move resolver logic into the resolver

Honestly not sure why it wasn't done like this before. This code is much
less awkward to follow and more compartmentalized.

These changes were mainly motivated by a clippy lint triggering on the
original code, which then made me wonder if I could get rid of some of
the `Box`ing. Turns out I could, and this is the result of that.

src/service/globals/mod.rs

@@ -109,11 +109,11 @@ impl Default for RotationHandler {
 
 pub struct Resolver {
     inner: GaiResolver,
-    overrides: Box<dyn Fn(&str) -> Option<SocketAddr> + Send + Sync>,
+    overrides: Arc<RwLock<TlsNameMap>>,
 }
 
 impl Resolver {
-    pub fn new(overrides: Box<dyn Fn(&str) -> Option<SocketAddr> + Send + Sync>) -> Resolver {
+    pub fn new(overrides: Arc<RwLock<TlsNameMap>>) -> Self {
         Resolver {
             inner: GaiResolver::new(),
             overrides,
@@ -123,16 +123,26 @@ impl Resolver {
 
 impl Resolve for Resolver {
     fn resolve(&self, name: Name) -> Resolving {
-        if let Some(addr) = (self.overrides)(name.as_str()) {
-            let once: Box<dyn Iterator<Item = SocketAddr> + Send> = Box::new(iter::once(addr));
-            return Box::pin(future::ready(Ok(once)));
-        }
-        let this = &mut self.inner.clone();
-        Box::pin(HyperService::<Name>::call(this, name).map(|result| {
-            result
-                .map(|addrs| -> Addrs { Box::new(addrs) })
-                .map_err(|err| -> Box<dyn StdError + Send + Sync> { Box::new(err) })
-        }))
+        self.overrides
+            .read()
+            .expect("lock should not be poisoned")
+            .get(name.as_str())
+            .and_then(|(override_name, port)| {
+                override_name.get(0).map(|first_name| {
+                    let x: Box<dyn Iterator<Item = SocketAddr> + Send> =
+                        Box::new(iter::once(SocketAddr::new(*first_name, *port)));
+                    let x: Resolving = Box::pin(future::ready(Ok(x)));
+                    x
+                })
+            })
+            .unwrap_or_else(|| {
+                let this = &mut self.inner.clone();
+                Box::pin(HyperService::<Name>::call(this, name).map(|result| {
+                    result
+                        .map(|addrs| -> Addrs { Box::new(addrs) })
+                        .map_err(|err| -> Box<dyn StdError + Send + Sync> { Box::new(err) })
+                }))
+            })
     }
 }
 
@@ -157,14 +167,8 @@ impl Service {
             .map(|secret| jsonwebtoken::DecodingKey::from_secret(secret.as_bytes()));
 
         let default_client = reqwest_client_builder(&config)?.build()?;
-        let name_override = Arc::clone(&tls_name_override);
         let federation_client = reqwest_client_builder(&config)?
-            .dns_resolver(Arc::new(Resolver::new(Box::new(move |domain| {
-                let read_guard = name_override.read().unwrap();
-                let (override_name, port) = read_guard.get(domain)?;
-                let first_name = override_name.get(0)?;
-                Some(SocketAddr::new(*first_name, *port))
-            }))))
+            .dns_resolver(Arc::new(Resolver::new(tls_name_override.clone())))
             .build()?;
 
         // Supported and stable room versions