Add shutDown method to PluginMessageHandler

Sources/SwiftCompilerPlugin/CompilerPlugin.swift

@@ -112,6 +112,6 @@ extension CompilerPlugin {
     let connection = try StandardIOMessageConnection()
     let provider = MacroProviderAdapter(plugin: Self())
     let impl = CompilerPluginMessageListener(connection: connection, provider: provider)
-    impl.main()
+    try impl.main()
   }
 }

Sources/SwiftCompilerPluginMessageHandling/CompilerPluginMessageHandler.swift

@@ -96,12 +96,13 @@ public class CompilerPluginMessageListener<Connection: MessageConnection, Handle
   ///
   /// On internal errors, such as I/O errors or JSON serialization errors, print
   /// an error message and `exit(1)`
-  public func main() {
+  public func main() throws {
     #if os(WASI)
     // Rather than blocking on read(), let the host tell us when there's data.
     readabilityHandler = { _ = self.handleNextMessage() }
     #else
     while handleNextMessage() {}
+    try self.handler.shutDown()
     #endif
   }
 
@@ -133,6 +134,12 @@ public class CompilerPluginMessageListener<Connection: MessageConnection, Handle
 public protocol PluginMessageHandler {
   /// Handles a single message received from the plugin host.
   func handleMessage(_ message: HostToPluginMessage) -> PluginToHostMessage
+
+  /// Deterministically and synchronously cleans up resources that cannot be dealt with in
+  /// a deinitializer due to possible errors thrown during the clean up. Usually this
+  /// includes closure of file handles, sockets, shutting down external processes and IPC
+  /// resources set up for these processes, etc.
+  func shutDown() throws
 }
 
 /// A `PluginMessageHandler` that uses a `PluginProvider`.
@@ -226,6 +233,10 @@ public class PluginProviderMessageHandler<Provider: PluginProvider>: PluginMessa
       return .loadPluginLibraryResult(loaded: diags.isEmpty, diagnostics: diags)
     }
   }
+
+  /// Empty implementation for the default message handler, since all resources are automatically
+  /// cleaned up in the synthesized initializer.
+  public func shutDown() throws {}
 }
 
 @_spi(PluginMessage)