Tolerate sending data after close(mode: .output) (#2421)

Motivation

We shouldn't crash on somewhat likely user error.

Modifications

Pass on writes after close(mode: .output) instead of crashing.

Result

User code is more robust to weird edge cases.

Sources/NIOHTTP1/HTTPServerPipelineHandler.swift

@@ -111,7 +111,11 @@ public final class HTTPServerPipelineHandler: ChannelDuplexHandler, RemovableCha
                 // We got a response while still receiving a request, which we have to
                 // wait for.
                 self = .requestEndPending
-            case .requestEndPending, .idle, .sentCloseOutput, .sentCloseOutputRequestEndPending:
+            case .sentCloseOutput, .sentCloseOutputRequestEndPending:
+                // This is a user error: they have sent close(mode: .output), but are continuing to write.
+                // The write will fail, so we can allow it to pass.
+                ()
+            case .requestEndPending, .idle:
                 preconditionFailure("Unexpectedly received a response in state \(self)")
             }
         }

Tests/NIOHTTP1Tests/HTTPServerPipelineHandlerTest.swift

@@ -1165,4 +1165,20 @@ class HTTPServerPipelineHandlerTest: XCTestCase {
                        [.channelRead(HTTPServerRequestPart.head(self.requestHead)),
                         .channelRead(HTTPServerRequestPart.end(nil))])
     }
+
+    func testWritesAfterCloseOutputAreDropped() throws {
+        // Send in a request
+        XCTAssertNoThrow(try self.channel.writeInbound(HTTPServerRequestPart.head(self.requestHead)))
+        XCTAssertNoThrow(try self.channel.writeInbound(HTTPServerRequestPart.end(nil)))
+
+        // Server sends a head.
+        XCTAssertNoThrow(try self.channel.writeOutbound(HTTPServerResponsePart.head(self.responseHead)))
+
+        // Now the server sends close output
+        XCTAssertNoThrow(try channel.close(mode: .output).wait())
+
+        // Now, in error, the server sends .body and .end. Both pass unannounced.
+        XCTAssertNoThrow(try self.channel.writeOutbound(HTTPServerResponsePart.body(.byteBuffer(ByteBuffer()))))
+        XCTAssertNoThrow(try self.channel.writeOutbound(HTTPServerResponsePart.end(nil)))
+    }
 }