use ensure init pattern to make sure root configuration fallback is respected

Author: zhiyuanliang-ms

src/Microsoft.FeatureManagement/ConfigurationFeatureDefinitionProvider.cs

@@ -28,6 +28,8 @@ public sealed class ConfigurationFeatureDefinitionProvider : IFeatureDefinitionP
         private readonly ConcurrentDictionary<string, Task<FeatureDefinition>> _definitions;
         private IDisposable _changeSubscription;
         private int _stale = 0;
+        private long _initialized = 0;
+        private readonly object _lock = new object();
         private readonly Func<string, Task<FeatureDefinition>> _getFeatureDefinitionFunc;
 
         const string ParseValueErrorString = "Invalid setting '{0}' with value '{1}' for feature '{2}'.";
@@ -49,10 +51,6 @@ public ConfigurationFeatureDefinitionProvider(IConfiguration configuration)
             {
                 return Task.FromResult(GetMicrosoftSchemaFeatureDefinition(featureName) ?? GetDotnetSchemaFeatureDefinition(featureName));
             };
-
-            _dotnetFeatureDefinitionSections = GetDotnetFeatureDefinitionSections();
-
-            _microsoftFeatureDefinitionSections = GetMicrosoftFeatureDefinitionSections();
         }
 
         /// <summary>
@@ -92,6 +90,8 @@ public Task<FeatureDefinition> GetFeatureDefinitionAsync(string featureName)
                 throw new ArgumentException($"The value '{ConfigurationPath.KeyDelimiter}' is not allowed in the feature name.", nameof(featureName));
             }
 
+            EnsureInit();
+
             if (Interlocked.Exchange(ref _stale, 0) != 0)
             {
                 _definitions.Clear();
@@ -115,6 +115,8 @@ public Task<FeatureDefinition> GetFeatureDefinitionAsync(string featureName)
         public async IAsyncEnumerable<FeatureDefinition> GetAllFeatureDefinitionsAsync()
 #pragma warning restore CS1998
         {
+            EnsureInit();
+
             if (Interlocked.Exchange(ref _stale, 0) != 0)
             {
                 _definitions.Clear();
@@ -163,6 +165,28 @@ public async IAsyncEnumerable<FeatureDefinition> GetAllFeatureDefinitionsAsync()
             }
         }
 
+        private void EnsureInit()
+        {
+            if (_initialized == 0)
+            {
+                IEnumerable<IConfigurationSection> dotnetFeatureDefinitionSections = GetDotnetFeatureDefinitionSections();
+
+                IEnumerable<IConfigurationSection> microsoftFeatureDefinitionSections = GetMicrosoftFeatureDefinitionSections();
+
+                lock (_lock)
+                {
+                    if (Interlocked.Read(ref _initialized) == 0)
+                    {
+                        _dotnetFeatureDefinitionSections = dotnetFeatureDefinitionSections;
+
+                        _microsoftFeatureDefinitionSections = microsoftFeatureDefinitionSections;
+
+                        Interlocked.Exchange(ref _initialized, 1);
+                    }
+                }
+            }
+        }
+
         private FeatureDefinition GetDotnetSchemaFeatureDefinition(string featureName)
         {
             IConfigurationSection dotnetFeatureDefinitionConfiguration = _dotnetFeatureDefinitionSections