Add json-log-path setting

[edolstra]

Motivation

The json-log-path setting specifies a path (which can be a regular file or Unix domain socket) that receives a copy of all Nix log messages (in the same format used by --log-format internal-json but without the @nix prefixes). Internally this is implemented using the new TeeLogger class that dispatches log messages to multiple underlying loggers (i.e. the JSON logger and the regular progress bar).

The main use case is to provide an easy way in CI to process all Nix log messages centrally to extract hash mismatches, build/substitution results, etc.

Context

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[Mic92]

cc @elikoga do you want to review this from an consumer perspective?

[elikoga]

I'm surprised #12647 is already merged, I thought it'd arrive with a PR like this, seems I did not look too closely two weeks ago

Additionally, it seems like

    resHashMismatch = 109,
    resBuildResult = 110,

from the big DetSys PR in src/libutil/logging.hh is not included in this PR, which is reasonable for it to come later but it means that this does not involve a change in my internal-json parser.

I will verified the following though , once compilation on my end is done:

• --log-format internal-json works as before
• json-log-path output looks useful/as expected

I see this PR useful for users of nix in a terminal that want to have additional information accessible after a nix command execution while seeing the normal human-readable output in the terminal. For that it works great. I use --log-format internal-json by completely wrapping the output and reproducing it from the structured data for user consumption so this does not apply to me directly.

[elikoga]

I believe json-log-path could be documented as a very very very verbose --verbose that does not make the terminal unreadable.

One should maybe warn the user of the resulting filesize, if one decides to run many commands with the setting and write the logs to a normal plain file without any compression, then the disk may run full.

-rw-r--r-- 1 elikoga users 131M Apr 12 01:13 nix.log

One build produced more than 100 megabytes of logs for me.

Useful feature in any case

[elikoga]

The design could suggest a new useful log message type like commandInvoked, which would allow one to distinguish when a new command output log exists in a log file with outputs from multiple different invocations. That's probably one for the future though.

[Mic92]

Suggested change

	A path to which JSON records of Nix's log output will be
	A file or unix socket path to which JSON records of Nix's log output will be

[Mic92]

Optional as suggested by the reviewer:

Suggested change

	(without the `@nix ` prefixes on each line).
	(without the `@nix ` prefixes on each line).
	When writing to a file be aware that the output can in the order of 100MBs per build.

[nixos-discourse]

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

https://discourse.nixos.org/t/determinate-nix-3-3-json-logging-a-new-experience-around-hash-mismatches-and-more/63142/4