don't let dotnetup failure count as a install script failure

eng/restore-toolset.ps1

@@ -242,11 +242,23 @@ function InstallDotNetSharedFrameworksWithInstallScript([string[]]$runtimeSpecs,
             $installArgs.Architecture = $architecture
         }
 
-        if (-not (Test-Path Variable:LASTEXITCODE)) { $global:LASTEXITCODE = 0 }
+        $global:LASTEXITCODE = 0
         & $installScript @installArgs
 
-        if ($lastExitCode -ne 0) {
-            throw "Failed to install shared framework $version to '$dotNetRoot' using dotnet install script for architecture '$architecture' (exit code '$lastExitCode')."
+        $installScriptExitCode = $LASTEXITCODE
+
+        # The install script is a PowerShell script, so $LASTEXITCODE is only updated when it
+        # invokes a native process or calls `exit`. A successful run that returns normally can
+        # leave a stale exit code, and a soft failure can leave 0. We reset $LASTEXITCODE above to
+        # neutralize a stale non-zero (false failure); pair that with a filesystem check that the
+        # expected shared-framework version actually landed to catch a false success (exit 0 but
+        # nothing installed).
+        $sharedFrameworkName = if ($component -eq 'aspnetcore') { 'Microsoft.AspNetCore.App' } elseif ($component -eq 'windowsdesktop') { 'Microsoft.WindowsDesktop.App' } else { 'Microsoft.NETCore.App' }
+        $frameworkInstalled = Test-Path -PathType Container (Join-Path $dotNetRoot "shared\$sharedFrameworkName\$version*")
+
+        if ($installScriptExitCode -ne 0 -or -not $frameworkInstalled) {
+            $architectureMessage = if ([string]::IsNullOrEmpty($architecture)) { "" } else { " for architecture '$architecture'" }
+            throw "Failed to install shared framework $version to '$dotNetRoot' using dotnet install script$architectureMessage (exit code '$installScriptExitCode', installed '$frameworkInstalled')."
         }
     }
 }

eng/restore-toolset.sh

@@ -151,13 +151,14 @@ function InstallDotNetSharedFrameworksWithInstallScript {
 
   for spec in "$@"; do
     local component="dotnet"
-    local install_version="$spec"
+    local version="$spec"
     if [[ "$spec" == *@* ]]; then
       component="${spec%@*}"
-      install_version="${spec#*@}"
+      version="${spec#*@}"
     fi
     # Map dotnetup channel (e.g. "9.0") to the specific version the install
     # script's --version parameter expects (e.g. "9.0.0").
+    local install_version="$version"
     if [[ "$install_version" =~ ^[0-9]+\.[0-9]+$ ]]; then
       install_version="$install_version.0"
     fi
@@ -167,11 +168,45 @@ function InstallDotNetSharedFrameworksWithInstallScript {
       install_args+=(--architecture "$arch")
     fi
 
+    # Disable errexit around the install-script call so the exit-code and filesystem checks
+    # below always run (mirrors the dotnetup invocation above). Capture $? before restoring
+    # errexit, since the restore itself runs commands that overwrite $?.
+    local restore_errexit=false
+    if [[ $- == *e* ]]; then
+      restore_errexit=true
+      set +e
+    fi
     bash "$install_script" "${install_args[@]}"
     local lastexitcode=$?
+    if [[ "$restore_errexit" == true ]]; then
+      set -e
+    fi
+
+    # bash $? is never stale (unlike PowerShell's $LASTEXITCODE), so the false-failure case
+    # does not apply here. But a soft failure could still exit 0 without installing, so pair
+    # the exit code with a filesystem check that the expected shared-framework version landed
+    # to catch a false success.
+    local shared_framework_name="Microsoft.NETCore.App"
+    case "$component" in
+      aspnetcore) shared_framework_name="Microsoft.AspNetCore.App" ;;
+      windowsdesktop) shared_framework_name="Microsoft.WindowsDesktop.App" ;;
+    esac
+    local framework_installed=false
+    if compgen -G "$dotnet_root/shared/$shared_framework_name/$version*" > /dev/null 2>&1; then
+      framework_installed=true
+    fi
+
+    # Promote a false success (exit 0 but nothing on disk) to a real failure.
+    if [[ $lastexitcode == 0 && "$framework_installed" != true ]]; then
+      lastexitcode=1
+    fi
 
     if [[ $lastexitcode != 0 ]]; then
-      echo "Failed to install shared framework spec '$spec' to '$dotnet_root' using dotnet install script for architecture '$arch' (exit code '$lastexitcode')."
+      local architecture_message=""
+      if [[ -n "$arch" ]]; then
+        architecture_message=" for architecture '$arch'"
+      fi
+      echo "Failed to install shared framework spec '$spec' to '$dotnet_root' using dotnet install script${architecture_message} (exit code '$lastexitcode', installed '$framework_installed')."
       ExitWithExitCode $lastexitcode
     fi
   done