metrics-exporter-prometheus: Add API to initialize HTTP listener from TCP listener

This commit adds a new API to the `PrometheusBuilder` that allows users to provide their own configured TCP listener.
The use case is to enable users to configure the TCP listener before passing it to the exporter - for example, to set socket options or
bind to port 0 and retrieve the port number assigned by the OS.

Signed-off-by: Michel Heily <michel.heily@wiz.io>

Author: michelhe

metrics-exporter-prometheus/src/exporter/builder.rs

@@ -105,6 +105,25 @@ impl PrometheusBuilder {
         self
     }
 
+    /// Same as `with_http_listener`, but allows to bring your own configured tcp listener.
+    #[cfg(feature = "http-listener")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "http-listener")))]
+    #[must_use]
+    pub fn with_http_listener_from_existing_listener(
+        mut self,
+        listener: tokio::net::TcpListener,
+    ) -> Self {
+        use std::sync::Arc;
+
+        // We need to wrap the listener in an Arc to allow for cloning.
+        let listener = Arc::new(listener);
+
+        self.exporter_config = ExporterConfig::HttpListener {
+            destination: super::ListenDestination::ExistingListener(listener),
+        };
+        self
+    }
+
     /// Configures the exporter to push periodic requests to a Prometheus [push gateway].
     ///
     /// Running in push gateway mode is mutually exclusive with the HTTP listener i.e. enabling the push gateway will
@@ -485,11 +504,25 @@ impl PrometheusBuilder {
                 #[cfg(feature = "http-listener")]
                 ExporterConfig::HttpListener { destination } => match destination {
                     super::ListenDestination::Tcp(listen_address) => {
-                        super::http_listener::new_http_listener(
-                            handle,
-                            listen_address,
-                            allowed_addresses,
-                        )?
+                        let listener = std::net::TcpListener::bind(listen_address)
+                            .and_then(|listener| {
+                                listener.set_nonblocking(true)?;
+                                Ok(listener)
+                            })
+                            .map_err(|e| BuildError::FailedToCreateHTTPListener(e.to_string()))?;
+                        let listener = tokio::net::TcpListener::from_std(listener).unwrap();
+                        super::http_listener::new_http_listener(handle, listener, allowed_addresses)
+                    }
+                    super::ListenDestination::ExistingListener(listener) => {
+                        use std::sync::Arc;
+
+                        // Should always succeed as we only created the Arc so the TcpListener can be stored in a Clone
+                        let listener = Arc::try_unwrap(listener).map_err(|_| {
+                            BuildError::FailedToCreateHTTPListener(
+                                "Failed to unwrap Arc<TcpListener>".to_string(),
+                            )
+                        })?;
+                        super::http_listener::new_http_listener(handle, listener, allowed_addresses)
                     }
                     #[cfg(feature = "uds-listener")]
                     super::ListenDestination::Uds(listen_path) => {

metrics-exporter-prometheus/src/exporter/http_listener.rs

@@ -1,5 +1,3 @@
-use std::net::SocketAddr;
-
 use http_body_util::Full;
 use hyper::{
     body::{Bytes, Incoming},
@@ -17,7 +15,7 @@ use tokio::net::{TcpListener, TcpStream};
 use tokio::net::{UnixListener, UnixStream};
 use tracing::warn;
 
-use crate::{common::BuildError, ExporterFuture, PrometheusHandle};
+use crate::{ExporterFuture, PrometheusHandle};
 
 struct HttpListeningExporter {
     handle: PrometheusHandle,
@@ -154,24 +152,16 @@ impl HttpListeningExporter {
 /// Will return Err if it cannot bind to the listen address
 pub(crate) fn new_http_listener(
     handle: PrometheusHandle,
-    listen_address: SocketAddr,
+    listener: TcpListener,
     allowed_addresses: Option<Vec<IpNet>>,
-) -> Result<ExporterFuture, BuildError> {
-    let listener = std::net::TcpListener::bind(listen_address)
-        .and_then(|listener| {
-            listener.set_nonblocking(true)?;
-            Ok(listener)
-        })
-        .map_err(|e| BuildError::FailedToCreateHTTPListener(e.to_string()))?;
-    let listener = TcpListener::from_std(listener).unwrap();
-
+) -> ExporterFuture {
     let exporter = HttpListeningExporter {
         handle,
         allowed_addresses,
         listener_type: ListenerType::Tcp(listener),
     };
 
-    Ok(Box::pin(async move { exporter.serve().await.map_err(super::ExporterError::HttpListener) }))
+    Box::pin(async move { exporter.serve().await.map_err(super::ExporterError::HttpListener) })
 }
 
 /// Creates an `ExporterFuture` implementing a http listener that serves prometheus metrics.

metrics-exporter-prometheus/src/exporter/mod.rs

@@ -2,12 +2,14 @@
 use http_listener::HttpListeningError;
 #[cfg(any(feature = "http-listener", feature = "push-gateway"))]
 use std::future::Future;
-#[cfg(feature = "http-listener")]
-use std::net::SocketAddr;
 #[cfg(any(feature = "http-listener", feature = "push-gateway"))]
 use std::pin::Pin;
 #[cfg(feature = "push-gateway")]
 use std::time::Duration;
+#[cfg(feature = "http-listener")]
+use std::{net::SocketAddr, sync::Arc};
+#[cfg(feature = "http-listener")]
+use tokio::net::TcpListener;
 
 #[cfg(feature = "push-gateway")]
 use hyper::Uri;
@@ -28,6 +30,7 @@ pub type ExporterFuture = Pin<Box<dyn Future<Output = Result<(), ExporterError>>
 #[derive(Clone, Debug)]
 enum ListenDestination {
     Tcp(SocketAddr),
+    ExistingListener(Arc<TcpListener>),
     #[cfg(feature = "uds-listener")]
     Uds(std::path::PathBuf),
 }