Wait for closeFuture instead of close promise in NIOAsyncChannel's executeThenClose (#3032)

After the changes introduced in
apple/swift-nio-http2#487, we need to make a
small change in the implementation of `NIOAsyncChannel` to wait on the
`closeFuture` instead of on `close`'s promise in the `executeThenClose`
implementation.

### Motivation:

`executeThenClose` shouldn't fail from errors arising from closing the
channel - at this point, the user of the channel cannot really do
anything, and since the channel has been closed, we should not fail
since resources have been cleaned up anyways.

### Modifications:

This PR changes the implementation of `NIOAsyncChannel` to wait on the
`closeFuture` instead of on `close`'s promise in the `executeThenClose`
implementation.
It also updates the docs for `closeFuture` to better explain when it
will be succeeded and why it won't ever be failed.


### Result:

`executeThenClose` won't throw errors upon closing.

Author: gjcairo

Sources/NIOCore/AsyncChannel/AsyncChannel.swift

@@ -301,15 +301,25 @@ public struct NIOAsyncChannel<Inbound: Sendable, Outbound: Sendable>: Sendable {
             }
         }
 
+        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 {
-            self._outbound.finish()
-            try await self.channel.close().get()
+            try await self.channel.closeFuture.get()
         } catch {
-            if let error = error as? ChannelError, error == .alreadyClosed {
-                return result
-            }
-            throw error
+            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
     }
 }

Sources/NIOCore/Channel.swift

@@ -107,6 +107,12 @@ public protocol Channel: AnyObject, ChannelOutboundInvoker, _NIOPreconcurrencySe
     var allocator: ByteBufferAllocator { get }
 
     /// The `closeFuture` will fire when the `Channel` has been closed.
+    ///
+    /// - Important: This future should never be failed: it signals when the channel has been closed, and this action should not fail,
+    ///              regardless of whether the close happenned cleanly or not.
+    ///              If you are interested in any errors thrown during `close` to diagnose any unclean channel closures, you
+    ///              should instead use the future returned from ``ChannelOutboundInvoker/close(mode:file:line:)-7hlgf``
+    ///              or pass a promise via ``ChannelOutboundInvoker/close(mode:promise:)``.
     var closeFuture: EventLoopFuture<Void> { get }
 
     /// The `ChannelPipeline` which handles all I/O events and requests associated with this `Channel`.