Use GitHub App authentication for promotion validation

.vault-config/product-builds-engkeyvault.yaml

@@ -57,6 +57,13 @@ secrets:
       hasWebhookSecret: false
       hasOAuthSecret: false
 
+  arcade-promotion-validation-app:
+    type: github-app-secret
+    parameters:
+      hasPrivateKey: true
+      hasWebhookSecret: false
+      hasOAuthSecret: false
+
   BotAccount-dotnet-maestro-bot:
     type: github-account
     parameters:

azure-pipelines.yml

@@ -69,3 +69,4 @@ extends:
           oneLocEnabled: true
         microbuildUseESRP: true
         runValidateSdk: ${{ parameters.runValidateSdk }}
+        runPromotionValidation: true

eng/build.yml

@@ -5,14 +5,16 @@ parameters:
 - name: microbuildUseESRP
   default: false
   type: boolean
-# When true (default), run the Validate Arcade SDK stage: three parallel validations of the newly
-# produced SDK - signing validation, self-build (self-bootstrap), and promotion to a test channel
-# with the new Arcade. Set to false as a bootstrap-breakage escape hatch to ship a new Arcade when
-# the current one cannot self-validate; the promotion to '.NET Eng - Latest' still runs on build
-# success.
+# When true (default), run the Validate Arcade SDK stage for the newly produced SDK. Set to false as
+# a bootstrap-breakage escape hatch to ship a new Arcade when the current one cannot self-validate;
+# the promotion to '.NET Eng - Latest' still runs on build success.
 - name: runValidateSdk
   default: true
   type: boolean
+# Promotion validation requires credentials restricted to the internal official pipeline.
+- name: runPromotionValidation
+  default: false
+  type: boolean
 # The channel successful builds are promoted to, and the source branch the promotion gate runs on.
 # Defined once here so they aren't duplicated throughout the promotion stage below.
 - name: promotionChannel
@@ -83,7 +85,8 @@ stages:
       parameters:
         microbuildUseESRP: ${{ parameters.microbuildUseESRP }}
     # Validate that build promotion works when driven by the newly built Arcade.
-    - template: /eng/validate-promotion.yml@self
+    - ${{ if eq(parameters.runPromotionValidation, true) }}:
+      - template: /eng/validate-promotion.yml@self
 
 - template: /eng/common/templates-official/post-build/post-build.yml@self
   parameters:

eng/validate-promotion.yml

@@ -17,7 +17,6 @@ jobs:
         name: ReleaseConfigs
         path: release_configs
     variables:
-    - group: Arcade-Promotion-GitHub
     - template: /eng/common/templates-official/variables/pool-providers.yml@self
     pool:
       name: $(DncEngInternalBuildPool)
@@ -45,6 +44,14 @@ jobs:
           $azdoToken = az account get-access-token --resource "499b84ac-1321-427f-aa17-267ca6975798" --query accessToken -o tsv
           if ($LASTEXITCODE -ne 0) { Write-Error "Failed to get AzDO token"; exit 1 }
           Write-Host "##vso[task.setvariable variable=WifAzDoToken;issecret=true]$azdoToken"
+    - template: /eng/common/templates-official/steps/get-github-app-token.yml@self
+      parameters:
+        azureSubscription: "dnceng-arcade-promotion-githubapp"
+        keyVaultName: EngKeyVault
+        appIdSecretName: arcade-promotion-validation-app-app-id
+        appPrivateKeySecretName: arcade-promotion-validation-app-app-private-key
+        installationOwner: dotnet
+        outputVariableName: GitHubAppInstallationToken
     - task: AzureCLI@2
       displayName: Validate promotion with the newly built Arcade
       inputs:
@@ -56,4 +63,4 @@ jobs:
           -BuildId $(BARBuildId)
           -Commit $(Build.SourceVersion)
           -AzdoToken $(WifAzDoToken)
-          -GitHubPat $(BotAccount-dotnet-maestro-bot-PAT)
+          -GitHubToken $(GitHubAppInstallationToken)

eng/validation/validate-promotion.ps1

@@ -20,7 +20,7 @@ Param(
   [Parameter(Mandatory=$true)][int]    $BuildId,     # BAR build id of the build being validated.
   [Parameter(Mandatory=$true)][string] $Commit,      # The commit that produced this build (Build.SourceVersion).
   [Parameter(Mandatory=$true)][string] $AzdoToken,   # AzDO OAuth/AAD access token (WIF), not a PAT; needs code read/write on the mirror.
-  [Parameter(Mandatory=$true)][string] $GitHubPat,   # GitHub credential for dependency metadata and common-script retrieval.
+  [Parameter(Mandatory=$true)][string] $GitHubToken, # Short-lived GitHub App installation token for dependency metadata and common scripts.
   [string] $AzdoOrg      = 'dnceng',
   [string] $AzdoProject  = 'internal',
   [string] $AzdoRepoName = 'dotnet-arcade',
@@ -95,7 +95,7 @@ try {
 
     # The --id form retrieves dependency metadata and common scripts from GitHub while applying
     # updates, so it requires a GitHub credential even when there are no coherent dependencies.
-    & $darc update-dependencies --id $BuildId --azdev-pat $AzdoToken --github-pat $GitHubPat --ci
+    & $darc update-dependencies --id $BuildId --azdev-pat $AzdoToken --github-pat $GitHubToken --ci
     if ($LASTEXITCODE -ne 0) { throw "darc update-dependencies failed." }
 
     # A no-op update means darc produced the same versions already pinned, which is unexpected for a