More informative errors from RegistryClient (#134)

Also creates an `ErrorResponse` type to model the errors typically
returned by a container registry.

Reference: https://distribution.github.io/distribution/spec/api/#errors

Example error message:
```
Error: HTTP request to https://ghcr.io/token?client_id=containerization-registry-client&service=ghcr.io&scope=repository:user/image:pull failed with response: 403 Forbidden. Reason: {"errors":[{"message":"requested access to the resource is denied","code":"DENIED"}]}
```

Signed-off-by: Aditya Ramani <a_ramani@apple.com>

Sources/ContainerizationOCI/Client/RegistryClient+Error.swift

@@ -14,19 +14,56 @@
 // limitations under the License.
 //===----------------------------------------------------------------------===//
 
+import AsyncHTTPClient
+import Foundation
 import NIOHTTP1
 
 extension RegistryClient {
     /// `RegistryClient` errors.
     public enum Error: Swift.Error, CustomStringConvertible {
-        case invalidStatus(url: String, HTTPResponseStatus)
+        case invalidStatus(url: String, HTTPResponseStatus, reason: String? = nil)
 
         /// Description of the errors.
         public var description: String {
             switch self {
-            case .invalidStatus(let u, let response):
-                return "HTTP request to \(u) failed with response: \(response.description)"
+            case .invalidStatus(let u, let response, let reason):
+                return "HTTP request to \(u) failed with response: \(response.description). Reason: \(reason ?? "Unknown")"
             }
         }
     }
+
+    /// The container registry typically returns actionable failure reasons in the response body
+    /// of the failing HTTP Request. This type models the structure of the error message.
+    /// Reference: https://distribution.github.io/distribution/spec/api/#errors
+    internal struct ErrorResponse: Codable {
+        let errors: [RemoteError]
+
+        internal struct RemoteError: Codable {
+            let code: String
+            let message: String
+            let detail: String?
+        }
+
+        internal static func fromResponseBody(_ body: HTTPClientResponse.Body) async -> ErrorResponse? {
+            guard var buffer = try? await body.collect(upTo: Int(1.mib())) else {
+                return nil
+            }
+            guard let bytes = buffer.readBytes(length: buffer.readableBytes) else {
+                return nil
+            }
+            let data = Data(bytes)
+            guard let jsonError = try? JSONDecoder().decode(ErrorResponse.self, from: data) else {
+                return nil
+            }
+            return jsonError
+        }
+
+        public var jsonString: String {
+            let data = try? JSONEncoder().encode(self)
+            guard let data else {
+                return "{}"
+            }
+            return String(data: data, encoding: .utf8) ?? "{}"
+        }
+    }
 }

Sources/ContainerizationOCI/Client/RegistryClient+Fetch.swift

@@ -49,7 +49,8 @@ extension RegistryClient {
         return try await request(components: components, method: .HEAD, headers: headers) { response in
             guard response.status == .ok else {
                 let url = components.url?.absoluteString ?? "unknown"
-                throw Error.invalidStatus(url: url, response.status)
+                let reason = await ErrorResponse.fromResponseBody(response.body)?.jsonString
+                throw Error.invalidStatus(url: url, response.status, reason: reason)
             }
 
             guard let digest = response.headers.first(name: "Docker-Content-Digest") else {
@@ -150,7 +151,8 @@ extension RegistryClient {
         try await request(components: components, headers: headers) { response in
             guard response.status == .ok else {
                 let url = components.url?.absoluteString ?? "unknown"
-                throw Error.invalidStatus(url: url, response.status)
+                let reason = await ErrorResponse.fromResponseBody(response.body)?.jsonString
+                throw Error.invalidStatus(url: url, response.status, reason: reason)
             }
 
             // How many bytes to expect

Sources/ContainerizationOCI/Client/RegistryClient+Push.swift

@@ -93,7 +93,8 @@ extension RegistryClient {
                 }
             } else if response.status != .notFound {
                 let url = components.url?.absoluteString ?? "unknown"
-                throw Error.invalidStatus(url: url, response.status)
+                let reason = await ErrorResponse.fromResponseBody(response.body)?.jsonString
+                throw Error.invalidStatus(url: url, response.status, reason: reason)
             }
         }
 
@@ -114,7 +115,8 @@ extension RegistryClient {
                     throw ContainerizationError(.exists, message: "Content already exists \(descriptor.digest)")
                 default:
                     let url = components.url?.absoluteString ?? "unknown"
-                    throw Error.invalidStatus(url: url, response.status)
+                    let reason = await ErrorResponse.fromResponseBody(response.body)?.jsonString
+                    throw Error.invalidStatus(url: url, response.status, reason: reason)
                 }
 
                 // Get the location to upload the blob.
@@ -149,7 +151,8 @@ extension RegistryClient {
                 break
             default:
                 let url = components.url?.absoluteString ?? "unknown"
-                throw Error.invalidStatus(url: url, response.status)
+                let reason = await ErrorResponse.fromResponseBody(response.body)?.jsonString
+                throw Error.invalidStatus(url: url, response.status, reason: reason)
             }
 
             guard descriptor.digest == response.headers.first(name: "Docker-Content-Digest") else {

Sources/ContainerizationOCI/Client/RegistryClient.swift

@@ -235,7 +235,8 @@ public final class RegistryClient: ContentClient {
         try await request(components: components, method: .GET, headers: headers) { response in
             guard response.status == .ok else {
                 let url = components.url?.absoluteString ?? "unknown"
-                throw Error.invalidStatus(url: url, response.status)
+                let reason = await ErrorResponse.fromResponseBody(response.body)?.jsonString
+                throw Error.invalidStatus(url: url, response.status, reason: reason)
             }
 
             var body = try await response.body.collect(upTo: self.bufferSize)
@@ -263,7 +264,8 @@ public final class RegistryClient: ContentClient {
         try await request(components: components) { response in
             guard response.status == .ok else {
                 let url = components.url?.absoluteString ?? "unknown"
-                throw Error.invalidStatus(url: url, response.status)
+                let reason = await ErrorResponse.fromResponseBody(response.body)?.jsonString
+                throw Error.invalidStatus(url: url, response.status, reason: reason)
             }
         }
     }