Support multiple interim responses

Sources/HTTP3/HTTP3Frame.swift

@@ -302,6 +302,21 @@ extension HTTP3PartialFrame {
     }
 }
 
+extension HTTP3Frame.Headers {
+    /// Whether these headers contain a `:status` pseudo-header field whose value is an informational 1xx status code.
+    ///
+    /// Returns `false` if the `:status` pseudo-header field is not present or if its value does not represent a 1xx
+    /// status code.
+    ///
+    /// - SeeAlso: https://www.rfc-editor.org/rfc/rfc9110.html#section-15.2.
+    package var representsInterimResponse: Bool {
+        guard let status = self.fields.first(where: { $0.name == .status })?.value, let code = Int(status) else {
+            return false
+        }
+        return (100..<200).contains(code)
+    }
+}
+
 extension HTTP3Frame {
     package static func data(_ payload: ByteBuffer) -> Self {
         .data(.init(payload: payload))

Sources/HTTP3/HTTP3FrameValidator.swift

@@ -228,8 +228,7 @@ package enum HTTP3FrameValidator: ~Copyable {
                 case .headers(let headers):
                     // A response MAY consist of multiple messages when and only when one or more interim responses (1xx; see Section 15.2 of [HTTP]) precede a final response to the same request.
                     // Interim responses do not contain content or trailer sections.
-                    let status = headers.fields.first(where: { $0.name == .status })?.value
-                    if status == "100" {
+                    if headers.representsInterimResponse {
                         // This header doesn't affect our state because it's interim. We just pass it through
                         self = .init(requestState: existingRequestState, responseState: existingResponseState)
                         return .forwardFrame(frame)

Sources/NIOHTTP3/HTTP3ToHTTPCodecs.swift

@@ -196,13 +196,13 @@ extension HTTPResponsePart: HTTPMessagePart {
 /// Use this to convert incoming frames into message parts.
 package struct HTTPMessageParsingStateMachine<Part: HTTPMessagePart> {
     enum State {
-        case idle
+        case awaitingFinalHead
         case processedHeaders
         case processedTrailers
         case previousError
     }
 
-    private var state = State.idle
+    private var state = State.awaitingFinalHead
 
     package init() {}
 
@@ -215,12 +215,19 @@ package struct HTTPMessageParsingStateMachine<Part: HTTPMessagePart> {
         switch self.state {
         case .previousError:
             return .none
-        case .idle:
+        case .awaitingFinalHead:
             switch frame {
             case .headers(let headers):
                 do {
                     let part = try Part.head(fields: headers.fields)
-                    self.state = .processedHeaders
+                    if headers.representsInterimResponse {
+                        // Multiple interim (1xx) responses can precede the final response; remain in the same state to
+                        // accept further interim responses or the final response. We can only reach this branch on the
+                        // response parsing side.
+                        self.state = .awaitingFinalHead
+                    } else {
+                        self.state = .processedHeaders
+                    }
                     return .returnPart(part)
                 } catch {
                     self.state = .previousError
@@ -261,7 +268,7 @@ package struct HTTPMessageParsingStateMachine<Part: HTTPMessagePart> {
 
     package mutating func inputClosed() -> InputClosedAction? {
         switch self.state {
-        case .idle:
+        case .awaitingFinalHead:
             self.state = .processedTrailers
             return .returnPart(.end())
         case .processedHeaders:

Tests/HTTP3Tests/HTTP3FrameValidatorTests.swift

@@ -306,19 +306,58 @@ struct HTTP3FrameValidatorTests {
     }
 
     @Test
-    func testDoubleResponse() {
+    func testInboundInterimResponse() {
+        var validator = HTTP3FrameValidator(streamType: .request, incoming: false)
+        validator.assertOutboundFramePassesThrough(Self.validRequestHeaders())
+
+        validator.assertInboundFramePassesThrough(.headers([.init(name: .status, value: "103")]))
+        validator.assertInboundFramePassesThrough(.headers([.init(name: .status, value: "200")]))
+        validator.assertInboundFramePassesThrough(.data(.init(bytes: [1, 2, 3])))
+        validator.assertInboundFramePassesThrough(Self.validTrailers())
+    }
+
+    @Test
+    func testInboundMultipleInterimResponse() {
         var validator = HTTP3FrameValidator(streamType: .request, incoming: false)
         validator.assertOutboundFramePassesThrough(Self.validRequestHeaders())  // write req headers
 
         // Some informational responses
         validator.assertInboundFramePassesThrough(.headers([.init(name: .status, value: "100")]))
         validator.assertInboundFramePassesThrough(.headers([.init(name: .status, value: "100")]))
+        validator.assertInboundFramePassesThrough(.headers([.init(name: .status, value: "102")]))
+        validator.assertInboundFramePassesThrough(.headers([.init(name: .status, value: "103")]))
         // Final response
         validator.assertInboundFramePassesThrough(.headers([.init(name: .status, value: "200")]))
         validator.assertInboundFramePassesThrough(.data(.init(bytes: [1, 2, 3])))
         validator.assertInboundFramePassesThrough(.headers([.init(name: .cookie, value: "test")]))
     }
 
+    @Test
+    func testOutboundInterimResponse() {
+        var validator = HTTP3FrameValidator(streamType: .request, incoming: true)
+        validator.assertInboundFramePassesThrough(Self.validRequestHeaders())
+
+        validator.assertOutboundFramePassesThrough(.headers([.init(name: .status, value: "103")]))
+        validator.assertOutboundFramePassesThrough(.headers([.init(name: .status, value: "200")]))
+        validator.assertOutboundFramePassesThrough(.data(.init(bytes: [1, 2, 3])))
+    }
+
+    @Test
+    func testOutboundMultipleInterimResponses() {
+        var validator = HTTP3FrameValidator(streamType: .request, incoming: true)
+        validator.assertInboundFramePassesThrough(Self.validRequestHeaders())
+
+        // Some informational responses
+        validator.assertOutboundFramePassesThrough(.headers([.init(name: .status, value: "100")]))
+        validator.assertOutboundFramePassesThrough(.headers([.init(name: .status, value: "100")]))
+        validator.assertOutboundFramePassesThrough(.headers([.init(name: .status, value: "102")]))
+        validator.assertOutboundFramePassesThrough(.headers([.init(name: .status, value: "103")]))
+        // Final response
+        validator.assertOutboundFramePassesThrough(.headers([.init(name: .status, value: "200")]))
+        validator.assertOutboundFramePassesThrough(.data(.init(bytes: [1, 2, 3])))
+        validator.assertOutboundFramePassesThrough(.headers([.init(name: .cookie, value: "test")]))
+    }
+
     // MARK: Control stream
 
     /// Anything other than settings is invalid to be the first frame.

Tests/NIOHTTP3Tests/HTTPMessageParsingTests.swift

@@ -153,6 +153,70 @@ struct HTTPMessageParsingTests {
         #expect(action3 == nil)
     }
 
+    let informationalEarlyHintsHead = HTTP3Frame.headers([.init(name: .status, value: "103")])
+
+    @Test
+    func testSingleInterimThenFinal() throws {
+        var machine = HTTPMessageParsingStateMachine<HTTPResponsePart>()
+
+        let action1 = machine.processFrame(frame: self.informationalEarlyHintsHead)
+        #expect(action1?.returnPart == .head(.init(status: .init(code: 103))))
+
+        let action2 = machine.processFrame(frame: self.validResponseHead)
+        #expect(action2?.returnPart == .head(.init(status: .ok)))
+
+        let action3 = machine.processFrame(frame: .data(.init()))
+        #expect(action3?.returnPart == .body(.init()))
+
+        let action4 = machine.processFrame(frame: self.validTrailers)
+        #expect(action4?.returnPart == .end([.init("test")!: "hello"]))
+    }
+
+    @Test
+    func testMultipleInterimThenFinal() throws {
+        var machine = HTTPMessageParsingStateMachine<HTTPResponsePart>()
+
+        let action1 = machine.processFrame(frame: self.informationalEarlyHintsHead)
+        #expect(action1?.returnPart == .head(.init(status: .init(code: 103))))
+
+        let action2 = machine.processFrame(frame: self.informationalEarlyHintsHead)
+        #expect(action2?.returnPart == .head(.init(status: .init(code: 103))))
+
+        let action3 = machine.processFrame(frame: self.validResponseHead)
+        #expect(action3?.returnPart == .head(.init(status: .ok)))
+
+        let action4 = machine.processFrame(frame: .data(.init()))
+        #expect(action4?.returnPart == .body(.init()))
+
+        let action5 = machine.processFrame(frame: self.validTrailers)
+        #expect(action5?.returnPart == .end([.init("test")!: "hello"]))
+    }
+
+    @Test
+    func testInvalidInterimHeaders() throws {
+        var machine = HTTPMessageParsingStateMachine<HTTPResponsePart>()
+
+        // 1xx response with a forbidden transfer-encoding field.
+        let action1 = machine.processFrame(
+            frame: .headers([
+                .init(name: .status, value: "103"),
+                .init(name: .transferEncoding, value: "chunked"),
+            ])
+        )
+        action1.assertError { error in
+            expectH3ErrorEqual(
+                error: error,
+                expectedCode: .malformedMessage,
+                expectedH3ErrorCode: .H3_MESSAGE_ERROR,
+                expectedMessage: "Invalid headers"
+            )
+        }
+
+        // After error, even valid final headers are ignored.
+        let action2 = machine.processFrame(frame: self.validResponseHead)
+        #expect(action2 == nil)
+    }
+
     // MARK: General request and response header validation
 
     @Test