The Nix daemon crashes when started without --experimental-features ca-derivations, but a client tries to force it

[thufschmitt]

Describe the bug

(privileged) clients can enable some experimental features on the daemon they talk to. This is generally fine, except for the ca-derivations which will cause the daemon to crash with something like src/libstore/sqlite.cc:92: nix::SQLiteStmt::Use::Use(nix::SQLiteStmt&): Assertion stmt.stmt' failed`.

Steps To Reproduce

• Start the Nix daemon without the ca-derivations experimental feature
• Run a (privileged) build with --extra-experimental-features ca-derivations

Expected behavior

Ideally this should transparently work as expected.
This might be a bit tricky because of the DB schema constraints, but we should at least yield a proper error in that case.

Additional context

The underlying issue is that enabling ca-derivations changes the underlying db schema (which can only happen at the begining of the process when we open the DB).
Then some precompiled SQL queries in LocalStore depend on the ca-specific schema, so they are left undefined if the experimental-feature isn’t set (here).
In a “normal” setting, this is fine because we’ll never use these as ca-derivations isn’t set. But since the client can enable the feature after-the-fact, we end-up using it anyways, leading to the assertion failure (which is mostly a glorified null pointer exception)

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/content-addressed-nix-call-for-testers/12881/119

[edolstra]

Probably the daemon should ignore the experimental-features setting sent by the client.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/content-addressed-nix-call-for-testers/12881/129