packer: keep main in app extensions

Documentation/xtool.docc/Appex.md

@@ -140,4 +140,6 @@ struct HelloWidget: Widget {
 
 > Note: Blindly refreshing once an hour isn't a great strategy in practice but it makes for a short snippet. I never said this was a tutorial on writing _good_ WidgetKit code; there's plenty of other resources online if that's your goal, including Apple's own [documentation](https://developer.apple.com/documentation/widgetkit).
 
+> Note: xtool links the extension with `_NSExtensionMain` as the entry point and forces the linker to keep the `main` symbol (`-u _main`), so the extension's library must define one: a `@main` type, as above, or top-level code. A library that only provides an `NSExtensionPrincipalClass` and no `main` fails to link with an undefined `_main` symbol.
+
 Finally, build and run with `xtool dev`. You should be able to locate the widget in your widget library and add it to your home screen!

Sources/PackLib/Packer.swift

@@ -233,27 +233,42 @@ public struct Packer: Sendable {
 }
 
 extension Plan.Product {
-    fileprivate var linkerSettings: String {
+    /// The `-Xlinker` and friends passed through `unsafeFlags` for this product.
+    var linkerFlags: [String] {
         switch self.type {
-        case .application: """
+        case .application: [
+            "-Xlinker", "-rpath", "-Xlinker", "@executable_path/Frameworks",
+        ]
+        case .appExtension: [
+            // Set the entry point to Foundation`_NSExtensionMain
+            "-Xlinker", "-e", "-Xlinker", "_NSExtensionMain",
+            // Keep `main`: nothing references it when the entry point is
+            // _NSExtensionMain, so the dead-strip would drop the whole extension.
+            // This makes a `main` symbol mandatory: the builder's executable target
+            // only has an empty stub.c, so it must come from the extension's own
+            // library (`@main` or top-level code). See Appex.md.
+            "-Xlinker", "-u", "-Xlinker", "_main",
+            // Include frameworks that the host app may use
+            "-Xlinker", "-rpath", "-Xlinker", "@executable_path/../../Frameworks",
+            // ...as well as our own
+            "-Xlinker", "-rpath", "-Xlinker", "@executable_path/Frameworks",
+        ]
+        }
+    }
+
+    var linkerSettings: String {
+        let flags = linkerFlags.map { "\"\($0)\"" }.joined(separator: ", ")
+        switch self.type {
+        case .application: return """
         [
-            .unsafeFlags([
-                "-Xlinker", "-rpath", "-Xlinker", "@executable_path/Frameworks",
-            ]),
+            .unsafeFlags([\(flags)]),
         ]
         """
-        case .appExtension: """
+        case .appExtension: return """
         [
             // Link to Foundation framework which implements the _NSExtensionMain entrypoint
             .linkedFramework("Foundation"),
-            .unsafeFlags([
-                // Set the entry point to Foundation`_NSExtensionMain
-                "-Xlinker", "-e", "-Xlinker", "_NSExtensionMain",
-                // Include frameworks that the host app may use
-                "-Xlinker", "-rpath", "-Xlinker", "@executable_path/../../Frameworks",
-                // ...as well as our own
-                "-Xlinker", "-rpath", "-Xlinker", "@executable_path/Frameworks",
-            ]),
+            .unsafeFlags([\(flags)]),
         ]
         """
         }

Tests/XToolTests/PackerTests.swift

@@ -13,3 +13,28 @@ import Testing
 
     #expect(Packer.cfBundleSupportedPlatforms(for: "unknown-triple") == ["iPhoneOS"])
 }
+
+private func product(_ type: Plan.ProductType) -> Plan.Product {
+    Plan.Product(
+        type: type,
+        product: "Example",
+        deploymentTarget: "17.0",
+        bundleID: "com.example.Example",
+        infoPlist: [:],
+        resources: []
+    )
+}
+
+@Test func appExtensionKeepsMainSymbolDespiteForeignEntryPoint() {
+    let flags = product(.appExtension).linkerFlags
+    #expect(flags.contains("_NSExtensionMain"))
+    // The entry point is Foundation's, so nothing references the extension's own `main`
+    // and the dead-strip would drop the whole extension unless it is forced to stay.
+    #expect(flags.contains(["-Xlinker", "-u", "-Xlinker", "_main"]))
+    // ...and the generated Package.swift snippet actually carries the quoted pair.
+    #expect(product(.appExtension).linkerSettings.contains(#""-u", "-Xlinker", "_main""#))
+}
+
+@Test func applicationDoesNotForceMainSymbol() {
+    #expect(!product(.application).linkerFlags.contains("-u"))
+}