Fail fast on variable conflicts with subscription config (#4915)

Co-authored-by: Ben Broderick Phillips <bebroder@microsoft.com>

eng/common/TestResources/SubConfig-Helpers.ps1

@@ -109,11 +109,17 @@ function SetSubscriptionConfiguration([object]$subscriptionConfiguration)
                 # Mark values as secret so we don't print json blobs containing secrets in the logs.
                 # Prepend underscore to the variable name, so we can still access the variable names via environment
                 # variables if they get set subsequently.
+                if ([Environment]::GetEnvironmentVariable($nestedPair.Name)) {
+                    throw "Environment variable '$($nestedPair.Name)' is already set. Check the tests.yml/ci.yml EnvVars parameter does not conflict with the subscription config json"
+                }
                 if (ShouldMarkValueAsSecret "AZURE_" $nestedPair.Name $nestedPair.Value) {
                     Write-Host "##vso[task.setvariable variable=_$($nestedPair.Name);issecret=true;]$($nestedPair.Value)"
                 }
             }
         } else {
+            if ([Environment]::GetEnvironmentVariable($pair.Name)) {
+                throw "Environment variable '$($pair.Name)' is already set. Check the tests.yml/ci.yml EnvVars parameter does not conflict with the subscription config json"
+            }
             if (ShouldMarkValueAsSecret "AZURE_" $pair.Name $pair.Value) {
                 Write-Host "##vso[task.setvariable variable=_$($pair.Name);issecret=true;]$($pair.Value)"
             }

eng/common/TestResources/build-test-resource-config.yml

@@ -5,6 +5,10 @@ parameters:
   - name: SubscriptionConfigurations
     type: object
     default: null
+  # EnvVars is used to help diagnose variable conflict issues early
+  - name: EnvVars
+    type: object
+    default: null
 
 steps:
   - ${{ if parameters.SubscriptionConfiguration }}:
@@ -16,6 +20,8 @@ steps:
         . ./eng/common/TestResources/SubConfig-Helpers.ps1
         SetSubscriptionConfiguration $config
       displayName: Initialize SubscriptionConfiguration variable
+      ${{ if parameters.EnvVars }}:
+        env: ${{ parameters.EnvVars }}
 
   - ${{ if parameters.SubscriptionConfigurations }}:
     - pwsh: |
@@ -36,3 +42,5 @@ steps:
           UpdateSubscriptionConfiguration $configBase $config
 
         displayName: Merge Test Resource Configurations
+        ${{ if parameters.EnvVars }}:
+          env: ${{ parameters.EnvVars }}