Fix encryption key cache design for AKV provider

src/Microsoft.Data.SqlClient/add-ons/AzureKeyVaultProvider/AzureSqlKeyCryptographer.cs

@@ -7,12 +7,12 @@
 using Azure.Security.KeyVault.Keys.Cryptography;
 using System;
 using System.Collections.Concurrent;
-using System.Threading.Tasks;
+using System.Threading;
 using static Azure.Security.KeyVault.Keys.Cryptography.SignatureAlgorithm;
 
 namespace Microsoft.Data.SqlClient.AlwaysEncrypted.AzureKeyVaultProvider
 {
-    internal class AzureSqlKeyCryptographer
+    internal sealed class AzureSqlKeyCryptographer : IDisposable
     {
         /// <summary>
         /// TokenCredential to be used with the KeyClient
@@ -25,16 +25,14 @@ internal class AzureSqlKeyCryptographer
         private readonly ConcurrentDictionary<Uri, KeyClient> _keyClientDictionary = new();
 
         /// <summary>
-        /// Holds references to the fetch key tasks and maps them to their corresponding Azure Key Vault Key Identifier (URI).
-        /// These tasks will be used for returning the key in the event that the fetch task has not finished depositing the 
-        /// key into the key dictionary.
+        /// Holds references to the Azure Key Vault keys and maps them to their corresponding Azure Key Vault Key Identifier (URI).
         /// </summary>
-        private readonly ConcurrentDictionary<string, Task<Azure.Response<KeyVaultKey>>> _keyFetchTaskDictionary = new();
+        private readonly ConcurrentDictionary<string, KeyVaultKey> _keyDictionary = new();
 
         /// <summary>
-        /// Holds references to the Azure Key Vault keys and maps them to their corresponding Azure Key Vault Key Identifier (URI).
+        /// SemaphoreSlim to ensure thread safety when accessing the key dictionary or making network calls to Azure Key Vault to fetch keys.
         /// </summary>
-        private readonly ConcurrentDictionary<string, KeyVaultKey> _keyDictionary = new();
+        private SemaphoreSlim _keyDictionarySemaphore = new(1, 1);
 
         /// <summary>
         /// Holds references to the Azure Key Vault CryptographyClient objects and maps them to their corresponding Azure Key Vault Key Identifier (URI).
@@ -50,20 +48,44 @@ internal AzureSqlKeyCryptographer(TokenCredential tokenCredential)
             TokenCredential = tokenCredential;
         }
 
+        /// <summary>
+        /// Disposes the SemaphoreSlim used for thread safety.
+        /// </summary>
+        public void Dispose()
+        {
+            _keyDictionarySemaphore.Dispose();
+        }
+
         /// <summary>
         /// Adds the key, specified by the Key Identifier URI, to the cache.
+        /// Validates the key type and fetches the key from Azure Key Vault if it is not already cached.
         /// </summary>
         /// <param name="keyIdentifierUri"></param>
         internal void AddKey(string keyIdentifierUri)
         {
-            if (TheKeyHasNotBeenCached(keyIdentifierUri))
+            // Allow only one thread to proceed to ensure thread safety
+            // as we will need to fetch key information from Azure Key Vault if the key is not found in cache.
+            _keyDictionarySemaphore.Wait();
+
+            try
             {
-                ParseAKVPath(keyIdentifierUri, out Uri vaultUri, out string keyName, out string keyVersion);
-                CreateKeyClient(vaultUri);
-                FetchKey(vaultUri, keyName, keyVersion, keyIdentifierUri);
-            }
+                if (!_keyDictionary.ContainsKey(keyIdentifierUri))
+                {
+                    ParseAKVPath(keyIdentifierUri, out Uri vaultUri, out string keyName, out string keyVersion);
+
+                    // Fetch the KeyClient for the Key vault URI.
+                    KeyClient keyClient = GetOrCreateKeyClient(vaultUri);
+
+                    // Fetch the key from Azure Key Vault.
+                    KeyVaultKey key = FetchKeyFromKeyVault(keyClient, keyName, keyVersion);
 
-            bool TheKeyHasNotBeenCached(string k) => !_keyDictionary.ContainsKey(k) && !_keyFetchTaskDictionary.ContainsKey(k);
+                    _keyDictionary.AddOrUpdate(keyIdentifierUri, key, (k, v) => key);
+                }
+            }
+            finally
+            {
+                _keyDictionarySemaphore.Release();
+            }
         }
 
         /// <summary>
@@ -75,18 +97,12 @@ internal KeyVaultKey GetKey(string keyIdentifierUri)
         {
             if (_keyDictionary.TryGetValue(keyIdentifierUri, out KeyVaultKey key))
             {
-                AKVEventSource.Log.TryTraceEvent("Fetched master key from cache");
+                AKVEventSource.Log.TryTraceEvent("Fetched key name={0} from cache", key.Name);
                 return key;
             }
 
-            if (_keyFetchTaskDictionary.TryGetValue(keyIdentifierUri, out Task<Azure.Response<KeyVaultKey>> task))
-            {
-                AKVEventSource.Log.TryTraceEvent("New Master key fetched.");
-                return Task.Run(() => task).GetAwaiter().GetResult();
-            }
-
             // Not a public exception - not likely to occur.
-            AKVEventSource.Log.TryTraceEvent("Master key not found.");
+            AKVEventSource.Log.TryTraceEvent("Key not found; URI={0}", keyIdentifierUri);
             throw ADP.MasterKeyNotFound(keyIdentifierUri);
         }
 
@@ -95,10 +111,7 @@ internal KeyVaultKey GetKey(string keyIdentifierUri)
         /// </summary>
         /// <param name="keyIdentifierUri">The key vault key identifier URI</param>
         /// <returns></returns>
-        internal int GetKeySize(string keyIdentifierUri)
-        {
-            return GetKey(keyIdentifierUri).Key.N.Length;
-        }
+        internal int GetKeySize(string keyIdentifierUri) =>  GetKey(keyIdentifierUri).Key.N.Length;
 
         /// <summary>
         /// Generates signature based on RSA PKCS#v1.5 scheme using a specified Azure Key Vault Key URL. 
@@ -142,49 +155,58 @@ private CryptographyClient GetCryptographyClient(string keyIdentifierUri)
 
             CryptographyClient cryptographyClient = new(GetKey(keyIdentifierUri).Id, TokenCredential);
             _cryptoClientDictionary.TryAdd(keyIdentifierUri, cryptographyClient);
-
             return cryptographyClient;
         }
 
         /// <summary>
-        /// 
+        /// Fetches the column encryption key from the Azure Key Vault.
         /// </summary>
-        /// <param name="vaultUri">The Azure Key Vault URI</param>
+        /// <param name="keyClient">The KeyClient instance</param>
         /// <param name="keyName">The name of the Azure Key Vault key</param>
         /// <param name="keyVersion">The version of the Azure Key Vault key</param>
-        /// <param name="keyResourceUri">The Azure Key Vault key identifier</param>
-        private void FetchKey(Uri vaultUri, string keyName, string keyVersion, string keyResourceUri)
+        private KeyVaultKey FetchKeyFromKeyVault(KeyClient keyClient, string keyName, string keyVersion)
         {
-            Task<Azure.Response<KeyVaultKey>> fetchKeyTask = FetchKeyFromKeyVault(vaultUri, keyName, keyVersion);
-            _keyFetchTaskDictionary.AddOrUpdate(keyResourceUri, fetchKeyTask, (k, v) => fetchKeyTask);
+            AKVEventSource.Log.TryTraceEvent("Fetching key name={0}", keyName);
 
-            fetchKeyTask
-                .ContinueWith(k => ValidateRsaKey(k.GetAwaiter().GetResult()))
-                .ContinueWith(k => _keyDictionary.AddOrUpdate(keyResourceUri, k.GetAwaiter().GetResult(), (key, v) => k.GetAwaiter().GetResult()));
+            Azure.Response<KeyVaultKey> keyResponse = keyClient?.GetKey(keyName, keyVersion);
 
-            Task.Run(() => fetchKeyTask);
+            // Handle the case where the key response is null or contains an error
+            // This can happen if the key does not exist or if there is an issue with the KeyClient.
+            // In such cases, we log the error and throw an exception.
+            if (keyResponse == null || keyResponse.Value == null || keyResponse.GetRawResponse().IsError)
+            {
+                AKVEventSource.Log.TryTraceEvent("Get Key failed to fetch Key from Azure Key Vault for key {0}, version {1}", keyName, keyVersion);
+                if (keyResponse?.GetRawResponse() is Azure.Response response)
+                {
+                    AKVEventSource.Log.TryTraceEvent("Response status {0} : {1}", response.Status, response.ReasonPhrase);
+                }
+                throw ADP.GetKeyFailed(keyName);
+            }
+
+            KeyVaultKey key = keyResponse.Value;
+
+            // Validate that the key is of type RSA
+            key = ValidateRsaKey(key);
+            return key;
         }
 
         /// <summary>
-        /// Looks up the KeyClient object by it's URI and then fetches the key by name.
+        /// Gets or creates a KeyClient for the specified Azure Key Vault URI.
         /// </summary>
-        /// <param name="vaultUri">The Azure Key Vault URI</param>
-        /// <param name="keyName">Then name of the key</param>
-        /// <param name="keyVersion">Then version of the key</param>
+        /// <param name="vaultUri">Key Identifier URL</param>
         /// <returns></returns>
-        private Task<Azure.Response<KeyVaultKey>> FetchKeyFromKeyVault(Uri vaultUri, string keyName, string keyVersion)
+        private KeyClient GetOrCreateKeyClient(Uri vaultUri)
         {
-            _keyClientDictionary.TryGetValue(vaultUri, out KeyClient keyClient);
-            AKVEventSource.Log.TryTraceEvent("Fetching requested master key: {0}", keyName);
-            return keyClient?.GetKeyAsync(keyName, keyVersion);
+            return _keyClientDictionary.GetOrAdd(
+                vaultUri, (_) => new KeyClient(vaultUri, TokenCredential));
         }
 
         /// <summary>
         /// Validates that a key is of type RSA
         /// </summary>
         /// <param name="key"></param>
         /// <returns></returns>
-        private KeyVaultKey ValidateRsaKey(KeyVaultKey key)
+        private static KeyVaultKey ValidateRsaKey(KeyVaultKey key)
         {
             if (key.KeyType != KeyType.Rsa && key.KeyType != KeyType.RsaHsm)
             {
@@ -195,26 +217,14 @@ private KeyVaultKey ValidateRsaKey(KeyVaultKey key)
             return key;
         }
 
-        /// <summary>
-        /// Instantiates and adds a KeyClient to the KeyClient dictionary
-        /// </summary>
-        /// <param name="vaultUri">The Azure Key Vault URI</param>
-        private void CreateKeyClient(Uri vaultUri)
-        {
-            if (!_keyClientDictionary.ContainsKey(vaultUri))
-            {
-                _keyClientDictionary.TryAdd(vaultUri, new KeyClient(vaultUri, TokenCredential));
-            }
-        }
-
         /// <summary>
         /// Validates and parses the Azure Key Vault URI and key name.
         /// </summary>
         /// <param name="masterKeyPath">The Azure Key Vault key identifier</param>
         /// <param name="vaultUri">The Azure Key Vault URI</param>
         /// <param name="masterKeyName">The name of the key</param>
         /// <param name="masterKeyVersion">The version of the key</param>
-        private void ParseAKVPath(string masterKeyPath, out Uri vaultUri, out string masterKeyName, out string masterKeyVersion)
+        private static void ParseAKVPath(string masterKeyPath, out Uri vaultUri, out string masterKeyName, out string masterKeyVersion)
         {
             Uri masterKeyPathUri = new(masterKeyPath);
             vaultUri = new Uri(masterKeyPathUri.GetLeftPart(UriPartial.Authority));

src/Microsoft.Data.SqlClient/add-ons/AzureKeyVaultProvider/LocalCache.cs

@@ -2,8 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using Microsoft.Extensions.Caching.Memory;
 using System;
+using Microsoft.Extensions.Caching.Memory;
 using static System.Math;
 
 namespace Microsoft.Data.SqlClient.AlwaysEncrypted.AzureKeyVaultProvider
@@ -92,6 +92,7 @@ internal TValue GetOrCreate(TKey key, Func<TValue> createItem)
 
         /// <summary>
         /// Determines whether the <see cref="LocalCache{TKey, TValue}">LocalCache</see> contains the specified key.
+        /// Used in unit tests to verify that the cache contains the expected entries.
         /// </summary>
         /// <param name="key"></param>
         /// <returns></returns>

src/Microsoft.Data.SqlClient/add-ons/AzureKeyVaultProvider/SqlColumnEncryptionAzureKeyVaultProvider.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Text;
+using System.Threading;
 using Azure.Core;
 using Azure.Security.KeyVault.Keys.Cryptography;
 using static Microsoft.Data.SqlClient.AlwaysEncrypted.AzureKeyVaultProvider.Validator;
@@ -55,6 +56,8 @@ public class SqlColumnEncryptionAzureKeyVaultProvider : SqlColumnEncryptionKeySt
 
         private readonly static KeyWrapAlgorithm s_keyWrapAlgorithm = KeyWrapAlgorithm.RsaOaep;
 
+        private SemaphoreSlim _cacheSemaphore = new(1, 1);
+
         /// <summary>
         /// List of Trusted Endpoints
         /// 
@@ -69,7 +72,7 @@ public class SqlColumnEncryptionAzureKeyVaultProvider : SqlColumnEncryptionKeySt
         /// <summary>
         /// A cache for storing the results of signature verification of column master key metadata.
         /// </summary>
-        private readonly LocalCache<Tuple<string, bool, string>, bool> _columnMasterKeyMetadataSignatureVerificationCache = 
+        private readonly LocalCache<Tuple<string, bool, string>, bool> _columnMasterKeyMetadataSignatureVerificationCache =
             new(maxSizeLimit: 2000) { TimeToLive = TimeSpan.FromDays(10) };
 
         /// <summary>
@@ -230,7 +233,7 @@ byte[] DecryptEncryptionKey()
                 // Get ciphertext
                 byte[] cipherText = new byte[cipherTextLength];
                 Array.Copy(encryptedColumnEncryptionKey, currentIndex, cipherText, 0, cipherTextLength);
-                
+
                 currentIndex += cipherTextLength;
 
                 // Get signature
@@ -394,17 +397,10 @@ private byte[] CompileMasterKeyMetadata(string masterKeyPath, bool allowEnclaveC
         /// <param name="source">An array of bytes to convert.</param>
         /// <returns>A string of hexadecimal characters</returns>
         /// <remarks>
-        /// Produces a string of hexadecimal character pairs preceded with "0x", where each pair represents the corresponding element in value; for example, "0x7F2C4A00".
+        /// Produces a string of hexadecimal character pairs preceded with "0x", where each pair represents the corresponding element in source; for example, "0x7F2C4A00".
         /// </remarks>
         private string ToHexString(byte[] source)
-        {
-            if (source is null)
-            {
-                return null;
-            }
-
-            return "0x" + BitConverter.ToString(source).Replace("-", "");
-        }
+            => source is null ? null : "0x" + BitConverter.ToString(source).Replace("-", "");
 
         /// <summary>
         /// Returns the cached decrypted column encryption key, or unwraps the encrypted column encryption key if not present.
@@ -415,8 +411,21 @@ private string ToHexString(byte[] source)
         /// <remarks>
         ///
         /// </remarks>
-        private byte[] GetOrCreateColumnEncryptionKey(string encryptedColumnEncryptionKey, Func<byte[]> createItem) 
-            => _columnEncryptionKeyCache.GetOrCreate(encryptedColumnEncryptionKey, createItem);
+        private byte[] GetOrCreateColumnEncryptionKey(string encryptedColumnEncryptionKey, Func<byte[]> createItem)
+        {
+            // Allow only one thread to access the cache at a time.
+            _cacheSemaphore.Wait();
+
+            try
+            {
+                return _columnEncryptionKeyCache.GetOrCreate(encryptedColumnEncryptionKey, createItem);
+            }
+            finally
+            {
+                // Release the semaphore to allow other threads to access the cache.
+                _cacheSemaphore.Release();
+            }
+        }
 
         /// <summary>
         /// Returns the cached signature verification result, or proceeds to verify if not present.

src/Microsoft.Data.SqlClient/add-ons/AzureKeyVaultProvider/Strings.resx

@@ -118,13 +118,16 @@
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
   <data name="NullOrWhitespaceForEach" xml:space="preserve">
-    <value>One or more of the elements in {0} are null or empty or consist of only whitespace.</value>
+    <value>One or more of the elements in '{0}' are null or empty or consist of only whitespace.</value>
   </data>
   <data name="CipherTextLengthMismatch" xml:space="preserve">
     <value>CipherText length does not match the RSA key size.</value>
   </data>
   <data name="EmptyArgumentInternal" xml:space="preserve">
-    <value>Internal error. Empty {0} specified.</value>
+    <value>Internal error. Empty '{0}' specified.</value>
+  </data>
+  <data name="GetKeyFailed" xml:space="preserve">
+    <value>Failed to fetch key from Azure Key Vault. Key: {0}.</value>
   </data>
   <data name="MasterKeyNotFound" xml:space="preserve">
     <value>The key with identifier '{0}' was not found.</value>