feat(tvix/tracing): http trace propagation r/8339

Introduces a helper function within tvix-tracing that returns a reqwest
tracing middleware that will ingest the traceparent if otlp is enabled.

It is feature flagged in tvix-tracing so not every consumer of that
library automatically has reqwest in its dependencies.

Tested using netcat to verify that the `traceparent` header is there if
otlp is enabled and missing if otlp feature is disabled.

Change-Id: I5abccae777b725f5ff7382e3686165383c477a39
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11886
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

5 files changed, 23 insertions, 5 deletions

diff --git a/tvix/tracing/Cargo.toml b/tvix/tracing/Cargo.toml
index bc9a8c3c7792..80892bf7a09f 100644
--- a/tvix/tracing/Cargo.toml
+++ b/tvix/tracing/Cargo.toml
@@ -22,6 +22,8 @@ opentelemetry-http = { version = "0.11.0", optional = true }
 tonic = { version = "0.11.0", optional = true }
 http  = { version = "0.2.11", optional = true }
 
+reqwest-tracing = { version = "0.4.8", default-features = false, optional = true }
+
 [features]
 default = []
 otlp = [
@@ -29,7 +31,8 @@ otlp = [
   "dep:opentelemetry",
   "dep:opentelemetry-otlp",
   "dep:opentelemetry_sdk",
-  "dep:opentelemetry-http"
+  "dep:opentelemetry-http",
+  "reqwest-tracing?/opentelemetry_0_22",
 ]
 tracy = [
   "dep:tracing-tracy"
@@ -38,6 +41,9 @@ tonic = [
   "dep:tonic",
   "dep:http",
 ]
+reqwest = [
+  "dep:reqwest-tracing",
+]
 
 [lints]
 workspace = true
diff --git a/tvix/tracing/default.nix b/tvix/tracing/default.nix
index dd7dc200f2c6..a4ac9de1c8bc 100644
--- a/tvix/tracing/default.nix
+++ b/tvix/tracing/default.nix
@@ -6,6 +6,6 @@
   meta.ci.targets = lib.filter (x: lib.hasPrefix "with-features" x || x == "no-features") (lib.attrNames passthru);
   passthru = depot.tvix.utils.mkFeaturePowerset {
     inherit (old) crateName;
-    features = [ "otlp" "tracy" ];
+    features = [ "otlp" "tracy" "tonic" "reqwest" ];
   };
 })
diff --git a/tvix/tracing/src/lib.rs b/tvix/tracing/src/lib.rs
index b914408ff4c7..35fdcda5a88a 100644
--- a/tvix/tracing/src/lib.rs
+++ b/tvix/tracing/src/lib.rs
@@ -17,7 +17,6 @@ use opentelemetry_sdk::{
 #[cfg(feature = "tracy")]
 use tracing_tracy::TracyLayer;
 
-#[cfg(feature = "tonic")] // TODO or http
 pub mod propagate;
 
 lazy_static! {
diff --git a/tvix/tracing/src/propagate/mod.rs b/tvix/tracing/src/propagate/mod.rs
index 42c532e9d8cf..9a7e4332b637 100644
--- a/tvix/tracing/src/propagate/mod.rs
+++ b/tvix/tracing/src/propagate/mod.rs
@@ -1,8 +1,8 @@
 #[cfg(feature = "tonic")]
 pub mod tonic;
 
-// TODO: Helper library for reqwest. We could use
-// https://github.com/TrueLayer/reqwest-middleware/tree/main/reqwest-tracing to realise this
+#[cfg(feature = "reqwest")]
+pub mod reqwest;
 
 // TODO: Helper library for axum or another http server, see
 // https://github.com/hseeberger/hello-tracing-rs/blob/main/hello-tracing-common/src/otel/http.rs
diff --git a/tvix/tracing/src/propagate/reqwest.rs b/tvix/tracing/src/propagate/reqwest.rs
new file mode 100644
index 000000000000..e2afb3e948c7
--- /dev/null
+++ b/tvix/tracing/src/propagate/reqwest.rs
@@ -0,0 +1,13 @@
+use reqwest_tracing::{SpanBackendWithUrl, TracingMiddleware};
+
+/// Returns a new tracing middleware which can be used with reqwest_middleware.
+/// It will then write the `traceparent` in the header on the request and additionally records the
+/// `url` into `http.url`.
+///
+/// If otlp feature is disabled, this will not insert a `traceparent` into the header. It will
+/// basically function as a noop.
+///
+/// `traceparent` => https://www.w3.org/TR/trace-context/#trace-context-http-headers-format
+pub fn tracing_middleware() -> TracingMiddleware<SpanBackendWithUrl> {
+    TracingMiddleware::<SpanBackendWithUrl>::new()
+}