Possibly defaulting native-compile-target-directory to XDG_CACHE_HOME/...

[Rob Browning]

We're still trying to resolve some native compilation related crashes in
Debian in the hope that we'll be able to have 28 in the next stable
release --with-native-compilation.  (Stage one of the freeze begins on
Jan 12.)

One remaining issue is that emacs crashes when HOME=/nope in some of our
test suites and perhapssome other packaging or install-related
activities.  For example: https://bugs.debian.org/1021842

(To refresh, Debian policies require packages not to write outside of a
 few specified locations in certain situations, e.g. package builds, and
 HOME is not one of the specified locations.)

I can't recall from previous discussions whether that crash would be
considered a bug, or whether emacs just requires a writable cache dir
now (which is fine if so).

But I have discovered that if we allow the redirection of the
native-compile-target-directory to XDG_CACHE_HOME via something like
this in comp.el:

  (defvar native-compile-target-directory (getenv "XDG_CACHE_HOME")
    "When non-nil force the target directory for the eln files being compiled.")

then we can resolve the issue:

  export HOME=/nope
  export XDG_CACHE_HOME="$(mktemp -d ...)"
  run-tests...

Of course, I assume the default above isn't quite right, i.e. if we were
to pursue this, I imagine we'd actually want to default
native-compile-target-directory to something more like
"$XDG_CACHE_HOME/emacs/eln-cache" when XDG_CACHE_DIR is set.

I also noticed that emacs already respects XDG_CONFIG_HOME, so I
imagined relying on XDG_CACHE_HOME might not be entirely suspect.

Thanks
-- 
Rob Browning
rlb @defaultvalue.org and @debian.org
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