Fixes for azurepipeline extend variable support

.github/workflows/nuget.yml

@@ -174,6 +174,7 @@ jobs:
         gharun --event azpipelines -C testworkflows/azpipelines/relative-and-absolute-paths -W testworkflows/azpipelines/relative-and-absolute-paths/pipeline.yml
         gharun --event azpipelines -C testworkflows/azpipelines/null-each-loop -W testworkflows/azpipelines/null-each-loop/pipeline.yml
         gharun --event azpipelines -C testworkflows/azpipelines/cross-repo-relative-paths/repo1 -W testworkflows/azpipelines/cross-repo-relative-paths/repo1/pipeline.yml --local-repository central/shared@main=testworkflows/azpipelines/cross-repo-relative-paths/repo2
+        gharun --event azpipelines -C testworkflows/azpipelines/extend-variables -W testworkflows/azpipelines/extend-variables/pipeline.yml
     - name: Windows Container Change Firewall Settings
       if: contains(matrix.os, 'windows') && !job.container
       run: |

src/Sdk/AzurePipelines/Pipeline.cs

@@ -134,8 +134,12 @@ public Pipeline Parse(Runner.Server.Azure.Devops.Context context, TemplateToken
                 }
             }
             if(parent.Variables != null) {
-                foreach(var cr in parent.Variables) {
-                    Variables[cr.Key] = cr.Value;
+                if (Variables == null) {
+                    Variables = parent.Variables;
+                    variablesMetaData = parent.variablesMetaData;
+                } else {
+                    // emulate the error message provided by Azure Pipelines
+                    throw new NotSupportedException("__built-in-schema.yml (Line: 40, Col:11): 'variables' is already defined");
                 }
             }
         }
@@ -208,4 +212,4 @@ public DictionaryContextData ToContextData() {
         return pipeline;
     }
 }
-}
+}

testworkflows/azpipelines/extend-variables/base.yml

@@ -0,0 +1,6 @@
+# not supported:
+#variables:
+#  provided: value
+
+steps:
+- script: exit 0

testworkflows/azpipelines/extend-variables/pipeline.yml

@@ -0,0 +1,5 @@
+variables:
+  pipeline: value
+
+extends:
+  template: base.yml