Introduce castOrConvertRPCError helper

Author: bauer-andreas

Sources/GRPCCore/Call/Client/Internal/ClientStreamExecutor.swift

@@ -26,7 +26,7 @@ internal enum ClientStreamExecutor {
   ///   - attempt: The attempt number for the RPC that will be executed.
   ///   - serializer: A request serializer.
   ///   - deserializer: A response deserializer.
-  ///   - stream: The stream to excecute the RPC on.
+  ///   - stream: The stream to execute the RPC on.
   /// - Returns: A streamed response.
   @inlinable
   static func execute<Input: Sendable, Output: Sendable, Bytes: GRPCContiguousBytes>(
@@ -95,12 +95,8 @@ internal enum ClientStreamExecutor {
     let result = await Result {
       try await stream.write(.metadata(request.metadata))
       try await request.producer(.map(into: stream) { .message(try serializer.serialize($0)) })
-    }.castError(to: RPCError.self) { other in
-      if let convertible = other as? any RPCErrorConvertible {
-        RPCError(convertible)
-      } else {
-        RPCError(code: .unknown, message: "Write failed.", cause: other)
-      }
+    }.castOrConvertRPCError { other in
+      RPCError(code: .unknown, message: "Write failed.", cause: other)
     }
 
     switch result {

Sources/GRPCCore/Call/Server/Internal/ServerRPCExecutor.swift

@@ -213,12 +213,8 @@ struct ServerRPCExecutor {
         return try await contents.producer(
           .serializingToRPCResponsePart(into: outbound, with: serializer)
         )
-      }.castError(to: RPCError.self) { error in
-        if let convertible = error as? (any RPCErrorConvertible) {
-          return RPCError(convertible)
-        } else {
-          return RPCError(code: .unknown, message: "", cause: error)
-        }
+      }.castOrConvertRPCError { error in
+        RPCError(code: .unknown, message: "", cause: error)
       }
 
       switch result {

Sources/GRPCCore/Internal/Result+Catching.swift

@@ -45,4 +45,25 @@ extension Result {
       return (error as? NewError) ?? buildError(error)
     }
   }
+
+  /// Attempt to map or convert the error to an `RPCError`.
+  ///
+  /// If the cast or conversion is not possible then the provided closure is used to create an error of the given type.
+  ///
+  /// - Parameter buildError: A closure which constructs the desired error if conversion is not possible.
+  @inlinable
+  @available(gRPCSwift 2.0, *)
+  func castOrConvertRPCError(
+    or buildError: (any Error) -> RPCError
+  ) -> Result<Success, RPCError> {
+    return self.mapError { error in
+      if let rpcError = error as? RPCError {
+        return rpcError
+      } else if let convertibleError = error as? any RPCErrorConvertible {
+        return RPCError(convertibleError)
+      } else {
+        return buildError(error)
+      }
+    }
+  }
 }