Infra changes to fail online builds when prebuilts exist (#17198)

eng/pipelines/templates/stages/vmr-build.yml

@@ -87,28 +87,28 @@ stages:
       useMonoRuntime: false              # 🚫
       withPreviousSDK: false             # 🚫
 
-  - template: ../jobs/vmr-build.yml
-    parameters:
-      # Changing the build name requires updating the referenced name in the source-build-sdk-diff-tests.yml pipeline
-      buildName: CentOSStream8_Offline_MsftSdk
-      isBuiltFromVmr: ${{ parameters.isBuiltFromVmr }}
-      vmrBranch: ${{ variables.VmrBranch }}
-      architecture: x64
-      pool:
-        name: ${{ variables.defaultPoolName }}
-        demands: ${{ variables.defaultPoolDemands }}
-      container: ${{ parameters.centOSStream8Container }}
-      buildFromArchive: true             # ✅
-      enablePoison: false                # 🚫
-      excludeOmniSharpTests: true        # ✅
-      runOnline: false                   # 🚫
-      useMonoRuntime: false              # 🚫
-      withPreviousSDK: false             # 🚫
-
   - ${{ if ne(variables['Build.Reason'], 'PullRequest') }}:
 
     # CI - Stage 1 x64 legs ------------------------------------
 
+    - template: ../jobs/vmr-build.yml
+      parameters:
+        # Changing the build name requires updating the referenced name in the source-build-sdk-diff-tests.yml pipeline
+        buildName: CentOSStream8_Offline_MsftSdk
+        isBuiltFromVmr: ${{ parameters.isBuiltFromVmr }}
+        vmrBranch: ${{ variables.VmrBranch }}
+        architecture: x64
+        pool:
+          name: ${{ variables.defaultPoolName }}
+          demands: ${{ variables.defaultPoolDemands }}
+        container: ${{ parameters.centOSStream8Container }}
+        buildFromArchive: true             # ✅
+        enablePoison: false                # 🚫
+        excludeOmniSharpTests: true        # ✅
+        runOnline: false                   # 🚫
+        useMonoRuntime: false              # 🚫
+        withPreviousSDK: false             # 🚫
+
     - template: ../jobs/vmr-build.yml
       parameters:
         # Changing the build name requires updating the referenced name in the source-build-sdk-diff-tests.yml pipeline

src/SourceBuild/content/build.proj

@@ -167,7 +167,8 @@
           AfterTargets="Build"
           DependsOnTargets="
             CheckIfPrebuiltsExistToPack;
-            CreatePrebuiltsTarballIfPrebuiltsExist"/>
+            CreatePrebuiltsTarballIfPrebuiltsExist;
+            ErrorOnPrebuilts"/>
 
   <Target Name="CheckIfPrebuiltsExistToPack">
     <!-- Directory existence doesn't mean there are files inside. Use a pattern to find files. -->
@@ -190,4 +191,9 @@
 
     <Message Text="Tarball '$(TarballFilePath)' was successfully created from '$(TarballWorkingDir)'" Importance="High" />
   </Target>
+
+  <Target Name="ErrorOnPrebuilts"
+          Condition="'@(PrebuiltFile->Count())' != '0' AND '$(SkipErrorOnPrebuilts)' != 'true'">
+    <Error Text="@(PrebuiltFile->Count()) Prebuilts Exist" />
+  </Target>
 </Project>