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mkDummySrc: preserve mode and then make writeable separately #964

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Merged
ipetkov merged 3 commits into from
Jan 30, 2026
+9 −1
Merged

mkDummySrc: preserve mode and then make writeable separately #964

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mkDummySrc: preserve mode and then make writeable separately
cole-h Jan 29, 2026
6440cb9
Add a comment on the load bearing chmod +w
ipetkov Jan 30, 2026
909c3bd
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ipetkov Jan 30, 2026
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10 changes: 9 additions & 1 deletion lib/mkDummySrc.nix
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Expand Up @@ -356,9 +356,17 @@ let
else
last nameWithoutHash;
in
# NB: A previous attempt used --no-preserve-mode + --preserve=xattr to prevent needing to change any
# xattrs at all, but this doesn't work when cp is built without xattr support. So instead we just
# create the effect of --no-preserve=mode without using that flag: making $out writeable,
# recursively.
#
# Without this, builds can fail when using macOS's Virtualization.framework to share the store into
# a Linux VM, because its implementation of virtiofs doesn't seem to support xattrs.
runCommand sourceName { } ''
mkdir -p $out
cp --recursive --no-preserve=mode,ownership ${cleanSrc}/. -t $out
cp --recursive --no-preserve=ownership ${cleanSrc}/. -t $out
chmod +w -R $out
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@dpc dpc Jan 29, 2026

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The exact effect of +w seems to be umask-dependent. Is that OK? Should it use more explicit way of setting the permissions?

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@ipetkov ipetkov Jan 30, 2026

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Looking at the generic stdenv setup (what runCommand eventually delegates to) it doesn't seem like umask is set (to something different than its default) so I don't imagine it would affect things here?

Either way if we get this wrong builds will fail if we try to clobber the files. Once the derivation finishes the write permissions will get stripped from the store path

${copyCargoLock}
${copyAndStubCargoTomls}
${extraDummyScript}
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