Remove use of tsc_await in RegistryManager.discover

Author: mattt

Diff for: Sources/PackageRegistry/RegistryManager.swift

@@ -62,36 +62,43 @@ public final class RegistryManager {
             }
         }
 
-        do {
-            let manager = try cache.memoize(url) {
-                let request = HTTPClient.Request(
-                    method: .head,
-                    url: url,
-                    headers: [
-                        "Accept": "application/vnd.swift.registry.v1"
-                    ]
-                )
-
-                let client = clientFactory(diagnosticsEngine)
-
-                let response = try tsc_await { client.execute(request, callback: $0) }
-                if response.statusCode == 303,
-                   response.headers.get("Content-Version").first == "1",
-                   let location = response.headers.get("Location").first,
-                   let redirectedURL = URL(string: location, relativeTo: url)
-                {
-                    return RegistryManager(registryBaseURL: redirectedURL, diagnosticEngine: diagnosticsEngine)
-                } else {
-                    throw RegistryError.invalidResponse
-                }
-            }
-
+        if let manager = cache[url] {
             queue.async {
                 completion(.success(manager))
             }
-        } catch {
-            queue.async {
-                completion(.failure(error))
+        } else {
+            let request = HTTPClient.Request(
+                method: .head,
+                url: url,
+                headers: [
+                    "Accept": "application/vnd.swift.registry.v1"
+                ]
+            )
+
+            let client = clientFactory(diagnosticsEngine)
+            client.execute(request) { result in
+                switch result {
+                case .success(let response):
+                    if response.statusCode == 303,
+                       response.headers.get("Content-Version").first == "1",
+                       let location = response.headers.get("Location").first,
+                       let redirectedURL = URL(string: location, relativeTo: url)
+                    {
+                        let manager = RegistryManager(registryBaseURL: redirectedURL, diagnosticEngine: diagnosticsEngine)
+                        self.cache[url] = manager
+                        queue.async {
+                            completion(.success(manager))
+                        }
+                    } else {
+                        queue.async {
+                            completion(.failure(RegistryError.invalidResponse))
+                        }
+                    }
+                case .failure(let error):
+                    queue.async {
+                        completion(.failure(error))
+                    }
+                }
             }
         }
     }