C API: Logging callbacks

[ehmry]

It would be ideal to have isolated logging streams that would be associated with either an evaluator state, or store, or a context. By logs I mean output from builtins.trace and from builders.

At the moment it would be a nice feature for me but if I find I really need it then I might attempt to implement it myself.

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[roberth]

Nix has a global logger, and it could provide a C API function to set it.

nix/src/libutil/logging.hh

Line 183 in 509be0e

extern Logger * logger;

This allows intercepting almost all output; certainly build logs and builtins.trace and friends.

Child process stderr (e.g. git clone) isn't always captured, but that probably isn't too much of a problem for most applications.

• Capture all non-interactive child process stderrs #11040

[nixos-discourse]

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

https://discourse.nixos.org/t/2024-07-10-nix-team-meeting-minutes-160/49101/1