[plugin] Line-buffer stdout in the plugin helper so output streams as produced

Sources/AWSLambdaPluginHelper/AWSLambdaPluginHelper.swift

@@ -24,6 +24,10 @@ struct AWSLambdaPluginHelper {
     }
 
     public static func main() async throws {
+        // Stream output line-by-line; SwiftPM connects the plugin's stdout to a pipe, which the C
+        // runtime would otherwise block-buffer until the process exits.
+        enableLineBufferedStdout()
+
         let args = CommandLine.arguments
         let helper = AWSLambdaPluginHelper()
 

Sources/AWSLambdaPluginHelper/StandardOutput.swift

@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftAWSLambdaRuntime open source project
+//
+// Copyright SwiftAWSLambdaRuntime project authors
+// Copyright (c) Amazon.com, Inc. or its affiliates.
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftAWSLambdaRuntime project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+// `stdout` is a libc global `var` (an `extern FILE *`) and so reading it trips the Swift 6
+// strict-concurrency check. `@preconcurrency` silences that diagnostic for libc symbols. It is
+// confined to this dedicated file — the rest of the target imports libc normally and keeps full
+// concurrency checking — to keep the suppression's blast radius as small as possible.
+#if os(macOS)
+@preconcurrency import Darwin.C
+#elseif canImport(Glibc)
+@preconcurrency import Glibc
+#elseif canImport(Musl)
+@preconcurrency import Musl
+#elseif os(Windows)
+@preconcurrency import ucrt
+#else
+#error("Unsupported platform")
+#endif
+
+/// Switches `stdout` to line buffering.
+///
+/// SwiftPM runs plugins with stdout connected to a pipe rather than a TTY, so the C runtime
+/// block-buffers stdout and the helper's output (and `--help` text) only appears once the process
+/// exits. Line buffering makes each printed line stream to the user as it is produced.
+///
+/// Must be called once, before any output is produced.
+func enableLineBufferedStdout() {
+    setvbuf(stdout, nil, _IOLBF, 0)
+}