Handle No List Permission for Secrets

When the KeyVault does not have list permissions fail gracefully
and wait for the individual GetSecret call to see if secret values can be retrieved.
#21

Author: rahulpnath

src/Azure/AzureKeyVaultConfigBuilder.cs

@@ -33,6 +33,7 @@ public class AzureKeyVaultConfigBuilder : KeyValueConfigBuilder
         private string _uri;
         private string _version;
         private bool _preload;
+        private bool _preloadFailed;
 
         private KeyVaultClient _kvClient;
         private List<string> _allKeys;
@@ -116,7 +117,7 @@ public override ICollection<KeyValuePair<string, string>> GetAllValues(string pr
 
         private async Task<string> GetValueAsync(string key)
         {
-            if (!_preload || _allKeys.Contains(key, StringComparer.OrdinalIgnoreCase))
+            if (!_preload || _preloadFailed || _allKeys.Contains(key, StringComparer.OrdinalIgnoreCase))
             {
                 try
                 {
@@ -145,20 +146,37 @@ private async Task<string> GetValueAsync(string key)
         private List<string> GetAllKeys()
         {
             List<string> keys = new List<string>(); // KeyVault keys are case-insensitive. There won't be case-duplicates. List<> should be fine.
-
-            // Get first page of secret keys
-            var allSecrets = Task.Run(async () => { return await _kvClient.GetSecretsAsync(_uri); }).Result;
-            foreach (var secretItem in allSecrets)
-                keys.Add(secretItem.Identifier.Name);
-
-            // If there more more pages, get those too
-            string nextPage = allSecrets.NextPageLink;
-            while (!String.IsNullOrWhiteSpace(nextPage))
+            try
             {
-                var moreSecrets = Task.Run(async () => { return await _kvClient.GetSecretsNextAsync(nextPage); }).Result;
-                foreach (var secretItem in moreSecrets)
+                // Get first page of secret keys
+                var allSecrets = Task.Run(async () => { return await _kvClient.GetSecretsAsync(_uri); }).Result;
+                foreach (var secretItem in allSecrets)
                     keys.Add(secretItem.Identifier.Name);
-                nextPage = moreSecrets.NextPageLink;
+
+                // If there more more pages, get those too
+                string nextPage = allSecrets.NextPageLink;
+                while (!String.IsNullOrWhiteSpace(nextPage))
+                {
+                    var moreSecrets = Task.Run(async () => { return await _kvClient.GetSecretsNextAsync(nextPage); }).Result;
+                    foreach (var secretItem in moreSecrets)
+                        keys.Add(secretItem.Identifier.Name);
+                    nextPage = moreSecrets.NextPageLink;
+                }
+            }
+            catch (AggregateException ae)
+            {
+                ae.Handle(ex =>
+                {
+                    var exAsKve = ex as KeyVaultErrorException;
+                    // If List Permission on Secrets in not available return empty list of keys
+                    if (exAsKve != null && exAsKve.Body.Error.Code == "Forbidden")
+                    {
+                        _preloadFailed = true;
+                        return true;
+                    }
+                    else
+                        return false;
+                });
             }
 
             return keys;