Re: Finalizing 'inhibit-automatic-native-compilation'

[Sean Whitton <spwhitton@spwhitton.name>]

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Hello,

[quoting whole message for the benefit of debian-emacsen]

On Fri 27 Jan 2023 at 02:57PM +02, Eli Zaretskii wrote:

> The variable 'inhibit-automatic-native-compilation' was introduced in
> last October, as result of various discussions on this list regarding
> the need to disable async native-compilation in some situations.
> Since its introduction was met with some opposition, in particular
> from Andrea, the final decision about whether this variable should
> stay in Emacs was deferred, with the purpose of collecting more data
> points and user experience.
>
> With the pretest of Emacs 29 just around the corner, I think now is
> the time to make that final decision.  With that in mind, I will first
> summarize the changes which this variable introduced into Emacs, and
> then ask for opinions regarding some of its aspects.
>
> This variable was introduced (under the name
> 'inhibit-native-compilation') in commit 5fec9182db.  In that commit:
>
>   . comp-trampoline-compile was changed to avoid writing the
>     trampolines to the eln-cache if this variable is non-nil (instead,
>     it writes the trampolines to a temporary-file directory, and
>     attempts to delete them after that, which on Posix platforms will
>     cause their deletion when Emacs which produced them exits, and on
>     Windows currently fails).
>   . In normal-top-level, we set this variable if the environment
>     variable EMACS_INHIBIT_AUTOMATIC_NATIVE_COMPILATION is set.
>   . In several places this variable either replaces
>     native-comp-deferred-compilation or has the same effect as the
>     latter (modulo the opposite meaning of nil/t value), therefore
>     disabling async compilation of *.el files that Emacs loads for
>     which there are no corresponding *.eln files.
>
> Here are the questions I think we want to be answered to finalize the
> implementation and effects of 'inhibit-automatic-native-compilation':
>
>   . Do people actually use 'inhibit-automatic-native-compilation' or
>     the corresponding environment variable?  If so, for what reasons,
>     and why tweaking 'native-comp-eln-load-path' to direct the *.eln
>     files to some other place, including the temporary-file directory,
>     was not enough?
>
>   . What do we want to do about compiling trampolines when
>     native-compilation is disabled?
>
>     Currently, 'inhibit-automatic-native-compilation' doesn't really
>     disable compilation of trampolines, it just causes them to be
>     written to a temporary location, and hopefully deleted when the
>     session ends.  This means that, for example, if the user has a
>     broken installation of GCC and Binutils, loading Lisp code that
>     uses advices will signal errors when Emacs compiles the
>     trampolines (because the compilation fails).
>     The alternative is to disable compilation of trampolines, but that
>     has a downside that advices for primitives will not have effect.
>     It is not clear to me which alternative is better, as they both
>     have failure modes.  Note that the build process was augmented so
>     you can say, after building Emacs as usual
>
>        make trampolines
>
>     and have all the trampolines for the built-in functions
>     (a.k.a. "primitives") compiled and written to the build tree, from
>     where they will be installed by "make install", thus minimizing
>     potential problems with the need to build trampolines when running
>     the installed Emacs.
>
>     If we leave the current build-trampolines-then-delete-them
>     machinery intact, is it a problem to have those trampolines not
>     deleted on MS-Windows?  They will then be left in the temporary
>     directory, and are supposed to be removed by system cleanup
>     processes, or maybe remain there forever.  Or do we have to add a
>     mechanism for deleting them at exit?
>
>   . Do we want to keep the EMACS_INHIBIT_AUTOMATIC_NATIVE_COMPILATION
>     environment variable?
>
>     I dislike having environment variables that alter Emacs behavior,
>     because environment variables are inherited by sub-processes.  So
>     having this variable set runs the risk of affecting all the
>     sub-processes, something that could be unexpected and is not easy
>     to prevent.  We had similar problems with EMACSLOADPATH, for
>     example, which is especially painful when building another version
>     of Emacs from a shell buffer inside Emacs.  This causes some
>     hard-to-debug problems.
>     So if this environment variable is largely unused, maybe we should
>     remove it, even if we keep 'inhibit-automatic-native-compilation'.

Debian is now relying on this environment variable.  It is going into
our next stable release, which is already partially frozen, and it has
already propagated to many (probably most) of our derivatives.
We took the decision to start using the variable after Lars said it was
going to stick around and, specifically, that it was therefore okay to
backport it to Emacs 28.

Over in our own channels, we had actually been planning to add our own
version of this environment variable, before it got added upstream.
In other words, we independently came to the conclusion that a mechanism
of this nature was what we needed; as Stefan says, one reason is that we
need to handle invocations of Emacs deep within third party Makefiles.

In the shorter term, were it to be removed, we'd probably patch it back
in with a DEBIAN_something name, rather than break all our packages
again, or, rather than delaying uploading Emacs 29 to Debian.
But in the longer term, I'm sure that we would be open to alternatives,
if you really do want to remove the variable.

(Just fyi, I am now co-maintaining Emacs in Debian alongside Rob.)

-- 
Sean Whitton

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