Add ability to fetch discovery URLs (#235)

## Description

This implements `Server::discovery_urls()` to retrieve the server's own
URL even when the port number was chosen randomly (i.e. the operating
system picked an arbitrary free TCP port to listen on).

Due to how `ServerRunner::run()` is implemented, this requires
`ServerRunner::run_until_cancelled()` for now.

Author: sgoll

Cargo.lock

@@ -21,16 +21,13 @@ futures-channel = "0.3.30"
 futures-core = { version = "0.3.30", default-features = false }
 log = "0.4.20"
 open62541-sys = "0.4.16"
+parking_lot = "0.12.4"
 paste = "1.0.14"
 serde = { version = "1.0.194", optional = true }
 serde_json = { version = "1.0.111", optional = true }
 thiserror = "2.0.3"
 time = { version = "0.3.38", optional = true }
-tokio = { version = "1.38.2", optional = true, features = [
-  "rt",
-  "sync",
-  "time",
-] }
+tokio = { version = "1.38.2", optional = true, features = ["rt", "sync", "time"] }
 uuid = { version = "1.16.0", optional = true }
 x509-certificate = { version = "0.24.0", optional = true }
 zeroize = { version = "1.8.1", optional = true }
@@ -49,13 +46,7 @@ tokio = { version = "1.35.1", features = ["macros", "rt-multi-thread"] }
 [features]
 default = ["serde", "time", "tokio", "uuid"]
 mbedtls = ["dep:zeroize", "open62541-sys/mbedtls"]
-serde = [
-  "dep:serde",
-  "dep:serde_json",
-  "time?/formatting",
-  "time?/serde",
-  "uuid?/serde",
-]
+serde = ["dep:serde", "dep:serde_json", "time?/formatting", "time?/serde", "uuid?/serde"]
 time = ["dep:time"]
 tokio = ["dep:tokio"]
 uuid = ["dep:uuid"]
@@ -199,6 +190,9 @@ required-features = ["mbedtls"]
 [[example]]
 name = "server_method_callback"
 
+[[example]]
+name = "server_random_port"
+
 [[example]]
 name = "ssl_create_certificate"
 required-features = ["mbedtls", "x509"]

Cargo.toml

@@ -0,0 +1,34 @@
+use std::thread;
+
+use open62541::ServerBuilder;
+
+fn main() {
+    env_logger::init();
+
+    let (server, runner) = ServerBuilder::default()
+        .server_urls(&["opc.tcp://127.0.0.1:0"])
+        .build();
+
+    let handle = thread::spawn(|| runner.run_until_cancelled(&mut || true).unwrap());
+
+    if let Some(urls) = server.discovery_urls() {
+        println!("Discovery URLs: {urls:?}");
+
+        if let Some(tail) = urls
+            .as_slice()
+            .first()
+            .and_then(|url| url.as_str())
+            .and_then(|url| url.strip_prefix("opc.tcp://127.0.0.1:"))
+        {
+            if let Ok(port) = tail.parse::<u16>() {
+                println!("Server running on port {port}");
+            } else {
+                println!("Unable to find port");
+            }
+        }
+    } else {
+        println!("No discovery URLs");
+    }
+
+    handle.join().unwrap();
+}