refactor API towards 1.0

motivation: define stable API in preperation 1.0 release

changes:
* require swift 5.7, remove redundant backwards compatibility code
* make LambdaHandler, EventLoopLambdaHandler, and ByteBufferLambdaHandler disjointed protocols to reduce API surface area
* create coding wrappers for LambdaHandler and EventLoopLambdaHandler to provide coding bridge
* reuse output ByteBuffer to reduce allocationsi
* add no-op initializer for simple landa use cases
* update callsites and tests

Sources/AWSLambdaRuntime/Lambda+Codable.swift

@@ -19,7 +19,45 @@ import class Foundation.JSONEncoder
 import NIOCore
 import NIOFoundationCompat
 
-// MARK: - Codable support
+// MARK: - LambdaHandler Codable support
+
+/// Implementation of  a`ByteBuffer` to `Event` decoding.
+@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+extension LambdaHandler where Event: Decodable {
+    @inlinable
+    public func decode(buffer: ByteBuffer) throws -> Event {
+        try self.decoder.decode(Event.self, from: buffer)
+    }
+}
+
+/// Implementation of  `Output` to `ByteBuffer` encoding.
+@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+extension LambdaHandler where Output: Encodable {
+    @inlinable
+    public func encode(value: Output, into buffer: inout ByteBuffer) throws {
+        try self.encoder.encode(value, into: &buffer)
+    }
+}
+
+/// Default `ByteBuffer` to `Event` decoder using Foundation's `JSONDecoder`.
+/// Advanced users that want to inject their own codec can do it by overriding these functions.
+@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+extension LambdaHandler where Event: Decodable {
+    public var decoder: LambdaCodableDecoder {
+        Lambda.defaultJSONDecoder
+    }
+}
+
+/// Default `Output` to `ByteBuffer` encoder using Foundation's `JSONEncoder`.
+/// Advanced users that want to inject their own codec can do it by overriding these functions.
+@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+extension LambdaHandler where Output: Encodable {
+    public var encoder: LambdaCodableEncoder {
+        Lambda.defaultJSONEncoder
+    }
+}
+
+// MARK: - EventLoopLambdaHandler Codable support
 
 /// Implementation of  a`ByteBuffer` to `Event` decoding.
 extension EventLoopLambdaHandler where Event: Decodable {
@@ -32,8 +70,8 @@ extension EventLoopLambdaHandler where Event: Decodable {
 /// Implementation of  `Output` to `ByteBuffer` encoding.
 extension EventLoopLambdaHandler where Output: Encodable {
     @inlinable
-    public func encode(allocator: ByteBufferAllocator, value: Output) throws -> ByteBuffer? {
-        try self.encoder.encode(value, using: allocator)
+    public func encode(value: Output, into buffer: inout ByteBuffer) throws {
+        try self.encoder.encode(value, into: &buffer)
     }
 }
 
@@ -58,7 +96,7 @@ public protocol LambdaCodableDecoder {
 }
 
 public protocol LambdaCodableEncoder {
-    func encode<T: Encodable>(_ value: T, using allocator: ByteBufferAllocator) throws -> ByteBuffer
+    func encode<T: Encodable>(_ value: T, into buffer: inout ByteBuffer) throws
 }
 
 extension Lambda {
@@ -68,11 +106,4 @@ extension Lambda {
 
 extension JSONDecoder: LambdaCodableDecoder {}
 
-extension JSONEncoder: LambdaCodableEncoder {
-    public func encode<T>(_ value: T, using allocator: ByteBufferAllocator) throws -> ByteBuffer where T: Encodable {
-        // nio will resize the buffer if necessary
-        var buffer = allocator.buffer(capacity: 1024)
-        try self.encode(value, into: &buffer)
-        return buffer
-    }
-}
+extension JSONEncoder: LambdaCodableEncoder {}

Sources/AWSLambdaRuntimeCore/Lambda+String.swift

@@ -13,25 +13,46 @@
 //===----------------------------------------------------------------------===//
 import NIOCore
 
-extension EventLoopLambdaHandler where Event == String {
+// MARK: - LambdaHandler String support
+
+@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+extension LambdaHandler where Event == String {
     /// Implementation of a `ByteBuffer` to `String` decoding.
     @inlinable
     public func decode(buffer: ByteBuffer) throws -> String {
-        var buffer = buffer
-        guard let string = buffer.readString(length: buffer.readableBytes) else {
-            fatalError("buffer.readString(length: buffer.readableBytes) failed")
+        guard let value = buffer.getString(at: buffer.readerIndex, length: buffer.readableBytes) else {
+            throw CodecError.invalidString
         }
-        return string
+        return value
     }
 }
 
-extension EventLoopLambdaHandler where Output == String {
+@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+extension LambdaHandler where Output == String {
     /// Implementation of `String` to `ByteBuffer` encoding.
     @inlinable
-    public func encode(allocator: ByteBufferAllocator, value: String) throws -> ByteBuffer? {
-        // FIXME: reusable buffer
-        var buffer = allocator.buffer(capacity: value.utf8.count)
+    public func encode(value: String, into buffer: inout ByteBuffer) throws {
+        buffer.writeString(value)
+    }
+}
+
+// MARK: - EventLoopLambdaHandler String support
+
+extension EventLoopLambdaHandler where Event == String {
+    /// Implementation of `String` to `ByteBuffer` encoding.
+    @inlinable
+    public func decode(buffer: ByteBuffer) throws -> String {
+        guard let value = buffer.getString(at: buffer.readerIndex, length: buffer.readableBytes) else {
+            throw CodecError.invalidString
+        }
+        return value
+    }
+}
+
+extension EventLoopLambdaHandler where Output == String {
+    /// Implementation of a `ByteBuffer` to `String` decoding.
+    @inlinable
+    public func encode(value: String, into buffer: inout ByteBuffer) throws {
         buffer.writeString(value)
-        return buffer
     }
 }

Sources/AWSLambdaRuntimeCore/Lambda.swift

@@ -33,9 +33,25 @@ public enum Lambda {
     ///     - handlerType: The Handler to create and invoke.
     ///
     /// - note: This is a blocking operation that will run forever, as its lifecycle is managed by the AWS Lambda Runtime Engine.
-    internal static func run<Handler: ByteBufferLambdaHandler>(
+
+    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    internal static func run<Handler: LambdaHandler>(
+        configuration: LambdaConfiguration = .init(),
+        handlerType: Handler.Type
+    ) -> Result<Int, Error> {
+        Self.run(configuration: configuration, handlerType: CodableLambdaHandler<Handler>.self)
+    }
+
+    internal static func run<Handler: EventLoopLambdaHandler>(
         configuration: LambdaConfiguration = .init(),
         handlerType: Handler.Type
+    ) -> Result<Int, Error> {
+        Self.run(configuration: configuration, handlerType: CodableEventLoopLambdaHandler<Handler>.self)
+    }
+
+    internal static func run(
+        configuration: LambdaConfiguration = .init(),
+        handlerType: (some ByteBufferLambdaHandler).Type
     ) -> Result<Int, Error> {
         let _run = { (configuration: LambdaConfiguration) -> Result<Int, Error> in
             Backtrace.install()
@@ -44,7 +60,7 @@ public enum Lambda {
 
             var result: Result<Int, Error>!
             MultiThreadedEventLoopGroup.withCurrentThreadAsEventLoop { eventLoop in
-                let runtime = LambdaRuntime<Handler>(eventLoop: eventLoop, logger: logger, configuration: configuration)
+                let runtime = LambdaRuntime(handlerType, eventLoop: eventLoop, logger: logger, configuration: configuration)
                 #if DEBUG
                 let signalSource = trap(signal: configuration.lifecycle.stopSignal) { signal in
                     logger.info("intercepted signal: \(signal)")
@@ -66,7 +82,6 @@ public enum Lambda {
                     result = lifecycleResult
                 }
             }
-
             logger.info("shutdown completed")
             return result
         }

Sources/AWSLambdaRuntimeCore/LambdaContext.swift

@@ -12,23 +12,17 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if compiler(>=5.6)
 @preconcurrency import Dispatch
-@preconcurrency import Logging
-@preconcurrency import NIOCore
-#else
-import Dispatch
 import Logging
 import NIOCore
-#endif
 
 // MARK: - InitializationContext
 
 /// Lambda runtime initialization context.
 /// The Lambda runtime generates and passes the `LambdaInitializationContext` to the Handlers
 /// ``ByteBufferLambdaHandler/makeHandler(context:)`` or ``LambdaHandler/init(context:)``
 /// as an argument.
-public struct LambdaInitializationContext: _AWSLambdaSendable {
+public struct LambdaInitializationContext: Sendable {
     /// `Logger` to log with.
     ///
     /// - note: The `LogLevel` can be configured using the `LOG_LEVEL` environment variable.
@@ -71,8 +65,8 @@ public struct LambdaInitializationContext: _AWSLambdaSendable {
 
 /// Lambda runtime context.
 /// The Lambda runtime generates and passes the `LambdaContext` to the Lambda handler as an argument.
-public struct LambdaContext: CustomDebugStringConvertible, _AWSLambdaSendable {
-    final class _Storage: _AWSLambdaSendable {
+public struct LambdaContext: CustomDebugStringConvertible, Sendable {
+    final class _Storage: Sendable {
         let requestID: String
         let traceID: String
         let invokedFunctionARN: String