PseudoHeaderFields adopting CoW to reduce size (#88)

HTTPRequest size 288 -> 16
HTTPResponse size 80 -> 16

rdar://144520456

Author: guoye-zhang

Sources/HTTPTypes/HTTPRequest.swift

@@ -146,53 +146,156 @@ public struct HTTPRequest: Sendable, Hashable {
 
     /// The pseudo header fields of a request.
     public struct PseudoHeaderFields: Sendable, Hashable {
+        private final class _Storage: @unchecked Sendable, Hashable {
+            var method: HTTPField
+            var scheme: HTTPField?
+            var authority: HTTPField?
+            var path: HTTPField?
+            var extendedConnectProtocol: HTTPField?
+
+            init(
+                method: HTTPField,
+                scheme: HTTPField?,
+                authority: HTTPField?,
+                path: HTTPField?,
+                extendedConnectProtocol: HTTPField?
+            ) {
+                self.method = method
+                self.scheme = scheme
+                self.authority = authority
+                self.path = path
+                self.extendedConnectProtocol = extendedConnectProtocol
+            }
+
+            func copy() -> Self {
+                .init(
+                    method: self.method,
+                    scheme: self.scheme,
+                    authority: self.authority,
+                    path: self.path,
+                    extendedConnectProtocol: self.extendedConnectProtocol
+                )
+            }
+
+            static func == (lhs: _Storage, rhs: _Storage) -> Bool {
+                lhs.method == rhs.method && lhs.scheme == rhs.scheme && lhs.authority == rhs.authority
+                    && lhs.path == rhs.path && lhs.extendedConnectProtocol == rhs.extendedConnectProtocol
+            }
+
+            func hash(into hasher: inout Hasher) {
+                hasher.combine(self.method)
+                hasher.combine(self.scheme)
+                hasher.combine(self.authority)
+                hasher.combine(self.path)
+                hasher.combine(self.extendedConnectProtocol)
+            }
+        }
+
+        private var _storage: _Storage
+
         /// The underlying ":method" pseudo header field.
         ///
         /// The value of this field must be a valid method.
         ///
         /// https://www.rfc-editor.org/rfc/rfc9110.html#name-methods
         public var method: HTTPField {
-            willSet {
+            get {
+                self._storage.method
+            }
+            set {
                 precondition(newValue.name == .method, "Cannot change pseudo-header field name")
                 precondition(HTTPField.isValidToken(newValue.rawValue._storage), "Invalid character in method field")
+
+                if !isKnownUniquelyReferenced(&self._storage) {
+                    self._storage = self._storage.copy()
+                }
+                self._storage.method = newValue
             }
         }
 
         /// The underlying ":scheme" pseudo header field.
         public var scheme: HTTPField? {
-            willSet {
+            get {
+                self._storage.scheme
+            }
+            set {
                 if let name = newValue?.name {
                     precondition(name == .scheme, "Cannot change pseudo-header field name")
                 }
+
+                if !isKnownUniquelyReferenced(&self._storage) {
+                    self._storage = self._storage.copy()
+                }
+                self._storage.scheme = newValue
             }
         }
 
         /// The underlying ":authority" pseudo header field.
         public var authority: HTTPField? {
-            willSet {
+            get {
+                self._storage.authority
+            }
+            set {
                 if let name = newValue?.name {
                     precondition(name == .authority, "Cannot change pseudo-header field name")
                 }
+
+                if !isKnownUniquelyReferenced(&self._storage) {
+                    self._storage = self._storage.copy()
+                }
+                self._storage.authority = newValue
             }
         }
 
         /// The underlying ":path" pseudo header field.
         public var path: HTTPField? {
-            willSet {
+            get {
+                self._storage.path
+            }
+            set {
                 if let name = newValue?.name {
                     precondition(name == .path, "Cannot change pseudo-header field name")
                 }
+
+                if !isKnownUniquelyReferenced(&self._storage) {
+                    self._storage = self._storage.copy()
+                }
+                self._storage.path = newValue
             }
         }
 
         /// The underlying ":protocol" pseudo header field.
         public var extendedConnectProtocol: HTTPField? {
-            willSet {
+            get {
+                self._storage.extendedConnectProtocol
+            }
+            set {
                 if let name = newValue?.name {
                     precondition(name == .protocol, "Cannot change pseudo-header field name")
                 }
+
+                if !isKnownUniquelyReferenced(&self._storage) {
+                    self._storage = self._storage.copy()
+                }
+                self._storage.extendedConnectProtocol = newValue
             }
         }
+
+        init(
+            method: HTTPField,
+            scheme: HTTPField?,
+            authority: HTTPField?,
+            path: HTTPField?,
+            extendedConnectProtocol: HTTPField? = nil
+        ) {
+            self._storage = .init(
+                method: method,
+                scheme: scheme,
+                authority: authority,
+                path: path,
+                extendedConnectProtocol: extendedConnectProtocol
+            )
+        }
     }
 
     /// The pseudo header fields.

Sources/HTTPTypes/HTTPResponse.swift

@@ -162,32 +162,88 @@ public struct HTTPResponse: Sendable, Hashable {
             let code =
                 Int(codeIterator.next()! - 48) * 100 + Int(codeIterator.next()! - 48) * 10
                 + Int(codeIterator.next()! - 48)
-            return Status(uncheckedCode: code, reasonPhrase: self.reasonPhrase)
+            return Status(uncheckedCode: code, reasonPhrase: self.pseudoHeaderFields.reasonPhrase)
         }
         set {
             self.pseudoHeaderFields.status.rawValue = ISOLatin1String(unchecked: newValue.fieldValue)
-            self.reasonPhrase = newValue.reasonPhrase
+            self.pseudoHeaderFields.reasonPhrase = newValue.reasonPhrase
         }
     }
 
     /// The pseudo header fields of a response.
     public struct PseudoHeaderFields: Sendable, Hashable {
+        private final class _Storage: @unchecked Sendable, Hashable {
+            var status: HTTPField
+            var reasonPhrase: String
+
+            init(status: HTTPField, reasonPhrase: String) {
+                self.status = status
+                self.reasonPhrase = reasonPhrase
+            }
+
+            func copy() -> Self {
+                .init(
+                    status: self.status,
+                    reasonPhrase: self.reasonPhrase
+                )
+            }
+
+            static func == (lhs: _Storage, rhs: _Storage) -> Bool {
+                lhs.status == rhs.status
+            }
+
+            func hash(into hasher: inout Hasher) {
+                hasher.combine(self.status)
+            }
+        }
+
+        private var _storage: _Storage
+
         /// The underlying ":status" pseudo header field.
         ///
         /// The value of this field must be 3 ASCII decimal digits.
         public var status: HTTPField {
-            willSet {
+            get {
+                self._storage.status
+            }
+            set {
                 precondition(newValue.name == .status, "Cannot change pseudo-header field name")
                 precondition(Status.isValidStatus(newValue.rawValue._storage), "Invalid status code")
+
+                if !isKnownUniquelyReferenced(&self._storage) {
+                    self._storage = self._storage.copy()
+                }
+                self._storage.status = newValue
             }
         }
+
+        var reasonPhrase: String {
+            get {
+                self._storage.reasonPhrase
+            }
+            set {
+                if !isKnownUniquelyReferenced(&self._storage) {
+                    self._storage = self._storage.copy()
+                }
+                self._storage.reasonPhrase = newValue
+            }
+        }
+
+        private init(status: HTTPField) {
+            self._storage = .init(status: status, reasonPhrase: "")
+        }
+
+        init(status: Status) {
+            self._storage = .init(
+                status: HTTPField(name: .status, uncheckedValue: ISOLatin1String(unchecked: status.fieldValue)),
+                reasonPhrase: status.reasonPhrase
+            )
+        }
     }
 
     /// The pseudo header fields.
     public var pseudoHeaderFields: PseudoHeaderFields
 
-    private var reasonPhrase: String
-
     /// The response header fields.
     public var headerFields: HTTPFields
 
@@ -196,20 +252,9 @@ public struct HTTPResponse: Sendable, Hashable {
     ///   - status: The status code and an optional reason phrase.
     ///   - headerFields: The response header fields.
     public init(status: Status, headerFields: HTTPFields = [:]) {
-        let statusField = HTTPField(name: .status, uncheckedValue: ISOLatin1String(unchecked: status.fieldValue))
-        self.pseudoHeaderFields = .init(status: statusField)
-        self.reasonPhrase = status.reasonPhrase
+        self.pseudoHeaderFields = .init(status: status)
         self.headerFields = headerFields
     }
-
-    public func hash(into hasher: inout Hasher) {
-        hasher.combine(self.pseudoHeaderFields)
-        hasher.combine(self.headerFields)
-    }
-
-    public static func == (lhs: HTTPResponse, rhs: HTTPResponse) -> Bool {
-        lhs.pseudoHeaderFields == rhs.pseudoHeaderFields && lhs.headerFields == rhs.headerFields
-    }
 }
 
 extension HTTPResponse: CustomDebugStringConvertible {
@@ -273,7 +318,7 @@ extension HTTPResponse: Codable {
     public func encode(to encoder: Encoder) throws {
         var container = encoder.container(keyedBy: CodingKeys.self)
         try container.encode(self.pseudoHeaderFields, forKey: .pseudoHeaderFields)
-        try container.encode(self.reasonPhrase, forKey: .reasonPhrase)
+        try container.encode(self.pseudoHeaderFields.reasonPhrase, forKey: .reasonPhrase)
         try container.encode(self.headerFields, forKey: .headerFields)
     }
 
@@ -288,7 +333,7 @@ extension HTTPResponse: Codable {
         guard Status.isValidReasonPhrase(reasonPhrase) else {
             throw DecodingError.invalidReasonPhrase(reasonPhrase)
         }
-        self.reasonPhrase = reasonPhrase
+        self.pseudoHeaderFields.reasonPhrase = reasonPhrase
         self.headerFields = try container.decode(HTTPFields.self, forKey: .headerFields)
     }
 }

Tests/HTTPTypesTests/HTTPTypesTests.swift

@@ -248,4 +248,10 @@ final class HTTPTypesTests: XCTestCase {
         XCTAssertEqual(trailerFields[HTTPField.Name("trailer1")!], "value1")
         XCTAssertEqual(trailerFields[HTTPField.Name("trailer2")!], "value2")
     }
+
+    func testTypeLayoutSize() {
+        XCTAssertEqual(MemoryLayout<HTTPRequest>.size, MemoryLayout<AnyObject>.size * 2)
+        XCTAssertEqual(MemoryLayout<HTTPResponse>.size, MemoryLayout<AnyObject>.size * 2)
+        XCTAssertEqual(MemoryLayout<HTTPFields>.size, MemoryLayout<AnyObject>.size)
+    }
 }