Fix SelectTests SDK bootstrap

Author: JamesNK

.github/actions/select-tests/action.yml

@@ -84,22 +84,23 @@ runs:
         persist-credentials: false
 
     # Minimal toolchain bootstrap -- not a full ./restore.sh. The repo SDK is .NET 11 while
-    # SelectTests intentionally targets .NET 10 for its Microsoft.Build ProjectGraph dependency,
-    # so install that one pinned runtime without restoring the rest of the repo toolsets.
+    # SelectTests intentionally targets .NET 10 for its Microsoft.Build ProjectGraph dependency.
+    # Install a .NET 10 SDK because MSBuildLocator needs its MSBuild assemblies; a runtime alone can
+    # execute SelectTests but leaves RegisterDefaults with no MSBuild instance to discover.
     - name: Install toolchain (minimal)
       if: ${{ inputs.setupDotNet == 'true' }}
       shell: bash
       run: |
         set -euo pipefail
         ./dotnet.sh --version
-        runtime_version="$(sed -n 's:.*<DotNetRuntimeNet10VersionForTesting>\(.*\)</DotNetRuntimeNet10VersionForTesting>.*:\1:p' eng/Versions.props)"
-        if [ -z "$runtime_version" ]; then
-          echo "::error::Could not read DotNetRuntimeNet10VersionForTesting from eng/Versions.props." >&2
+        sdk_version="$(sed -n 's:.*<DotNetSdkNet10VersionForTesting>\(.*\)</DotNetSdkNet10VersionForTesting>.*:\1:p' eng/Versions.props)"
+        if [ -z "$sdk_version" ]; then
+          echo "::error::Could not read DotNetSdkNet10VersionForTesting from eng/Versions.props." >&2
           exit 1
         fi
         ./eng/common/dotnet-install.sh \
-          -runtime dotnet \
-          -version "$runtime_version"
+          -runtime sdk \
+          -version "$sdk_version"
 
     - name: Select relevant tests
       id: select

eng/Versions.props

@@ -14,6 +14,7 @@
     <DotNetRuntimeNet10VersionForTesting>10.0.8</DotNetRuntimeNet10VersionForTesting>
     <DotNetSdkNet8VersionForTesting>8.0.415</DotNetSdkNet8VersionForTesting>
     <DotNetSdkNet9VersionForTesting>9.0.306</DotNetSdkNet9VersionForTesting>
+    <DotNetSdkNet10VersionForTesting>10.0.400</DotNetSdkNet10VersionForTesting>
     <DotNetSdkNet11VersionForTesting>11.0.100-rc.1.26419.129</DotNetSdkNet11VersionForTesting>
     <!-- Keep these aliases for global.json and non-template test infrastructure. -->
     <DotNetRuntimePreviousVersionForTesting>$(DotNetRuntimeNet8VersionForTesting)</DotNetRuntimePreviousVersionForTesting>

tests/Infrastructure.Tests/TestTriggerMap/SelectTestsWorkflowTests.cs

@@ -33,17 +33,19 @@ public void SelectTestsActionGatesForceAllOnBooleanInputNotPrBody()
     }
 
     // The repo SDK can move ahead of the target framework used by SelectTests. The minimal bootstrap
-    // must install that runtime through Arcade's wrapper using the wrapper's single-dash switches;
-    // unsupported raw dotnet-install switches make the setup job exit before test selection runs.
+    // must install that SDK through Arcade's wrapper because MSBuildLocator needs the SDK's MSBuild
+    // assemblies; installing only the runtime can execute SelectTests but cannot build its project graph.
     [Fact]
-    public void SelectTestsActionInstallsPinnedRuntimeWithArcadeWrapperArguments()
+    public void SelectTestsActionInstallsPinnedSdkWithArcadeWrapperArguments()
     {
         var action = File.ReadAllText(SelectTestsActionPath);
 
-        Assert.Contains("<DotNetRuntimeNet10VersionForTesting>", File.ReadAllText(VersionsPropsPath));
+        Assert.Contains("<DotNetSdkNet10VersionForTesting>", File.ReadAllText(VersionsPropsPath));
+        Assert.Contains("<DotNetSdkNet10VersionForTesting>", action);
         Assert.Contains("./eng/common/dotnet-install.sh", action);
-        Assert.Contains("-runtime dotnet", action);
-        Assert.Contains("-version \"$runtime_version\"", action);
+        Assert.Contains("-runtime sdk", action);
+        Assert.Contains("-version \"$sdk_version\"", action);
+        Assert.DoesNotContain("-runtime dotnet", action);
         Assert.DoesNotContain("--install-dir", action);
         Assert.DoesNotContain("--skip-non-versioned-files", action);
     }