's ABI does change, a Lisp update can fix the
problem. On the other hand, if a strongly-coupled module becomes
incompatible with the Emacs core, the remedy involves recompiling either
Emacs or the module. It's much easier to update Lisp than to rebuild
native code.

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