bug#48079: Temporary files while building after native-comp merge

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Andrea Corallo [2022-01-06 16:10:37] wrote:
> Eli Zaretskii <eliz@gnu.org> writes:
>>> In the plain old byte-compiler, these files are *very* short lived
>>> because they're not created during the compilation itself but only at
>>> the very end when we save the result to a file (and we just do it by
>>> first saving to `foo.elcNNMMPP` and then renaming that to `foo.elc`).
>>> 
>>> Now with `batch-byte-native-compile-for-bootstrap` apparently we "suspend
>>> the byte-compiler" right in the middle of this small time window, i.e. after
>>> writing to `foo.elcNNMMPP` but before its renamed.  Then we call the
>>> native compiler and only once the native compiler is done, we resume the
>>> byte compilation which just renames the file and exits.
>>> 
>>> If that understanding is correct, then I think we may be able to fix the
>>> problem by just changing the moment at which we suspend the byte-compiler:
>>> suspend it *before* it writes to `foo.elcNNMMPP`.
>>
>> Are you sure your description above is accurate?

No, but I haven't heard any indication to the contrary either.

>> We have a backtrace in bug#48978 that shows when we create the
>> .elcXXX temporary file.  My reading of that backtrace is that it's
>> the other way around: native-compilation invokes byte-compile-file,
>> which compiles the Lisp into bytecode, creates the file with
>> make-temp-file, and writes out the bytecode.  It is true that we then
>> defer renaming of the temporary file in this case

I don't see why you say "the other way around".  Maybe you're putting
more emphasis on some detail of what I wrote than what I intended.
The core os what I wrote is that we should change the code so that the
native compilation takes place before the `.elcNNMMPP` file is written
rather than after.

AFAICT the native compiler does not read/need that file because it gets
the same information in a more convenient format straight from
`bytecomp.el`.

> That's correct,

I don't know what "That" refers to.


        Stefan