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nix::fetchers::CacheImpl uses $XDG_CACHE_HOME, or its default based on
$HOME, to store its SQLite database. If the current process can't write
to that directory for whatever reason, though, any eval-time fetching
would fail just initializing the cache.
With this change, IO errors initializing the fetcher cache are logged
but ignored, and nix::fetchers::CacheImpl falls back to an in-memory¹
database instead.
Notably, this will fix any uses eval fetching while Lix itself is being
run in a derivation builder (such as during tests), as the derivation
builder does not set $XDG_CACHE_HOME, and sets $HOME to the non-existent
directory /homeless-shelter.
Before:
$ env -u XDG_CACHE_HOME HOME=/homeless-shelter nix -Lv eval --impure -E 'fetchTarball "https://git.lix.systems/lix-project/lix/archive/main.tar.gz"'
error:
… while calling the 'fetchTarball' builtin
at «string»:1:1:
1| fetchTarball "https://git.lix.systems/lix-project/lix/archive/main.tar.gz"
| ^
error: creating directory '/homeless-shelter': Permission denied
After:
$ env -u XDG_CACHE_HOME HOME=/homeless-shelter nix -Lv eval --impure -E 'fetchTarball "https://git.lix.systems/lix-project/lix/archive/main.tar.gz"'
warning: ignoring error initializing Lix fetcher cache: error: creating directory '/homeless-shelter': Permission denied
"/nix/store/s9lxdnn0awp37n560bg4fgr497ah4hvw-source"
¹: https://www.sqlite.org/inmemorydb.html
Change-Id: I15c38c9baaf215fc6e192b8a4c70b9692a69bc22
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