Because the nix rocksdb build depends on the jemalloc feature, you need
to use a different devshell when passing --all-features to cargo than
the default.
Some of the features affect nix dependencies, so we need to have a
full feature list available when constructing the nix derivation. This
incidentally fixes the bug where we weren't enabling jemalloc on rocksdb
in CI/devshells, because jemalloc is now a default feature. It does not
fix the more general class of that issue, where CI is performing an
`--all-features` build in a nix devshell built for default-features.
I am now passing `--no-default-features` to cargo, and having it use our
unified feature list rather than duplicating the unification inside cargo.
Without setting JEMALLOC_OVERRIDE, we end up linking to two different
jemalloc builds. Once dynamically, as a transitive dependency through
rocksdb, and a second time to the static jemalloc that tikv-jemalloc-sys
builds.
This fixes dynamically-linked jemalloc builds, for the reasons described
in <https://github.com/girlbossceo/conduwuit/pull/400#issue-2316700200>.
This prevents us from needing to recompile the dependencies when that
environment variable changes, which generally changes on every commit,
which is far more frequently than the dependencies are actually changed.
Users of the nix package can now just use `.override` to choose what
features they want.
This also makes RocksDB automatically use jemalloc when Grapevine is
configured to use jemalloc.
