feat(tvix/tracing): gRPC trace context propagation r/8299

This introduces optional helper function in tvix/tracing for trace
propagation and uses these helper in the `tvix-store`.

The GRPCBlobService, GRPCDirectoryService and GRPCPathInfoService now
accept a generic client, meaning the client can be generated with either
`::new` or `::with_interceptor`.

This was tested and validated by starting a `tvix-store daemon` and
`tvix-store import`.

Change-Id: I4b194483bf09266820104b4b56e4a135dca2b77a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11863
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

2 files changed, 24 insertions, 10 deletions

diff --git a/tvix/castore/src/directoryservice/from_addr.rs b/tvix/castore/src/directoryservice/from_addr.rs
index ee675ca68a9f..9aa01df171d7 100644
--- a/tvix/castore/src/directoryservice/from_addr.rs
+++ b/tvix/castore/src/directoryservice/from_addr.rs
@@ -63,8 +63,12 @@ pub async fn from_addr(uri: &str) -> Result<Box<dyn DirectoryService>, crate::Er
             // - In the case of unix sockets, there must be a path, but may not be a host.
             // - In the case of non-unix sockets, there must be a host, but no path.
             // Constructing the channel is handled by tvix_castore::channel::from_url.
-            let client = DirectoryServiceClient::new(crate::tonic::channel_from_url(&url).await?);
-            Box::new(GRPCDirectoryService::from_client(client))
+            Box::new(GRPCDirectoryService::from_client(
+                DirectoryServiceClient::with_interceptor(
+                    crate::tonic::channel_from_url(&url).await?,
+                    tvix_tracing::propagate::tonic::send_trace,
+                ),
+            ))
         }
         scheme if scheme.starts_with("objectstore+") => {
             // We need to convert the URL to string, strip the prefix there, and then
diff --git a/tvix/castore/src/directoryservice/grpc.rs b/tvix/castore/src/directoryservice/grpc.rs
index 24e498a997ef..54d7575adcc1 100644
--- a/tvix/castore/src/directoryservice/grpc.rs
+++ b/tvix/castore/src/directoryservice/grpc.rs
@@ -9,31 +9,41 @@ use tokio::spawn;
 use tokio::sync::mpsc::UnboundedSender;
 use tokio::task::JoinHandle;
 use tokio_stream::wrappers::UnboundedReceiverStream;
-use tonic::async_trait;
-use tonic::Code;
-use tonic::{transport::Channel, Status};
+use tonic::{async_trait, Code, Status};
 use tracing::{instrument, warn, Instrument as _};
 
 /// Connects to a (remote) tvix-store DirectoryService over gRPC.
 #[derive(Clone)]
-pub struct GRPCDirectoryService {
+pub struct GRPCDirectoryService<T>
+where
+    T: Clone,
+{
     /// The internal reference to a gRPC client.
     /// Cloning it is cheap, and it internally handles concurrent requests.
-    grpc_client: proto::directory_service_client::DirectoryServiceClient<Channel>,
+    grpc_client: proto::directory_service_client::DirectoryServiceClient<T>,
 }
 
-impl GRPCDirectoryService {
+impl<T> GRPCDirectoryService<T>
+where
+    T: tonic::client::GrpcService<tonic::body::BoxBody> + Clone,
+{
     /// construct a [GRPCDirectoryService] from a [proto::directory_service_client::DirectoryServiceClient].
     /// panics if called outside the context of a tokio runtime.
     pub fn from_client(
-        grpc_client: proto::directory_service_client::DirectoryServiceClient<Channel>,
+        grpc_client: proto::directory_service_client::DirectoryServiceClient<T>,
     ) -> Self {
         Self { grpc_client }
     }
 }
 
 #[async_trait]
-impl DirectoryService for GRPCDirectoryService {
+impl<T> DirectoryService for GRPCDirectoryService<T>
+where
+    T: tonic::client::GrpcService<tonic::body::BoxBody> + Send + Sync + Clone + 'static,
+    T::ResponseBody: tonic::codegen::Body<Data = tonic::codegen::Bytes> + Send + 'static,
+    <T::ResponseBody as tonic::codegen::Body>::Error: Into<tonic::codegen::StdError> + Send,
+    T::Future: Send,
+{
     #[instrument(level = "trace", skip_all, fields(directory.digest = %digest))]
     async fn get(
         &self,