Global logger

[jaytaph]

There are certain points in the engine where we need or want to log information back to a user. This could be warnings and errors during tokenizing/parsing data, or other information (for instance, when we query a hostname, and the dns is not found).

We should be able to have a central based log, where we can log these things into. Furthermore, things like the parser probably want to "duplicate" the logs so parsing of a document will result in a document/dom-tree and a separate log of all things that failed.

We could have a central logger that works a bit like syslog, and underwater passes it through different log mechanism like syslog, direct to file, or even post it directly over http for instance.

I don't have enough experience with rust to know if these kind of loggers are already available, or that we should create our own system. Note that i think it would make sense to have the logging be done outside the main thread: from the engine, we do a log.log(Log::HTML5_PARSE, Log::WARN, "error while parsing something on line 1, colum 5"), and this will return immediately while the actual storing of this into syslog, file, json, http, whereever is done in the background.

[jaytaph]

There is something we could potentially use: https://docs.rs/log/latest/log/

[blaumeise20]

This log crate is actually the "official" Rust logging frontend. You can specify a custom log handler behind that, which gets called with the log information. Pretty much every crate that logs something uses log, and that way you get all the logs including where they came from into one central position. You might have heard about env_logger, which is nothing more than a logging backend for log itself.

[jaytaph]

so, would we have a "log", and let the UA decide what to do with it? So we do something like warning!("something's off here"), and that just be it. I need to read into the log, but it would be nice if we can group logs (html5.error, css.warn, net.info) etc..

[blaumeise20]

Yes, exactly.

Regarding the grouping, you can either determine it by the location it came from, or you can attach a "target":

info!(target: "net", "Requested {path}");

See https://docs.rs/log/latest/log/macro.info.html for more info.

(It is even possible to associate structured data with a message, which requires a feature in the log crate though)

[jaytaph]

This seems exactly what we are looking for :-)

👍

[emwalker]

(For more sophisticated stuff, there's also the tracing crate.)

[jaytaph]

@emwalker @blaumeise20 would it be interesting to implement this directly into the parser engine at the moment?

[emwalker]

I would add it. It is a low stakes decision that is easy to revisit. It can be added in the middle of an unrelated PR:

$ cargo add log

// ...
info!("expected to see foo {:?}, but saw bar instead {:?}", foo, bar);

See this example.

[emwalker]

Later on we can dial this in.