From 3059490cb12f7f9c591a59c76d36af2e2e491a5e Mon Sep 17 00:00:00 2001
From: Jonathan Peppers <jonathan.peppers@microsoft.com>
Date: Tue, 2 Jun 2026 16:46:35 -0500
Subject: [PATCH] [Xamarin.Android.Build.Tasks] Fix _CreateAar race under
 parallel build (#11527)

Fixes: https://github.com/dotnet/android/issues/11514

Under parallel build (`-m`), library projects with `<GeneratePackageOnBuild>true</GeneratePackageOnBuild>` can intermittently fail with:

```
error XARLP7024: System.IO.IOException: The process cannot access the file
'.../<assembly>.aar' because it is being used by another process.
   at Microsoft.Android.Build.Tasks.Files.HashFile(...)
   at Xamarin.Android.Tasks.ResolveLibraryProjectImports.Extract(...)
```

The reporter's `mini.binlog` shows `_CreateAar` running in two distinct project instances of the same library csproj, and a local repro shows it running **three** times for a single `dotnet build`:

* Once from `BuildDependsOn`.
* Once from `_UpdateAndroidResourcesDependsOn` on the regular `Build` chain.
* Once more from `_UpdateAndroidResourcesDependsOn` on a Pack-dispatched project instance entered via `_GetFrameworkAssemblyReferences` (NuGet pack's per-TFM inner-target dispatch). That instance has different global properties, so MSBuild does not dedupe it and may run it on a fresh worker node, opening a write/write or write/read race against consumers that read the `.aar` via `Files.HashFile`.

`_UpdateAndroidResources` is the aapt2/resource-designer step; producing the publish artifact (`.aar`) is unrelated. Remove `_CreateAar` from `_UpdateAndroidResourcesDependsOn`. The AAR is still produced because `_CreateAar` remains in `BuildDependsOn` for non-application projects, and Pack depends on `Build`, so `dotnet build` and `dotnet pack` both still create it.

This collapses the count to a single invocation per build, eliminating the race window and the redundant AAR rewrites at the root rather than papering over them with reader retries or atomic writes.

Added `IncrementalBuildTest.CreateAarRunsOnceWithGeneratePackageOnBuild`: builds a library with `GeneratePackageOnBuild=true` at `LoggerVerbosity.Detailed` and asserts `Target "_CreateAar" in file ...` appears exactly once in the build log.

Verified against the issue's repro project: count drops from **3 to 1** with the fix.
---
 .../Microsoft.Android.Sdk.BuildOrder.targets  |  2 +-
 .../IncrementalBuildTest.cs                   | 27 +++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.BuildOrder.targets b/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.BuildOrder.targets
index 9bb66ab4073..724c4a3de7b 100644
--- a/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.BuildOrder.targets
+++ b/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.BuildOrder.targets
@@ -174,7 +174,6 @@ properties that determine build ordering.
       _CheckForDeletedResourceFile;
       _ComputeAndroidResourcePaths;
       _UpdateAndroidResgen;
-      _CreateAar;
     </_UpdateAndroidResourcesDependsOn>
     <CompileDependsOn>
       _SetupMSBuildAllProjects;
@@ -244,6 +243,7 @@ properties that determine build ordering.
     <_IncludeAarInNuGetPackageDependsOn>
       _BuildAndroidGradleProjects;
       _CategorizeAndroidLibraries;
+      _CreateAar;
     </_IncludeAarInNuGetPackageDependsOn>
   </PropertyGroup>
 
diff --git a/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/IncrementalBuildTest.cs b/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/IncrementalBuildTest.cs
index dbb0bd836f3..c2b61a40093 100644
--- a/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/IncrementalBuildTest.cs
+++ b/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/IncrementalBuildTest.cs
@@ -522,6 +522,33 @@ public void LibraryProjectTargetsDoNotBreak ()
 			}
 		}
 
+		[Test]
+		public void CreateAarRunsOnceWithGeneratePackageOnBuild ()
+		{
+			// https://github.com/dotnet/android/issues/11514
+			// `_CreateAar` was being invoked twice for a single library build when
+			// `GeneratePackageOnBuild=true`: once via `BuildDependsOn`, and again via
+			// NuGet pack's per-TFM dispatch which entered the project at
+			// `_GetFrameworkAssemblyReferences`. That target transitively pulled in
+			// `UpdateAndroidResources` -> `_UpdateAndroidResources` -> `_CreateAar`
+			// via `_UpdateAndroidResourcesDependsOn`. The second invocation can race
+			// the first writer when parallel builds (-m) consumers concurrently read
+			// the .aar via `Files.HashFile`, producing `XARLP7024`.
+			var proj = new XamarinAndroidLibraryProject ();
+			proj.SetProperty ("GeneratePackageOnBuild", "true");
+			using (var b = CreateDllBuilder ()) {
+				b.Verbosity = LoggerVerbosity.Detailed;
+				Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+				// MSBuild emits "Building target X completely" only when the target
+				// actually runs (not when it is entered then skipped due to empty
+				// Inputs/Outputs). Counting that message gives us the number of real
+				// `_CreateAar` executions, which is what races on disk in #11514.
+				int count = b.LastBuildOutput.Count (l => l.Contains ("Building target \"_CreateAar\""));
+				Assert.AreEqual (1, count,
+					$"`_CreateAar` should only execute once per build of a library project; was {count}.");
+			}
+		}
+
 		[Test]
 		public void ManifestMergerIncremental ()
 		{