WinUIBackend: Attach to parent console on launch (fixes stdout/err)

stdout and stderr used to go nowhere for apps compiled as GUI apps
(i.e. linked with `/SUBSYSTEM:WINDOWS`). They now go to the parent's
console.

Author: stackotter

Sources/WinUIBackend/Console.swift

@@ -0,0 +1,61 @@
+import WinSDK
+import Foundation
+
+extension WinUIBackend {
+    /// Attaches the app's standard IO streams to the parent's console.
+    ///
+    /// This allows the stdout/stderr of SwiftCrossUI GUI apps to be
+    /// viewed by starting them from the command line, even when they're
+    /// built and linked as /SUBSYSTEM:WINDOWS apps (GUI apps). Without
+    /// this fix the output of GUI apps is basically impossible to access.
+    ///
+    /// Adapted from: https://stackoverflow.com/a/55875595/8268001
+    static func attachToParentConsole() throws {
+        try Self.releaseConsole()
+        // -1 attaches to parent's console
+        if AttachConsole(DWORD(bitPattern: -1)) {
+            try Self.adjustConsoleBuffer(1024)
+            try Self.redirectConsoleIO()
+        }
+    }
+    
+    /// Releases existing files associated with the app's standard IO streams.
+    private static func releaseConsole() throws {
+        var fp = UnsafeMutablePointer<FILE>?.none
+        guard
+            freopen_s(&fp, "NUL:", "r", stdin) == 0,
+            freopen_s(&fp, "NUL:", "w", stdout) == 0,
+            freopen_s(&fp, "NUL:", "w", stdout) == 0,
+            FreeConsole()
+        else {
+            throw Error(message: "Failed to release existing console")
+        }
+    }
+
+    /// Redirect the application's standard IO streams to the current console.
+    private static func redirectConsoleIO() throws {
+        var fp = UnsafeMutablePointer<FILE>?.none
+        guard
+            freopen_s(&fp, "CONIN$", "r", stdin) == 0,
+            freopen_s(&fp, "CONOUT$", "w", stdout) == 0,
+            freopen_s(&fp, "CONOUT$", "w", stderr) == 0
+        else {
+            throw Error(message: "Failed to redirect console IO")
+        }
+    }
+
+    /// Adjusts the size of the app's console output buffer.
+    private static func adjustConsoleBuffer(_ minLength: SHORT) throws {
+        let handle = GetStdHandle(STD_OUTPUT_HANDLE)
+        var consoleInfo = CONSOLE_SCREEN_BUFFER_INFO();
+        guard GetConsoleScreenBufferInfo(handle, &consoleInfo) else {
+            throw Error(message: "Failed to get console screen buffer info")
+        }
+        if consoleInfo.dwSize.Y < minLength {
+            consoleInfo.dwSize.Y = minLength
+        }
+        guard SetConsoleScreenBufferSize(handle, consoleInfo.dwSize) else {
+            throw Error(message: "Failed to set console screen buffer size")
+        }
+    }
+}

Sources/WinUIBackend/WinUIBackend.swift

@@ -66,7 +66,26 @@ public final class WinUIBackend: AppBackend {
         internalState = InternalState()
     }
 
+    struct Error: LocalizedError {
+        var message: String
+
+        var errorDescription: String? {
+            message
+        }
+    }
+
     public func runMainLoop(_ callback: @escaping () -> Void) {
+        do {
+            try Self.attachToParentConsole()
+        } catch {
+            // We essentially just ignore if this fails because it's just a QoL
+            // debugging feature, and if it fails then any warning we print likely
+            // won't get seen anyway. But I don't trust my Windows knowledge enough
+            // to assert that it's impossible to view logs on failure, so let's
+            // print a warning anyway.
+            print("Warning: Failed to attach to parent console: \(error.localizedDescription)")
+        }
+
         WinUIApplication.callback = { application in
             // Toggle Switch has annoying default 'internal margins' (not Control
             // margins that we can set directly) that we can luckily get rid of by