crypto: cipher: introduce context free function

Refactors the qcrypto_cipher_free(), splits it into two parts. One
is gcrypt/nettle__cipher_free_ctx() to free the special context.

This makes code more clear, what's more, it would be used by the
later patch.

Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

crypto/cipher-gcrypt.c

@@ -64,6 +64,22 @@ struct QCryptoCipherGcrypt {
     uint8_t *iv;
 };
 
+static void gcrypt_cipher_free_ctx(QCryptoCipherGcrypt *ctx,
+                                   QCryptoCipherMode mode)
+{
+    if (!ctx) {
+        return;
+    }
+
+    gcry_cipher_close(ctx->handle);
+    if (mode == QCRYPTO_CIPHER_MODE_XTS) {
+        gcry_cipher_close(ctx->tweakhandle);
+    }
+    g_free(ctx->iv);
+    g_free(ctx);
+}
+
+
 QCryptoCipher *qcrypto_cipher_new(QCryptoCipherAlgorithm alg,
                                   QCryptoCipherMode mode,
                                   const uint8_t *key, size_t nkey,
@@ -228,29 +244,18 @@ QCryptoCipher *qcrypto_cipher_new(QCryptoCipherAlgorithm alg,
     return cipher;
 
  error:
-    gcry_cipher_close(ctx->handle);
-    if (cipher->mode == QCRYPTO_CIPHER_MODE_XTS) {
-        gcry_cipher_close(ctx->tweakhandle);
-    }
-    g_free(ctx);
+    gcrypt_cipher_free_ctx(ctx, mode);
     g_free(cipher);
     return NULL;
 }
 
 
 void qcrypto_cipher_free(QCryptoCipher *cipher)
 {
-    QCryptoCipherGcrypt *ctx;
     if (!cipher) {
         return;
     }
-    ctx = cipher->opaque;
-    gcry_cipher_close(ctx->handle);
-    if (cipher->mode == QCRYPTO_CIPHER_MODE_XTS) {
-        gcry_cipher_close(ctx->tweakhandle);
-    }
-    g_free(ctx->iv);
-    g_free(ctx);
+    gcrypt_cipher_free_ctx(cipher->opaque, cipher->mode);
     g_free(cipher);
 }
 

crypto/cipher-nettle.c

@@ -249,6 +249,19 @@ bool qcrypto_cipher_supports(QCryptoCipherAlgorithm alg,
 }
 
 
+static void nettle_cipher_free_ctx(QCryptoCipherNettle *ctx)
+{
+    if (!ctx) {
+        return;
+    }
+
+    g_free(ctx->iv);
+    g_free(ctx->ctx);
+    g_free(ctx->ctx_tweak);
+    g_free(ctx);
+}
+
+
 QCryptoCipher *qcrypto_cipher_new(QCryptoCipherAlgorithm alg,
                                   QCryptoCipherMode mode,
                                   const uint8_t *key, size_t nkey,
@@ -440,10 +453,7 @@ void qcrypto_cipher_free(QCryptoCipher *cipher)
     }
 
     ctx = cipher->opaque;
-    g_free(ctx->iv);
-    g_free(ctx->ctx);
-    g_free(ctx->ctx_tweak);
-    g_free(ctx);
+    nettle_cipher_free_ctx(ctx);
     g_free(cipher);
 }