Have more tags in the in_flight_requests meter (eg "service")

The host tag is not currently included, but could be added later with
some refactorings if we need it.

flake.lock

@@ -10,17 +10,17 @@
         "rust-overlay": "rust-overlay"
       },
       "locked": {
-        "lastModified": 1666087781,
-        "narHash": "sha256-trKVdjMZ8mNkGfLcY5LsJJGtdV3xJDZnMVrkFjErlcs=",
-        "owner": "Alexis211",
+        "lastModified": 1713199118,
+        "narHash": "sha256-MlLdAvk+zXCFUy280sY6LqtykqWXIkKVXo72J7a6HlU=",
+        "owner": "cargo2nix",
         "repo": "cargo2nix",
-        "rev": "a7a61179b66054904ef6a195d8da736eaaa06c36",
+        "rev": "1efb03f2f794ad5eed17e807e858c4da001dbc3e",
         "type": "github"
       },
       "original": {
-        "owner": "Alexis211",
+        "owner": "cargo2nix",
         "repo": "cargo2nix",
-        "rev": "a7a61179b66054904ef6a195d8da736eaaa06c36",
+        "rev": "1efb03f2f794ad5eed17e807e858c4da001dbc3e",
         "type": "github"
       }
     },
@@ -60,11 +60,11 @@
         "systems": "systems"
       },
       "locked": {
-        "lastModified": 1694529238,
-        "narHash": "sha256-zsNZZGTGnMOf9YpHKJqMSsa0dXbfmxeoJ7xHlrt+xmY=",
+        "lastModified": 1710146030,
+        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
         "owner": "numtide",
         "repo": "flake-utils",
-        "rev": "ff7b65b44d01cf9ba6a71320833626af21126384",
+        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
         "type": "github"
       },
       "original": {
@@ -75,11 +75,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1696234590,
-        "narHash": "sha256-mgOzQYTvaTT4bFopVOadlndy2RPwLy60rDjIWOGujwo=",
+        "lastModified": 1714030708,
+        "narHash": "sha256-JOGPOxa8N6ySzB7SQBsh0OVz+UXZriyahgvfNHMIY0Y=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "f902cb49892d300ff15cb237e48aa1cad79d68c3",
+        "rev": "b0d52b31f7f4d80f8bf38f0253652125579c35ff",
         "type": "github"
       },
       "original": {
@@ -91,17 +91,17 @@
     },
     "nixpkgs_2": {
       "locked": {
-        "lastModified": 1665657542,
-        "narHash": "sha256-mojxNyzbvmp8NtVtxqiHGhRfjCALLfk9i/Uup68Y5q8=",
+        "lastModified": 1707091808,
+        "narHash": "sha256-LahKBAfGbY836gtpVNnWwBTIzN7yf/uYM/S0g393r0Y=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "a3073c49bc0163fea6a121c276f526837672b555",
+        "rev": "9f2ee8c91ac42da3ae6c6a1d21555f283458247e",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "a3073c49bc0163fea6a121c276f526837672b555",
+        "rev": "9f2ee8c91ac42da3ae6c6a1d21555f283458247e",
         "type": "github"
       }
     },
@@ -117,16 +117,17 @@
         "nixpkgs": "nixpkgs"
       },
       "locked": {
-        "lastModified": 1682389182,
-        "narHash": "sha256-8t2nmFnH+8V48+IJsf8AK51ebXNlVbOSVYOpiqJKvJE=",
+        "lastModified": 1707271822,
+        "narHash": "sha256-/DZsoPH5GBzOpVEGz5PgJ7vh8Q6TcrJq5u8FcBjqAfI=",
         "owner": "oxalica",
         "repo": "rust-overlay",
-        "rev": "74f1a64dd28faeeb85ef081f32cad2989850322c",
+        "rev": "7a94fe7690d2bdfe1aab475382a505e14dc114a6",
         "type": "github"
       },
       "original": {
         "owner": "oxalica",
         "repo": "rust-overlay",
+        "rev": "7a94fe7690d2bdfe1aab475382a505e14dc114a6",
         "type": "github"
       }
     },

flake.nix

@@ -1,22 +1,29 @@
 {
-  description = "A very basic flake";
+  description = "Tricot, a reverse proxy with consul integration";
 
-  inputs.nixpkgs.url = "github:NixOS/nixpkgs/a3073c49bc0163fea6a121c276f526837672b555";
+  # Nixpkgs 23.11 as of 2024-02-07, has rustc v1.73
+  inputs.nixpkgs.url =
+    "github:NixOS/nixpkgs/9f2ee8c91ac42da3ae6c6a1d21555f283458247e";
   inputs.cargo2nix = {
-    # As of 2022-10-18: two small patches over unstable branch, one for clippy and one to fix feature detection
-    url = "github:Alexis211/cargo2nix/a7a61179b66054904ef6a195d8da736eaaa06c36";
+    # cargo2nix as of 2024-04-25
+    url = "github:cargo2nix/cargo2nix/1efb03f2f794ad5eed17e807e858c4da001dbc3e";
 
-    # Rust overlay as of 2023-04-25
+    # Rust overlay as of 2024-02-07
     inputs.rust-overlay.url =
-      "github:oxalica/rust-overlay/74f1a64dd28faeeb85ef081f32cad2989850322c";
+      "github:oxalica/rust-overlay/7a94fe7690d2bdfe1aab475382a505e14dc114a6";
 
     inputs.nixpkgs.follows = "nixpkgs";
   };
 
   outputs = { self, nixpkgs, cargo2nix }:
   let
+    targetHost = "x86_64-unknown-linux-musl";
     pkgs = import nixpkgs {
       system = "x86_64-linux";
+      crossSystem = {
+        config = targetHost;
+        isStatic = true;
+      };
       overlays = [ cargo2nix.overlays.default ];
     };
     packageFun = import ./Cargo.nix;
@@ -26,6 +33,7 @@
       let
         packageSet = pkgs.rustBuilder.makePackageSet ({
           inherit packageFun rustVersion;
+          target = targetHost;
         } // args);
       in
         packageSet.workspace.tricot {

src/https.rs

@@ -41,9 +41,21 @@ pub struct HttpsConfig {
 struct HttpsMetrics {
 	requests_received: metrics::Counter<u64>,
 	requests_served: metrics::Counter<u64>,
+	requests_in_flight: metrics::UpDownCounter<i64>,
 	request_proxy_duration: metrics::Histogram<f64>,
 }
 
+struct InFlightGuard<'a, 'b> {
+	metrics: &'a HttpsMetrics,
+	tags: &'b [KeyValue],
+}
+
+impl<'a, 'b> Drop for InFlightGuard<'a, 'b> {
+	fn drop(&mut self) {
+		self.metrics.requests_in_flight.add(-1, self.tags)
+	}
+}
+
 pub async fn serve_https(
 	config: HttpsConfig,
 	cert_store: Arc<CertStore>,
@@ -62,6 +74,10 @@ pub async fn serve_https(
 			.u64_counter("https_requests_served")
 			.with_description("Total number of requests served over HTTPS")
 			.init(),
+		requests_in_flight: meter
+			.i64_up_down_counter("https_requests_in_flight")
+			.with_description("Current number of requests handled over HTTPS")
+			.init(),
 		request_proxy_duration: meter
 			.f64_histogram("https_request_proxy_duration")
 			.with_description("Duration between time when request was received, and time when backend returned status code and headers")
@@ -178,9 +194,7 @@ async fn handle_request(
 			.unwrap_or_default(),
 	);
 
-	metrics
-		.requests_received
-		.add(1, &[host_tag, method_tag.clone()]);
+	metrics.requests_received.add(1, &[host_tag, method_tag.clone()]);
 
 	let mut tags = vec![method_tag];
 	let resp = select_target_and_proxy(
@@ -272,6 +286,17 @@ async fn select_target_and_proxy(
 		);
 		proxy_to.calls_in_progress.fetch_add(1, Ordering::SeqCst);
 
+		let tags_in_flight = &tags.clone();
+		metrics.requests_in_flight.add(1, &tags_in_flight);
+		// The guard ensures that we decrement requests_in_flight in all cases where
+		// the current tasks ends, including the case where it gets canceled and
+		// doesn't run to completion (which may happen e.g. if it timeouts).
+		// (Crucially we create the guard before the first .await in this function.)
+		let _guard = InFlightGuard {
+			metrics: &metrics,
+			tags: &tags_in_flight,
+		};
+
 		// Forward to backend
 		debug!("{}{} -> {}", host, path, proxy_to);
 		trace!("Request: {:?}", req);