Import System package when available (#34)

* Import System package when available

* Add isResourceTemporarilyUnavailable helper method to reduce number of imports of System / SystemPackage

Sources/MCP/Base/Error.swift

@@ -1,5 +1,11 @@
 import Foundation
 
+#if canImport(System)
+    import System
+#else
+    @preconcurrency import SystemPackage
+#endif
+
 /// A model context protocol error.
 public enum Error: Sendable {
     // Standard JSON-RPC 2.0 errors (-32700 to -32603)
@@ -29,6 +35,20 @@ public enum Error: Sendable {
         case .transportError: return -32001
         }
     }
+
+    /// Check if an error represents a "resource temporarily unavailable" condition
+    public static func isResourceTemporarilyUnavailable(_ error: Swift.Error) -> Bool {
+        #if canImport(System)
+            if let errno = error as? System.Errno, errno == .resourceTemporarilyUnavailable {
+                return true
+            }
+        #else
+            if let errno = error as? SystemPackage.Errno, errno == .resourceTemporarilyUnavailable {
+                return true
+            }
+        #endif
+        return false
+    }
 }
 
 // MARK: LocalizedError

Sources/MCP/Base/Transports.swift

@@ -1,9 +1,14 @@
 import Darwin
 import Logging
-import SystemPackage
 
 import struct Foundation.Data
 
+#if canImport(System)
+    import System
+#else
+    @preconcurrency import SystemPackage
+#endif
+
 /// Protocol defining the transport layer for MCP communication
 public protocol Transport: Actor {
     var logger: Logger { get }
@@ -107,7 +112,7 @@ public actor StdioTransport: Transport {
                         messageContinuation.yield(message)
                     }
                 }
-            } catch let error as Errno where error == .resourceTemporarilyUnavailable {
+            } catch let error where Error.isResourceTemporarilyUnavailable(error) {
                 try? await Task.sleep(nanoseconds: 10_000_000)  // 10ms backoff
                 continue
             } catch {
@@ -147,7 +152,7 @@ public actor StdioTransport: Transport {
                 if written > 0 {
                     remaining = remaining.dropFirst(written)
                 }
-            } catch let error as Errno where error == .resourceTemporarilyUnavailable {
+            } catch let error where Error.isResourceTemporarilyUnavailable(error) {
                 try await Task.sleep(nanoseconds: 10_000_000)  // 10ms backoff
                 continue
             } catch {

Sources/MCP/Client/Client.swift

@@ -1,5 +1,4 @@
 import Logging
-import SystemPackage
 
 import struct Foundation.Data
 import struct Foundation.Date
@@ -180,7 +179,7 @@ public actor Client {
                                 "Unexpected message received by client", metadata: metadata)
                         }
                     }
-                } catch let error as Errno where error == .resourceTemporarilyUnavailable {
+                } catch let error where Error.isResourceTemporarilyUnavailable(error) {
                     try? await Task.sleep(nanoseconds: 10_000_000)  // 10ms
                     continue
                 } catch {

Sources/MCP/Server/Server.swift

@@ -1,5 +1,4 @@
 import Logging
-import SystemPackage
 
 import struct Foundation.Data
 import struct Foundation.Date
@@ -202,7 +201,7 @@ public actor Server {
                             }
                             throw Error.parseError("Invalid message format")
                         }
-                    } catch let error as Errno where error == .resourceTemporarilyUnavailable {
+                    } catch let error where Error.isResourceTemporarilyUnavailable(error) {
                         // Resource temporarily unavailable, retry after a short delay
                         try? await Task.sleep(nanoseconds: 10_000_000)  // 10ms
                         continue

Tests/MCPTests/RoundtripTests.swift

@@ -1,10 +1,15 @@
 import Foundation
 import Logging
-import SystemPackage
 import Testing
 
 @testable import MCP
 
+#if canImport(System)
+    import System
+#else
+    @preconcurrency import SystemPackage
+#endif
+
 @Suite("Roundtrip Tests")
 struct RoundtripTests {
     @Test(

Tests/MCPTests/TransportTests.swift

@@ -1,9 +1,14 @@
 import Foundation
-import SystemPackage
 import Testing
 
 @testable import MCP
 
+#if canImport(System)
+    import System
+#else
+    @preconcurrency import SystemPackage
+#endif
+
 @Suite("Stdio Transport Tests")
 struct StdioTransportTests {
     @Test("Connection")