EnvFilter overwrites all attempts to set a default value

[tv42]

Bug Report

Version

├── tracing v0.1.26
│   ├── tracing-attributes v0.1.15 (proc-macro)
│   └── tracing-core v0.1.18
├── tracing-subscriber v0.2.19
│   ├── tracing v0.1.26 (*)
│   ├── tracing-core v0.1.18 (*)
│   ├── tracing-log v0.1.2
│   │   └── tracing-core v0.1.18 (*)
│   └── tracing-serde v0.1.2
│       └── tracing-core v0.1.18 (*)

Platform

Linux 5.4.109 x86_64

Description

EnvFilter has no way of setting defaults.

.add_directive is wrong (and the examples lead to bad behavior) because it overrides $RUST_LOG, as it is added to the end of whatever the user specified, overriding e.g. RUST_LOG=trace.

SubscriberBuilder::with_max_level is wrong because .with_env_filter explicit overwrites that, as per documentation.

EnvFilter internally defaults to ERROR (not documented anywhere).

There is no clean way to achieve

1. Default level INFO
2. User can override that with RUST_LOG=warn

This is related to #1400 but I don't think the fallback value necessarily solves the problem (consider desired application default level INFO and RUST_LOG=some_crate::error) and I don't want the problem be marked closed because a proposed solution is rejected.

[tv42]

Here's the ugly workaround:

const DEFAULT_LEVEL: tracing::Level = tracing::Level::INFO;
let input = std::env::var("RUST_LOG").expect("fixme");
let mut directives = DEFAULT_LEVEL.to_string();
if !input.is_empty() {
    directives.push(',');
    directives.push_str(&input);
}
let filter = tracing_subscriber::EnvFilter::try_new(directives).expect("fixme");
tracing_subscriber::fmt()
    .with_env_filter(filter)
    .init();

[Imberflur]

A feature that solves this would be quite useful. We have a similar workaround: https://gitlab.com/veloren/veloren/-/blob/cba946c07b5ec42fb5c47b1c59f5bc8def40e837/common/frontend/src/lib.rs#L87

[vilgotf]

This is already supported, just needs to be better documented.

tracing_subscriber::fmt()
	.with_env_filter(
		EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new("info")),
	)
	.init();

[Imberflur]

This is already supported, just needs to be better documented.

tracing_subscriber::fmt()
	.with_env_filter(
		EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new("info")),
	)
	.init();

@vilgotf I'm looking back over this and I think it doesn't address my specific use case of having several directives that are applied before the ones from RUST_LOG

[Imberflur]

Thinking about this further, I think it could be convenient to have a method to parse and apply directives from an env var to an existing EnvFilter instance (with errors returned if something fails to parse or the env var is missing or not unicode).

Another idea that came to mind is an add_directive_if_missing/add_directive_no_overwrite method.