Merged PR 797274: Allow loading hosted pool configuration file from env variables

Public/Src/Cache/BuildCacheResource/Helper/BuildCacheResourceHelper.cs

@@ -41,9 +41,7 @@ namespace BuildXL.Cache.BuildCacheResource.Helper
                 throw new ArgumentNullException(nameof(content), "The containing JSON is null");
             }
 
-            var hostedPoolBuildCacheConfiguration = new HostedPoolBuildCacheConfiguration { AssociatedBuildCaches = buildCaches! };
-
-            return hostedPoolBuildCacheConfiguration;
+            return new HostedPoolBuildCacheConfiguration { AssociatedBuildCaches = buildCaches! };
         }
 
         /// <summary>
@@ -53,7 +51,7 @@ namespace BuildXL.Cache.BuildCacheResource.Helper
         public static bool TrySelectBuildCache(this HostedPoolBuildCacheConfiguration hostedPoolBuildCacheConfiguration, string? hostedPoolActiveBuildCacheName, out BuildCacheConfiguration? buildCacheConfiguration)
         {
             // If an active cache is not provided, the first one in the collection of associated caches is the default
-            if (hostedPoolBuildCacheConfiguration is null)
+            if (string.IsNullOrEmpty(hostedPoolActiveBuildCacheName))
             {
                 buildCacheConfiguration = hostedPoolBuildCacheConfiguration!.AssociatedBuildCaches.First();
                 return true;

Public/Src/Cache/VerticalStore/MemoizationStoreAdapter/BlobCacheConfig.cs

@@ -177,6 +177,18 @@ namespace BuildXL.Cache.MemoizationStoreAdapter
         [DefaultValue(false)]
         public bool IsReadOnly { get; set; }
 
+        /// <summary>
+        /// Authenticate by using a file that contains a <see cref="HostedPoolBuildCacheConfigurationFile"/>
+        /// </summary>
+        /// <remarks>
+        /// The preferred authentication method is to use a managed identity (<see cref="StorageAccountEndpoint"/>
+        /// and <see cref="ManagedIdentityId"/>). However, this is unsupported for sharded scenarios and isn't
+        /// available outside of Azure. Use <see cref="ConnectionStringFileEnvironmentVariableName"/> if that's
+        /// your use-case.
+        /// </remarks>
+        [DefaultValue("BlobCacheFactoryHostedPoolConfigurationFile")]
+        public string HostedPoolBuildCacheConfigurationFileEnvironmentVariableName { get; set; }
+
         /// <summary>
         /// When not null, we are running on the context of 1ESHP and a set of cache resources are associated to the pool. The value of this string points
         /// to the JSON file describing this topology.

Public/Src/Cache/VerticalStore/MemoizationStoreAdapter/BlobCacheFactory.cs

@@ -43,9 +43,15 @@ namespace BuildXL.Cache.MemoizationStoreAdapter
             var context = new OperationContext(tracingContext);
 
             // Case where an ADO build cache resource is configured
-            if (configuration.HostedPoolBuildCacheConfigurationFile != null)
+            var hostedPoolConfigurationFilePath = configuration.HostedPoolBuildCacheConfigurationFile;
+            if (string.IsNullOrEmpty(hostedPoolConfigurationFilePath) && !string.IsNullOrEmpty(configuration.HostedPoolBuildCacheConfigurationFileEnvironmentVariableName))
             {
-                var hostedPoolBuildCacheConfiguration = BuildCacheResourceHelper.LoadFromJSONAsync(configuration.HostedPoolBuildCacheConfigurationFile).GetAwaiter().GetResult();
+                hostedPoolConfigurationFilePath = Environment.GetEnvironmentVariable(configuration.HostedPoolBuildCacheConfigurationFileEnvironmentVariableName);
+            }
+
+            if (!string.IsNullOrEmpty(hostedPoolConfigurationFilePath))
+            {
+                var hostedPoolBuildCacheConfiguration = BuildCacheResourceHelper.LoadFromJSONAsync(hostedPoolConfigurationFilePath).GetAwaiter().GetResult();
 
                 if (!hostedPoolBuildCacheConfiguration.TrySelectBuildCache(configuration.HostedPoolActiveBuildCacheName, out var selectedBuildCacheConfiguration))
                 {