feat(tvix/tracing): Allow configuring additional layers

This will be used by tvix-daemon to write tracing data into the active
client's connection socket.

Change-Id: I8889dd0a638e004ee2c8cb312946b029c9779313
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12734
Tested-by: BuildkiteCI
Autosubmit: Vladimir Kryachko <v.kryachko@gmail.com>
Reviewed-by: flokli <flokli@flokli.de>

tvix/tracing/src/lib.rs

@@ -5,7 +5,11 @@ use tracing::level_filters::LevelFilter;
 use tracing_indicatif::{
     filter::IndicatifFilter, util::FilteredFormatFields, writer, IndicatifLayer, IndicatifWriter,
 };
-use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt, EnvFilter, Layer};
+use tracing_subscriber::{
+    layer::{Identity, SubscriberExt},
+    util::SubscriberInitExt,
+    EnvFilter, Layer, Registry,
+};
 
 #[cfg(feature = "otlp")]
 use opentelemetry::{
@@ -175,74 +179,82 @@ impl TracingBuilder {
     /// It will also configure otlp if the feature is enabled and a service_name was provided. It
     /// will then correctly setup a channel which is later used for flushing the provider.
     pub fn build(self) -> Result<TracingHandle, Error> {
+        self.build_with_additional(Identity::new())
+    }
+
+    /// Similar to `build()` but allows passing in an additional tracing [`Layer`].
+    ///
+    /// This method is generic over the `Layer` to avoid the runtime cost of dynamic dispatch.
+    /// While it only allows passing a single `Layer`, it can be composed of multiple ones:
+    ///
+    /// ```ignore
+    /// build_with_additional(
+    ///   fmt::layer()
+    ///     .and_then(some_other_layer)
+    ///     .and_then(yet_another_layer)
+    ///     .with_filter(my_filter)
+    /// )
+    /// ```
+    /// [`Layer`]: tracing_subscriber::layer::Layer
+    pub fn build_with_additional<L>(self, additional_layer: L) -> Result<TracingHandle, Error>
+    where
+        L: Layer<Registry> + Send + Sync + 'static,
+    {
         // Set up the tracing subscriber.
         let indicatif_layer = IndicatifLayer::new().with_progress_style(PB_SPINNER_STYLE.clone());
         let stdout_writer = indicatif_layer.get_stdout_writer();
         let stderr_writer = indicatif_layer.get_stderr_writer();
-        let subscriber = tracing_subscriber::registry()
+
-            .with(
+        let layered = tracing_subscriber::fmt::Layer::new()
-                EnvFilter::builder()
+            .fmt_fields(FilteredFormatFields::new(
-                    .with_default_directive(LevelFilter::INFO.into())
+                tracing_subscriber::fmt::format::DefaultFields::new(),
-                    .from_env()
+                |field| field.name() != "indicatif.pb_show",
-                    .expect("invalid RUST_LOG"),
+            ))
-            )
+            .with_writer(indicatif_layer.get_stderr_writer())
-            .with(
+            .compact()
-                tracing_subscriber::fmt::Layer::new()
+            .and_then((self.progess_bar).then(|| {
-                    .fmt_fields(FilteredFormatFields::new(
-                        tracing_subscriber::fmt::format::DefaultFields::new(),
-                        |field| field.name() != "indicatif.pb_show",
-                    ))
-                    .with_writer(indicatif_layer.get_stderr_writer())
-                    .compact(),
-            )
-            .with((self.progess_bar).then(|| {
                 indicatif_layer.with_filter(
                     // only show progress for spans with indicatif.pb_show field being set
                     IndicatifFilter::new(false),
                 )
             }));
+        #[cfg(feature = "tracy")]
+        let layered = layered.and_then(TracyLayer::default());
+
+        let mut tx: Option<mpsc::Sender<Option<oneshot::Sender<()>>>> = None;
 
         // Setup otlp if a service_name is configured
         #[cfg(feature = "otlp")]
-        {
+        let layered = layered.and_then({
             if let Some(service_name) = self.service_name {
                 // register a text map propagator for trace propagation
                 opentelemetry::global::set_text_map_propagator(TraceContextPropagator::new());
 
-                let (tracer, tx) = gen_otlp_tracer(service_name.to_string());
+                let (tracer, sender) = gen_otlp_tracer(service_name.to_string());
+                tx = Some(sender);
                 // Create a tracing layer with the configured tracer
-                let layer = tracing_opentelemetry::layer().with_tracer(tracer);
+                Some(tracing_opentelemetry::layer().with_tracer(tracer))
-
+            } else {
-                #[cfg(feature = "tracy")]
+                None
-                {
-                    subscriber
-                        .with(TracyLayer::default())
-                        .with(Some(layer))
-                        .try_init()?;
-                }
-
-                #[cfg(not(feature = "tracy"))]
-                {
-                    subscriber.with(Some(layer)).try_init()?;
-                }
-                return Ok(TracingHandle {
-                    tx: Some(tx),
-                    stdout_writer,
-                    stderr_writer,
-                });
             }
-        }
+        });
-        #[cfg(feature = "tracy")]
+
-        {
+        let layered = layered.with_filter(
-            subscriber.with(TracyLayer::default()).try_init()?;
+            EnvFilter::builder()
-        }
+                .with_default_directive(LevelFilter::INFO.into())
-        #[cfg(not(feature = "tracy"))]
+                .from_env()
-        {
+                .expect("invalid RUST_LOG"),
-            subscriber.try_init()?;
+        );
-        }
+
+        tracing_subscriber::registry()
+            // TODO: if additional_layer has global filters, there is a risk that it will disable the "default" ones,
+            // while it could be solved by registering `additional_layer` last, it requires boxing `additional_layer`.
+            .with(additional_layer)
+            .with(layered)
+            .try_init()?;
 
         Ok(TracingHandle {
-            tx: None,
+            tx,
             stdout_writer,
             stderr_writer,
         })