parse configured EnvFilter once

This allows the error handling to be done upfront instead of for each
use. In particular, the `toml` error now points to the span of text in
the config file where the misconfigured EnvFilter value is. This is much
better than the previous error that did not indicate what was actually
causing it to happen.

src/config.rs

@@ -8,8 +8,10 @@ use serde::Deserialize;
 
 use crate::error;
 
+mod env_filter_clone;
 mod proxy;
 
+use env_filter_clone::EnvFilterClone;
 use proxy::ProxyConfig;
 
 #[allow(clippy::struct_excessive_bools)]
@@ -69,7 +71,7 @@ pub(crate) struct Config {
     #[serde(default = "default_trusted_servers")]
     pub(crate) trusted_servers: Vec<OwnedServerName>,
     #[serde(default = "default_log")]
-    pub(crate) log: String,
+    pub(crate) log: EnvFilterClone,
     #[serde(default)]
     pub(crate) turn_username: String,
     #[serde(default)]
@@ -146,8 +148,10 @@ fn default_trusted_servers() -> Vec<OwnedServerName> {
     vec![OwnedServerName::try_from("matrix.org").unwrap()]
 }
 
-fn default_log() -> String {
-    "warn,state_res=warn,_=off".to_owned()
+fn default_log() -> EnvFilterClone {
+    "warn,state_res=warn,_=off"
+        .parse()
+        .expect("hardcoded env filter should be valid")
 }
 
 fn default_turn_ttl() -> u64 {

src/config/env_filter_clone.rs

@@ -0,0 +1,44 @@
+//! A workaround for [`EnvFilter`] not directly implementing [`Clone`]
+//!
+//! This will be unnecessary after [tokio-rs/tracing#2956][0] is merged.
+//!
+//! [0]: https://github.com/tokio-rs/tracing/pull/2956
+#![warn(missing_docs, clippy::missing_docs_in_private_items)]
+
+use std::str::FromStr;
+
+use serde::{de, Deserialize, Deserializer};
+use tracing_subscriber::EnvFilter;
+
+/// A workaround for [`EnvFilter`] not directly implementing [`Clone`]
+///
+/// Use [`FromStr`] or [`Deserialize`] to construct this type, then [`From`] or
+/// [`Into`] to convert it into an [`EnvFilter`] when needed.
+#[derive(Debug)]
+pub(crate) struct EnvFilterClone(String);
+
+impl FromStr for EnvFilterClone {
+    type Err = <EnvFilter as FromStr>::Err;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        EnvFilter::from_str(s)?;
+        Ok(Self(s.to_owned()))
+    }
+}
+
+impl From<&EnvFilterClone> for EnvFilter {
+    fn from(other: &EnvFilterClone) -> Self {
+        EnvFilter::from_str(&other.0)
+            .expect("env filter syntax should have been validated already")
+    }
+}
+
+impl<'de> Deserialize<'de> for EnvFilterClone {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let s = String::deserialize(deserializer)?;
+        Self::from_str(&s).map_err(de::Error::custom)
+    }
+}

src/error.rs

@@ -61,9 +61,6 @@ pub(crate) enum Observability {
     #[error("opentelemetry error")]
     Otel(#[from] opentelemetry::trace::TraceError),
 
-    #[error("invalid log filter syntax")]
-    EnvFilter(#[from] tracing_subscriber::filter::ParseError),
-
     #[error("failed to install global default tracing subscriber")]
     SetSubscriber(#[from] tracing::subscriber::SetGlobalDefaultError),
 

src/observability.rs

@@ -82,8 +82,6 @@ pub(crate) enum FoundIn {
 
 /// Initialize observability
 pub(crate) fn init(config: &Config) -> Result<Guard, error::Observability> {
-    let config_filter_layer = || EnvFilter::try_new(&config.log);
-
     let jaeger_layer = config
         .allow_jaeger
         .then(|| {
@@ -101,7 +99,7 @@ pub(crate) fn init(config: &Config) -> Result<Guard, error::Observability> {
             let telemetry = tracing_opentelemetry::layer().with_tracer(tracer);
 
             Ok::<_, error::Observability>(
-                telemetry.with_filter(config_filter_layer()?),
+                telemetry.with_filter(EnvFilter::from(&config.log)),
             )
         })
         .transpose()?;
@@ -114,7 +112,7 @@ pub(crate) fn init(config: &Config) -> Result<Guard, error::Observability> {
             let flame_layer = flame_layer.with_empty_samples(false);
 
             Ok::<_, error::Observability>((
-                flame_layer.with_filter(config_filter_layer()?),
+                flame_layer.with_filter(EnvFilter::from(&config.log)),
                 guard,
             ))
         })
@@ -122,7 +120,7 @@ pub(crate) fn init(config: &Config) -> Result<Guard, error::Observability> {
         .unzip();
 
     let fmt_layer = tracing_subscriber::fmt::Layer::new()
-        .with_filter(config_filter_layer()?);
+        .with_filter(EnvFilter::from(&config.log));
 
     let subscriber = Registry::default()
         .with(jaeger_layer)