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

@@ -55,47 +55,94 @@
         "type": "github"
       }
     },
+    "flake-utils_2": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1710146030,
+        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1731094700,
-        "narHash": "sha256-lSiVjHP7sgnCt2hZabnq+tCLmBerDKmAdd2CS6BrBjw=",
+        "lastModified": 1714030708,
+        "narHash": "sha256-JOGPOxa8N6ySzB7SQBsh0OVz+UXZriyahgvfNHMIY0Y=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "551ba0fa7653afb9d590db225c3bcbccf68931c0",
+        "rev": "b0d52b31f7f4d80f8bf38f0253652125579c35ff",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "nixpkgs_2": {
+      "locked": {
+        "lastModified": 1707091808,
+        "narHash": "sha256-LahKBAfGbY836gtpVNnWwBTIzN7yf/uYM/S0g393r0Y=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "9f2ee8c91ac42da3ae6c6a1d21555f283458247e",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "551ba0fa7653afb9d590db225c3bcbccf68931c0",
+        "rev": "9f2ee8c91ac42da3ae6c6a1d21555f283458247e",
         "type": "github"
       }
     },
     "root": {
       "inputs": {
         "cargo2nix": "cargo2nix",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs_2"
       }
     },
     "rust-overlay": {
       "inputs": {
-        "nixpkgs": [
-          "cargo2nix",
-          "nixpkgs"
-        ]
+        "flake-utils": "flake-utils_2",
+        "nixpkgs": "nixpkgs"
       },
       "locked": {
-        "lastModified": 1731032894,
-        "narHash": "sha256-dQSyYPmrQiPr+PGEd+K8038rubFGz7G/dNXVeaGWE0w=",
+        "lastModified": 1707271822,
+        "narHash": "sha256-/DZsoPH5GBzOpVEGz5PgJ7vh8Q6TcrJq5u8FcBjqAfI=",
         "owner": "oxalica",
         "repo": "rust-overlay",
-        "rev": "d52f2a4c103a0acf09ded857b9e2519ae2360e59",
+        "rev": "7a94fe7690d2bdfe1aab475382a505e14dc114a6",
         "type": "github"
       },
       "original": {
         "owner": "oxalica",
         "repo": "rust-overlay",
-        "rev": "d52f2a4c103a0acf09ded857b9e2519ae2360e59",
+        "rev": "7a94fe7690d2bdfe1aab475382a505e14dc114a6",
+        "type": "github"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
         "type": "github"
       }
     }

flake.nix

@@ -1,20 +1,16 @@
 {
   description = "Tricot, a reverse proxy with consul integration";
 
-  # Nixpkgs 24.05 as of 2024-11-08, has rustc v1.77.2
+  # Nixpkgs 23.11 as of 2024-02-07, has rustc v1.73
   inputs.nixpkgs.url =
-    "github:NixOS/nixpkgs/551ba0fa7653afb9d590db225c3bcbccf68931c0";
+    "github:NixOS/nixpkgs/9f2ee8c91ac42da3ae6c6a1d21555f283458247e";
   inputs.cargo2nix = {
     # cargo2nix as of 2024-04-25
-    # NB: upgrading to a more recent commit of cargo2nix will not work (as of 2024-11-08),
-    # because the patch making openssl-sys cross-compilation work has been reverted.
-    # (patch: https://github.com/cargo2nix/cargo2nix/pull/237,
-    #  revert: https://github.com/cargo2nix/cargo2nix/commit/cfd086deb565314f3a11b5bb25807a3ce17315d4)
     url = "github:cargo2nix/cargo2nix/1efb03f2f794ad5eed17e807e858c4da001dbc3e";
 
-    # Rust overlay as of 2024-11-08
+    # Rust overlay as of 2024-02-07
     inputs.rust-overlay.url =
-      "github:oxalica/rust-overlay/d52f2a4c103a0acf09ded857b9e2519ae2360e59";
+      "github:oxalica/rust-overlay/7a94fe7690d2bdfe1aab475382a505e14dc114a6";
 
     inputs.nixpkgs.follows = "nixpkgs";
   };
@@ -31,7 +27,7 @@
       overlays = [ cargo2nix.overlays.default ];
     };
     packageFun = import ./Cargo.nix;
-    rustVersion = "1.77.2";
+    rustVersion = "1.68.0";
 
     compile = args: compileMode:
       let
@@ -49,13 +45,5 @@
     debug.x86_64-linux.tricot = compile { release = false; } "build";
     packages.x86_64-linux.tricot = compile { release = true; } "build";
     packages.x86_64-linux.default = self.packages.x86_64-linux.tricot;
-
-    docker = pkgs.dockerTools.buildImage {
-      name = "tricot";
-      config = {
-        contents = [ pkgs.cacert ];
-        Cmd = [ "${self.packages.x86_64-linux.default}/bin/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);

src/main.rs

@@ -132,7 +132,6 @@ async fn main() {
 	let opt = Opt::from_args();
 
 	info!("Starting Tricot");
-	println!("Starting Tricot");
 
 	let (exit_signal, provoke_exit) = watch_ctrl_c();
 	let exit_on_err = move |err: anyhow::Error| {