feat(transport): add user-agent header to client requests. (#457)

* Add a default user-agent header to outgoing requests.
* The user agent can be configured through the `Channel` builder.

fixes #453

tonic/src/transport/channel/endpoint.rs

@@ -6,7 +6,10 @@ use super::ClientTlsConfig;
 use crate::transport::service::TlsConnector;
 use crate::transport::Error;
 use bytes::Bytes;
-use http::uri::{InvalidUri, Uri};
+use http::{
+    uri::{InvalidUri, Uri},
+    HeaderValue,
+};
 use std::{
     convert::{TryFrom, TryInto},
     fmt,
@@ -20,6 +23,7 @@ use tower_make::MakeConnection;
 #[derive(Clone)]
 pub struct Endpoint {
     pub(crate) uri: Uri,
+    pub(crate) user_agent: Option<HeaderValue>,
     pub(crate) timeout: Option<Duration>,
     pub(crate) concurrency_limit: Option<usize>,
     pub(crate) rate_limit: Option<(u64, Duration)>,
@@ -74,6 +78,30 @@ impl Endpoint {
         Ok(Self::from(uri))
     }
 
+    /// Set a custom user-agent header.
+    ///
+    /// `user_agent` will be prepended to Tonic's default user-agent string (`tonic/x.x.x`).
+    /// It must be a value that can be converted into a valid  `http::HeaderValue` or building
+    /// the endpoint will fail.
+    /// ```
+    /// # use tonic::transport::Endpoint;
+    /// # let mut builder = Endpoint::from_static("https://example.com");
+    /// builder.user_agent("Greeter").expect("Greeter should be a valid header value");
+    /// // user-agent: "Greeter tonic/x.x.x"
+    /// ```
+    pub fn user_agent<T>(self, user_agent: T) -> Result<Self, Error>
+    where
+        T: TryInto<HeaderValue>,
+    {
+        user_agent
+            .try_into()
+            .map(|ua| Endpoint {
+                user_agent: Some(ua),
+                ..self
+            })
+            .map_err(|_| Error::new_invalid_user_agent())
+    }
+
     /// Apply a timeout to each request.
     ///
     /// ```
@@ -276,6 +304,7 @@ impl From<Uri> for Endpoint {
     fn from(uri: Uri) -> Self {
         Self {
             uri,
+            user_agent: None,
             concurrency_limit: None,
             rate_limit: None,
             timeout: None,

tonic/src/transport/error.rs

@@ -21,6 +21,7 @@ struct ErrorImpl {
 pub(crate) enum Kind {
     Transport,
     InvalidUri,
+    InvalidUserAgent,
 }
 
 impl Error {
@@ -43,10 +44,15 @@ impl Error {
         Error::new(Kind::InvalidUri)
     }
 
+    pub(crate) fn new_invalid_user_agent() -> Self {
+        Error::new(Kind::InvalidUserAgent)
+    }
+
     fn description(&self) -> &str {
         match &self.inner.kind {
             Kind::Transport => "transport error",
             Kind::InvalidUri => "invalid URI",
+            Kind::InvalidUserAgent => "user agent is not a valid header value",
         }
     }
 }

tonic/src/transport/service/connection.rs

@@ -1,4 +1,4 @@
-use super::{layer::ServiceBuilderExt, reconnect::Reconnect, AddOrigin};
+use super::{layer::ServiceBuilderExt, reconnect::Reconnect, AddOrigin, UserAgent};
 use crate::{body::BoxBody, transport::Endpoint};
 use http::Uri;
 use hyper::client::conn::Builder;
@@ -55,6 +55,7 @@ impl Connection {
 
         let stack = ServiceBuilder::new()
             .layer_fn(|s| AddOrigin::new(s, endpoint.uri.clone()))
+            .layer_fn(|s| UserAgent::new(s, endpoint.user_agent.clone()))
             .optional_layer(endpoint.timeout.map(TimeoutLayer::new))
             .optional_layer(endpoint.concurrency_limit.map(ConcurrencyLimitLayer::new))
             .optional_layer(endpoint.rate_limit.map(|(l, d)| RateLimitLayer::new(l, d)))

tonic/src/transport/service/mod.rs

@@ -8,6 +8,7 @@ mod reconnect;
 mod router;
 #[cfg(feature = "tls")]
 mod tls;
+mod user_agent;
 
 pub(crate) use self::add_origin::AddOrigin;
 pub(crate) use self::connection::Connection;
@@ -18,3 +19,4 @@ pub(crate) use self::layer::ServiceBuilderExt;
 pub(crate) use self::router::{Or, Routes};
 #[cfg(feature = "tls")]
 pub(crate) use self::tls::{TlsAcceptor, TlsConnector};
+pub(crate) use self::user_agent::UserAgent;

tonic/src/transport/service/user_agent.rs

@@ -0,0 +1,70 @@
+use http::{header::USER_AGENT, HeaderValue, Request};
+use std::task::{Context, Poll};
+use tower_service::Service;
+
+const TONIC_USER_AGENT: &str = concat!("tonic/", env!("CARGO_PKG_VERSION"));
+
+#[derive(Debug)]
+pub(crate) struct UserAgent<T> {
+    inner: T,
+    user_agent: HeaderValue,
+}
+
+impl<T> UserAgent<T> {
+    pub(crate) fn new(inner: T, user_agent: Option<HeaderValue>) -> Self {
+        let user_agent = user_agent
+            .map(|value| {
+                let mut buf = Vec::new();
+                buf.extend(value.as_bytes());
+                buf.push(b' ');
+                buf.extend(TONIC_USER_AGENT.as_bytes());
+                HeaderValue::from_bytes(&buf).expect("user-agent should be valid")
+            })
+            .unwrap_or(HeaderValue::from_static(TONIC_USER_AGENT));
+
+        Self { inner, user_agent }
+    }
+}
+
+impl<T, ReqBody> Service<Request<ReqBody>> for UserAgent<T>
+where
+    T: Service<Request<ReqBody>>,
+{
+    type Response = T::Response;
+    type Error = T::Error;
+    type Future = T::Future;
+
+    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.inner.poll_ready(cx)
+    }
+
+    fn call(&mut self, mut req: Request<ReqBody>) -> Self::Future {
+        req.headers_mut()
+            .insert(USER_AGENT, self.user_agent.clone());
+
+        self.inner.call(req)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    struct Svc;
+
+    #[test]
+    fn sets_default_if_no_custom_user_agent() {
+        assert_eq!(
+            UserAgent::new(Svc, None).user_agent,
+            HeaderValue::from_static(TONIC_USER_AGENT)
+        )
+    }
+
+    #[test]
+    fn prepends_custom_user_agent_to_default() {
+        assert_eq!(
+            UserAgent::new(Svc, Some(HeaderValue::from_static("Greeter 1.1"))).user_agent,
+            HeaderValue::from_str(&format!("Greeter 1.1 {}", TONIC_USER_AGENT)).unwrap()
+        )
+    }
+}