Resolve issue with readLazyAssembly returning stale value (#26642)

* Resolve issue with readLazyAssembly returning stale value

* Address feedback from peer review

* Update test to exercise loaded package

src/Components/Web.JS/src/Platform/Mono/MonoPlatform.ts

@@ -334,6 +334,10 @@ function createEmscriptenModuleInstance(resourceLoader: WebAssemblyResourceLoade
       return BINDING.js_to_mono_obj(Promise.resolve(0));
     }
 
+    const lazyResources: {
+      assemblies?: (ArrayBuffer | null)[],
+      pdbs?: (ArrayBuffer | null)[]
+    } = {};
     window['Blazor']._internal.getLazyAssemblies = (assembliesToLoadDotNetArray: System_Array<System_String>): System_Object => {
       const assembliesToLoad = BINDING.mono_array_to_js_array<System_String, string>(assembliesToLoadDotNetArray);
       const lazyAssemblies = resourceLoader.bootConfig.resources.lazyAssembly;
@@ -364,31 +368,40 @@ function createEmscriptenModuleInstance(resourceLoader: WebAssemblyResourceLoade
         .map(assembly => resourceLoader.loadResource(assembly, `_framework/${assembly}`, lazyAssemblies[assembly], 'assembly'))
         .map(async resource => (await resource.response).arrayBuffer()));
 
+
       return BINDING.js_to_mono_obj(
         Promise.all([resourcePromises, pdbPromises]).then(values => {
-          const resourcesToLoad = values[0];
+          lazyResources["assemblies"] = values[0];
-          const pdbsToLoad = values[1];
+          lazyResources["pdbs"] = values[1];
-          if (resourcesToLoad.length) {
+          if (lazyResources["assemblies"].length) {
             window['Blazor']._internal.readLazyAssemblies = () => {
-              const assemblyBytes = BINDING.mono_obj_array_new(resourcesToLoad.length);
+              const { assemblies } = lazyResources;
-              for (let i = 0; i < resourcesToLoad.length; i++) {
+              if (!assemblies) {
-                const assembly = resourcesToLoad[i] as ArrayBuffer;
+                return BINDING.mono_obj_array_new(0);
+              }
+              const assemblyBytes = BINDING.mono_obj_array_new(assemblies.length);
+              for (let i = 0; i < assemblies.length; i++) {
+                const assembly = assemblies[i] as ArrayBuffer;
                 BINDING.mono_obj_array_set(assemblyBytes, i, BINDING.js_typed_array_to_array(new Uint8Array(assembly)));
               }
               return assemblyBytes;
             };
 
             window['Blazor']._internal.readLazyPdbs = () => {
-              const pdbBytes = BINDING.mono_obj_array_new(resourcesToLoad.length);
+              const { assemblies, pdbs } = lazyResources;
-              for (let i = 0; i < resourcesToLoad.length; i++) {
+              if (!assemblies) {
-                const pdb = pdbsToLoad && pdbsToLoad[i] ? new Uint8Array(pdbsToLoad[i] as ArrayBufferLike) : new Uint8Array();
+                return BINDING.mono_obj_array_new(0);
+              }
+              const pdbBytes = BINDING.mono_obj_array_new(assemblies.length);
+              for (let i = 0; i < assemblies.length; i++) {
+                const pdb = pdbs && pdbs[i] ? new Uint8Array(pdbs[i] as ArrayBufferLike) : new Uint8Array();
                 BINDING.mono_obj_array_set(pdbBytes, i, BINDING.js_typed_array_to_array(pdb));
               }
               return pdbBytes;
             };
           }
 
-          return resourcesToLoad.length;
+          return lazyResources["assemblies"].length;
         }));
     }
   });

src/Components/test/E2ETest/Tests/WebAssemblyLazyLoadTest.cs

@@ -49,7 +49,7 @@ namespace Microsoft.AspNetCore.Components.E2ETest.Tests
             // Visit the route for the lazy-loaded assembly
             SetUrlViaPushState("/WithLazyAssembly");
 
-            var button = app.FindElement(By.Id("use-package-button"));
+            var button = Browser.Exists(By.Id("use-package-button"));
 
             // Now we should have requested the DLL
             Assert.True(HasLoadedAssembly("Newtonsoft.Json.dll"));
@@ -100,11 +100,11 @@ namespace Microsoft.AspNetCore.Components.E2ETest.Tests
             // Now the assembly has been loaded
             Assert.True(HasLoadedAssembly("LazyTestContentPackage.dll"));
 
-            var button = app.FindElement(By.Id("go-to-lazy-route"));
+            var button = Browser.Exists(By.Id("go-to-lazy-route"));
             button.Click();
 
             // Navigating the lazy-loaded route should show its content
-            var renderedElement = app.FindElement(By.Id("lazy-page"));
+            var renderedElement = Browser.Exists(By.Id("lazy-page"));
             Assert.True(renderedElement.Displayed);
         }
 
@@ -123,6 +123,39 @@ namespace Microsoft.AspNetCore.Components.E2ETest.Tests
             AssertLogContainsCriticalMessages("DoesNotExist.dll must be marked with 'BlazorWebAssemblyLazyLoad' item group in your project file to allow lazy-loading.");
         }
 
+        [Fact]
+        public void CanLazyLoadViaLinkChange()
+        {
+            // Navigate to a page without any lazy-loaded dependencies
+            SetUrlViaPushState("/");
+            var app = Browser.MountTestComponent<TestRouterWithLazyAssembly>();
+
+            // We start off with no lazy assemblies loaded
+            Assert.False(HasLoadedAssembly("LazyTestContentPackage.dll"));
+            Assert.False(HasLoadedAssembly("Newtonsoft.Json.dll"));
+
+            // Click the first link and verify that it worked as expected
+            var lazyAssemblyLink = Browser.Exists(By.Id("with-lazy-assembly"));
+            lazyAssemblyLink.Click();
+            var pkgButton = Browser.Exists(By.Id("use-package-button"));
+            Assert.True(HasLoadedAssembly("Newtonsoft.Json.dll"));
+            pkgButton.Click();
+
+            // Navigate to the next page and verify that it loaded its assembly
+            var lazyRoutesLink = Browser.Exists(By.Id("with-lazy-routes"));
+            lazyRoutesLink.Click();
+            Browser.Exists(By.Id("lazy-load-msg"));
+            Assert.True(HasLoadedAssembly("LazyTestContentPackage.dll"));
+
+            // Interact with that assembly to ensure it was loaded properly
+            var button = Browser.Exists(By.Id("go-to-lazy-route"));
+            button.Click();
+
+            // Navigating the lazy-loaded route should show its content
+            var renderedElement = Browser.Exists(By.Id("lazy-page"));
+            Assert.True(renderedElement.Displayed);
+        }
+
         private string SetUrlViaPushState(string relativeUri)
         {
             var pathBaseWithoutHash = ServerPathBase.Split('#')[0];

src/Components/test/testassets/BasicTestApp/RouterTest/Links.razor

@@ -20,7 +20,8 @@
     <li><NavLink href="/subdir/WithParameters/Name/Abc/LastName/McDef">With more parameters</NavLink></li>
     <li><NavLink href="/subdir/LongPage1">Long page 1</NavLink></li>
     <li><NavLink href="/subdir/LongPage2">Long page 2</NavLink></li>
-    <li><NavLink href="/subdir/WithLazyAssembly">With lazy assembly</NavLink></li>
+    <li><NavLink href="/subdir/WithLazyAssembly" id="with-lazy-assembly">With lazy assembly</NavLink></li>
+    <li><NavLink href="/subdir/WithLazyLoadedRoutes" id="with-lazy-routes">With lazy loaded routes</NavLink></li>
     <li><NavLink href="PreventDefaultCases">preventDefault cases</NavLink></li>
     <li><NavLink>Null href never matches</NavLink></li>
 </ul>