Constrained Result to be Sendable from the new executeThenClose.

Sources/NIOCore/AsyncChannel/AsyncChannel.swift

@@ -335,7 +335,7 @@ public struct NIOAsyncChannel<Inbound: Sendable, Outbound: Sendable>: Sendable {
         isolatedTo actor: isolated (any Actor)? = #isolation,
         _ body: (_ inbound: NIOAsyncChannelInboundStream<Inbound>, _ outbound: NIOAsyncChannelOutboundWriter<Outbound>)
             async throws -> Result
-    ) async throws -> Result {
+    ) async throws -> Result where Result: Sendable {
         let result: Result
         do {
             result = try await body(self._inbound, self._outbound)
@@ -385,9 +385,38 @@ extension NIOAsyncChannel {
     public func executeThenClose<Result>(
         _ body: (_ inbound: NIOAsyncChannelInboundStream<Inbound>) async throws -> Result
     ) async throws -> Result where Outbound == Never {
-        try await self.executeThenClose { inbound, _ in
-            try await body(inbound)
+        let result: Result
+        do {
+            result = try await body(self._inbound)
+        } catch let bodyError {
+            do {
+                self._outbound.finish()
+                try await self.channel.close().get()
+                throw bodyError
+            } catch {
+                throw bodyError
+            }
+        }
+
+        self._outbound.finish()
+        // We ignore errors from close, since all we care about is that the channel has been closed
+        // at this point.
+        self.channel.close(promise: nil)
+        // `closeFuture` should never be failed, so we could ignore the error. However, do an
+        // assertionFailure to guide bad Channel implementations that are incorrectly failing this
+        // future to stop failing it.
+        do {
+            try await self.channel.closeFuture.get()
+        } catch {
+            assertionFailure(
+                """
+                The channel's closeFuture should never be failed, but it was failed with error: \(error).
+                This is an error in the channel's implementation.
+                Refer to `Channel/closeFuture`'s documentation for more information.
+                """
+            )
         }
+        return result
     }
 
     #if compiler(>=6.0)
@@ -401,7 +430,7 @@ extension NIOAsyncChannel {
     public func executeThenClose<Result>(
         isolatedTo actor: isolated (any Actor)? = #isolation,
         _ body: (_ inbound: NIOAsyncChannelInboundStream<Inbound>) async throws -> Result
-    ) async throws -> Result where Outbound == Never {
+    ) async throws -> Result where Outbound == Never, Result: Sendable {
         try await self.executeThenClose { inbound, _ in
             try await body(inbound)
         }