From 6147c9744911181d4a2c0791a21fcafbe4349f1a Mon Sep 17 00:00:00 2001
From: Larry Ewing <lewing@gmail.com>
Date: Sat, 18 Jul 2026 17:21:16 -0500
Subject: [PATCH] [wasm] Fix AOT + BlazorWebAssemblyLazyLoad startup crash

When an app is published with AOT and uses BlazorWebAssemblyLazyLoad, the
Mono runtime crashes during startup (appdomain.c assertion / interp.c NIY
'should not be reached').

Root cause: an AOT image hard-binds to every assembly it references when the
image is loaded (see load_aot_module in aot-runtime.c, which eagerly calls
load_image for all referenced images unless aot-lazy-assembly-load is set). A
lazy-loaded assembly is not present at runtime startup, so any AOT image that
references it is marked 'unusable because dependency <asm> is not found' and its
methods fall back to the interpreter, which then hits an unsupported construct
and aborts.

Enable the runtime's 'aot-lazy-assembly-load' option automatically from the
WebAssembly SDK when AOT is combined with BlazorWebAssemblyLazyLoad, so
referenced lazy assemblies are only bound when they are actually loaded.

Also add AOT + lazy-load regression coverage to LazyLoadingTests, which
previously only exercised the interpreter (Debug) configuration.

Fixes #125794

(cherry picked from commit 3816e04674acccc8120fb1680c3cf99eb388613d)
---
 ...rosoft.NET.Sdk.WebAssembly.Browser.targets |  6 +++++
 .../wasm/Wasm.Build.Tests/LazyLoadingTests.cs | 22 +++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets b/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets
index ef3a27f424f8aa..0c0df98871681b 100644
--- a/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets
+++ b/src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets
@@ -197,6 +197,12 @@ Copyright (c) .NET Foundation. All rights reserved.
       <_WasmEnableWebcil Condition="'$(_WasmEnableWebcil)' == ''">true</_WasmEnableWebcil>
       <_BlazorWebAssemblyJiterpreter>$(BlazorWebAssemblyJiterpreter)</_BlazorWebAssemblyJiterpreter>
       <_BlazorWebAssemblyRuntimeOptions>$(BlazorWebAssemblyRuntimeOptions)</_BlazorWebAssemblyRuntimeOptions>
+      <!-- With AOT, each AOT image hard-binds to every assembly it references when the image is
+           loaded. A BlazorWebAssemblyLazyLoad assembly is not present at runtime startup, so any
+           AOT image that references it would be marked unusable and its methods would fall back to
+           the interpreter (which can then hit unsupported constructs and abort). Enable lazy AOT
+           assembly loading so referenced lazy assemblies are only bound when they are loaded. -->
+      <_BlazorWebAssemblyRuntimeOptions Condition="'$(RunAOTCompilation)' == 'true' and @(BlazorWebAssemblyLazyLoad->Count()) != 0 and !$(_BlazorWebAssemblyRuntimeOptions.Contains('--aot-lazy-assembly-load'))">$([System.String]::Concat('$(_BlazorWebAssemblyRuntimeOptions)', ' --aot-lazy-assembly-load').Trim())</_BlazorWebAssemblyRuntimeOptions>
       <_WasmInlineBootConfig>$(WasmInlineBootConfig)</_WasmInlineBootConfig>
       <_WasmInlineBootConfig Condition="'$(_WasmInlineBootConfig)' == '' and '$(_TargetingNET100OrLater)' == 'true' and '$(WasmBootConfigFileName)' == ''">true</_WasmInlineBootConfig>
       <_WasmInlineBootConfig Condition="'$(_WasmInlineBootConfig)' == ''">false</_WasmInlineBootConfig>
diff --git a/src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs b/src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs
index 65011962374221..9dc68a80e2a5c0 100644
--- a/src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs
+++ b/src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs
@@ -50,6 +50,28 @@ public async Task LoadLazyAssemblyBeforeItIsNeeded(string lazyLoadingTestExtensi
         }
     }
 
+    [Fact]
+    [TestCategory("native-mono")]
+    public async Task LoadLazyAssemblyWithAOT()
+    {
+        // Regression coverage for https://github.com/dotnet/runtime/issues/125794:
+        // AOT combined with BlazorWebAssemblyLazyLoad must still initialize the runtime
+        // and lazily load assemblies at runtime. AOT is only supported when publishing
+        // in Release, so this exercises the publish + AOT + lazy-load combination that
+        // the Debug build/run tests above do not cover.
+        Configuration config = Configuration.Release;
+        ProjectInfo info = CopyTestAsset(config, aot: true, TestAsset.WasmBasicTestApp, "LazyLoadingTestsAOT");
+        PublishProject(info, config, new PublishOptions(AOT: true, ExtraMSBuildArgs: "-p:TestLazyLoading=true"));
+
+        RunResult result = await RunForPublishWithWebServer(new BrowserRunOptions(
+            config,
+            AOT: true,
+            TestScenario: "LazyLoadingTest"
+        ));
+
+        Assert.True(result.TestOutput.Any(m => m.Contains("FirstName")), "The lazy loading test didn't emit expected message with JSON");
+    }
+
     [Fact]
     public async Task FailOnMissingLazyAssembly()
     {