[msbuild] Merge runtimeconfig.dev.json into the runtime configuration. Fixes #26330

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -26,6 +26,7 @@
 	<UsingTask Runtime="$(_TaskRuntime)" TaskName="Xamarin.MacDev.Tasks.InstallNameTool" AssemblyFile="$(_TaskAssemblyName)" />
 	<UsingTask Runtime="$(_TaskRuntime)" TaskName="Xamarin.MacDev.Tasks.LinkNativeCode" AssemblyFile="$(_TaskAssemblyName)" />
 	<UsingTask Runtime="$(_TaskRuntime)" TaskName="Xamarin.MacDev.Tasks.MergeAppBundles" AssemblyFile="$(_TaskAssemblyName)" />
+	<UsingTask Runtime="$(_TaskRuntime)" TaskName="Xamarin.MacDev.Tasks.MergeRuntimeConfigFiles" AssemblyFile="$(_TaskAssemblyName)" />
 	<UsingTask Runtime="$(_TaskRuntime)" TaskName="Xamarin.MacDev.Tasks.MobileILStrip" AssemblyFile="$(_TaskAssemblyName)" />
 	<UsingTask Runtime="$(_TaskRuntime)" TaskName="Xamarin.MacDev.Tasks.MacDevMessage" AssemblyFile="$(_TaskAssemblyName)" />
 	<UsingTask Runtime="$(_TaskRuntime)" TaskName="Xamarin.MacDev.Tasks.PostTrimmingProcessing" AssemblyFile="$(_TaskAssemblyName)" />
@@ -1593,10 +1594,13 @@
 	<!-- App bundle creation tasks -->
 
 	<Target Name="_CreateRuntimeConfiguration"
-		Inputs="$(ProjectRuntimeConfigFilePath)"
+		Inputs="$(ProjectRuntimeConfigFilePath);$(ProjectRuntimeConfigDevFilePath)"
 		Outputs="$(_ParsedRuntimeConfigFilePath)"
 		DependsOnTargets="GenerateBuildRuntimeConfigurationFiles;_ComputePublishLocation"
 		>
+		<PropertyGroup>
+			<_MergedRuntimeConfigFilePath>$(DeviceSpecificIntermediateOutputPath)runtimeconfig.merged.json</_MergedRuntimeConfigFilePath>
+		</PropertyGroup>
 		<ItemGroup>
 			<!-- List all the properties passed to xamarin_bridge_vm_initialize in xamarin_vm_initialize (in runtime.m) -->
 			<_RuntimeConfigReservedProperties Include="APP_PATHS" />
@@ -1608,9 +1612,25 @@
 			<_RuntimeConfigReservedProperties Include="SYSTEM_CORELIB_DIRECTORY" />
 			<_RuntimeConfigReservedProperties Include="STARTUP_HOOKS" />
 		</ItemGroup>
-		<RuntimeConfigParserTask
+
+		<!-- Merge the runtimeconfig.dev.json (Debug-only switches such as Hot Reload) on top of the main
+		     runtimeconfig.json, because we hand the runtime configuration directly to the runtime and thus
+		     bypass hostfxr, which is what would normally merge these files. -->
+		<MergeRuntimeConfigFiles
 			Condition="'$(GenerateRuntimeConfigurationFiles)' == 'true'"
 			RuntimeConfigFile="$(ProjectRuntimeConfigFilePath)"
+			RuntimeConfigDevFile="$(ProjectRuntimeConfigDevFilePath)"
+			OutputFile="$(_MergedRuntimeConfigFilePath)"
+			>
+		</MergeRuntimeConfigFiles>
+
+		<ItemGroup Condition="'$(GenerateRuntimeConfigurationFiles)' == 'true'">
+			<FileWrites Include="$(_MergedRuntimeConfigFilePath)" />
+		</ItemGroup>
+
+		<RuntimeConfigParserTask
+			Condition="'$(GenerateRuntimeConfigurationFiles)' == 'true'"
+			RuntimeConfigFile="$(_MergedRuntimeConfigFilePath)"
 			OutputFile="$(_ParsedRuntimeConfigFilePath)"
 			RuntimeConfigReservedProperties="@(_RuntimeConfigReservedProperties)"
 			>

msbuild/Xamarin.Localization.MSBuild/MSBStrings.resx

@@ -1705,4 +1705,9 @@
         <value>Codesign failed with 'errSecInternalComponent'. This usually means the keychain is locked, which is common when building over SSH. Unlock the keychain first, for example by running 'security unlock-keychain ~/Library/Keychains/login.keychain-db'.</value>
         <comment>Shown when codesign fails with the 'errSecInternalComponent' error, which typically indicates a locked keychain.</comment>
     </data>
+    <data name="E7185" xml:space="preserve">
+        <value>The runtime configuration file '{0}' is not a valid JSON object.</value>
+        <comment>Shown when the main runtime configuration file (*.runtimeconfig.json) does not contain a valid JSON object at its root.
+{0} - The path to the runtime configuration file.</comment>
+    </data>
 </root>

msbuild/Xamarin.MacDev.Tasks/Tasks/MergeRuntimeConfigFiles.cs

@@ -0,0 +1,72 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System.IO;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+using Microsoft.Build.Framework;
+
+#nullable enable
+
+namespace Xamarin.MacDev.Tasks {
+	// This task merges the 'runtimeOptions.configProperties' from a '*.runtimeconfig.dev.json' file on top of
+	// the ones from the main '*.runtimeconfig.json' file, writing the merged result to an output file.
+	//
+	// We do this because we hand the runtime configuration directly to the runtime (bypassing hostfxr), and
+	// hostfxr is what would normally merge the dev file (which contains Debug-only switches such as Hot Reload)
+	// into the main runtime configuration. The dev values win, matching hostfxr's behavior.
+	public class MergeRuntimeConfigFiles : XamarinTask {
+		[Required]
+		public string RuntimeConfigFile { get; set; } = "";
+
+		public string? RuntimeConfigDevFile { get; set; }
+
+		[Required]
+		public string OutputFile { get; set; } = "";
+
+		static readonly JsonDocumentOptions documentOptions = new JsonDocumentOptions {
+			AllowTrailingCommas = true,
+			CommentHandling = JsonCommentHandling.Skip,
+		};
+
+		public override bool Execute ()
+		{
+			var mainNode = JsonNode.Parse (File.ReadAllText (RuntimeConfigFile), documentOptions: documentOptions);
+			if (mainNode is not JsonObject mainObject) {
+				Log.LogError (MSBStrings.E7185 /* The runtime configuration file '{0}' is not a valid JSON object. */, RuntimeConfigFile);
+				return false;
+			}
+
+			if (!string.IsNullOrEmpty (RuntimeConfigDevFile) && File.Exists (RuntimeConfigDevFile)) {
+				var devNode = JsonNode.Parse (File.ReadAllText (RuntimeConfigDevFile!), documentOptions: documentOptions);
+				var devConfigProperties = (devNode as JsonObject)? ["runtimeOptions"]? ["configProperties"] as JsonObject;
+				if (devConfigProperties is not null && devConfigProperties.Count > 0) {
+					var runtimeOptions = mainObject ["runtimeOptions"] as JsonObject;
+					if (runtimeOptions is null) {
+						runtimeOptions = new JsonObject ();
+						mainObject ["runtimeOptions"] = runtimeOptions;
+					}
+
+					var configProperties = runtimeOptions ["configProperties"] as JsonObject;
+					if (configProperties is null) {
+						configProperties = new JsonObject ();
+						runtimeOptions ["configProperties"] = configProperties;
+					}
+
+					foreach (var property in devConfigProperties) {
+						configProperties [property.Key] = property.Value is null ? null : JsonNode.Parse (property.Value.ToJsonString ());
+					}
+				}
+			}
+
+			var outputDirectory = Path.GetDirectoryName (OutputFile);
+			if (!string.IsNullOrEmpty (outputDirectory))
+				Directory.CreateDirectory (outputDirectory!);
+
+			File.WriteAllText (OutputFile, mainObject.ToJsonString (new JsonSerializerOptions { WriteIndented = true }));
+
+			return !Log.HasLoggedErrors;
+		}
+	}
+}

tests/msbuild/Xamarin.MacDev.Tasks.Tests/TaskTests/MergeRuntimeConfigFilesTaskTest.cs

@@ -0,0 +1,114 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System.IO;
+
+using NUnit.Framework;
+
+#nullable enable
+
+namespace Xamarin.MacDev.Tasks {
+
+	// Note: we can't use System.Text.Json types here to inspect the output, because the
+	// Xamarin.MacDev.Tasks assembly under test is ILMerged and also exposes System.Text.Json,
+	// which causes ambiguous type references. We assert on the raw JSON text instead.
+	[TestFixture]
+	public class MergeRuntimeConfigFilesTaskTest : TestBase {
+
+		string RunMerge (string mainJson, string? devJson)
+		{
+			var tmp = Cache.CreateTemporaryDirectory ();
+			var mainFile = Path.Combine (tmp, "app.runtimeconfig.json");
+			File.WriteAllText (mainFile, mainJson);
+
+			string? devFile = null;
+			if (devJson is not null) {
+				devFile = Path.Combine (tmp, "app.runtimeconfig.dev.json");
+				File.WriteAllText (devFile, devJson);
+			}
+
+			var outputFile = Path.Combine (tmp, "obj", "runtimeconfig.merged.json");
+
+			var task = CreateTask<MergeRuntimeConfigFiles> ();
+			task.RuntimeConfigFile = mainFile;
+			task.RuntimeConfigDevFile = devFile;
+			task.OutputFile = outputFile;
+
+			ExecuteTask (task);
+
+			Assert.That (outputFile, Does.Exist, "output file created");
+			return File.ReadAllText (outputFile);
+		}
+
+		[Test]
+		public void MergesDevPropertiesOntoMain ()
+		{
+			var mainJson = @"{
+	""runtimeOptions"": {
+		""tfm"": ""net10.0"",
+		""configProperties"": {
+			""OnlyInMain"": true,
+			""InBoth"": ""main-value""
+		}
+	}
+}";
+			var devJson = @"{
+	""runtimeOptions"": {
+		""configProperties"": {
+			""OnlyInDev"": ""dev-only"",
+			""InBoth"": ""dev-value""
+		}
+	}
+}";
+
+			var merged = RunMerge (mainJson, devJson);
+
+			// (a) properties only in main are preserved
+			Assert.That (merged, Does.Contain ("\"OnlyInMain\": true"), "OnlyInMain preserved");
+			// (b) properties only in dev are added
+			Assert.That (merged, Does.Contain ("\"OnlyInDev\": \"dev-only\""), "OnlyInDev added");
+			// (c) properties in both are taken from dev (dev wins)
+			Assert.That (merged, Does.Contain ("\"InBoth\": \"dev-value\""), "dev wins for InBoth");
+			Assert.That (merged, Does.Not.Contain ("main-value"), "main value for InBoth is gone");
+		}
+
+		[Test]
+		public void NoDevFile ()
+		{
+			var mainJson = @"{
+	""runtimeOptions"": {
+		""configProperties"": {
+			""OnlyInMain"": true
+		}
+	}
+}";
+
+			var merged = RunMerge (mainJson, null);
+
+			Assert.That (merged, Does.Contain ("\"OnlyInMain\": true"), "OnlyInMain preserved");
+		}
+
+		[Test]
+		public void DevFileWithoutConfigProperties ()
+		{
+			var mainJson = @"{
+	""runtimeOptions"": {
+		""configProperties"": {
+			""OnlyInMain"": true
+		}
+	}
+}";
+			var devJson = @"{
+	""runtimeOptions"": {
+		""additionalProbingPaths"": [ ""/some/path"" ]
+	}
+}";
+
+			var merged = RunMerge (mainJson, devJson);
+
+			Assert.That (merged, Does.Contain ("\"OnlyInMain\": true"), "OnlyInMain preserved");
+			// The dev file had no configProperties, so nothing from it should have leaked in.
+			Assert.That (merged, Does.Not.Contain ("additionalProbingPaths"), "dev-only non-configProperties content is not merged");
+		}
+	}
+}