nbd/server: structurize simple reply header sending

Use packed structure instead of pointer arithmetics.

Also, merge two redundant traces into one.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20171012095319.136610-5-vsementsov@virtuozzo.com>
[eblake: tweak and mention impact on traces, fix errp usage]
Signed-off-by: NEric Blake <eblake@redhat.com>

include/block/nbd.h

@@ -63,6 +63,12 @@ struct NBDReply {
 };
 typedef struct NBDReply NBDReply;
 
+typedef struct NBDSimpleReply {
+    uint32_t magic;  /* NBD_SIMPLE_REPLY_MAGIC */
+    uint32_t error;
+    uint64_t handle;
+} QEMU_PACKED NBDSimpleReply;
+
 /* Transmission (export) flags: sent from server to client during handshake,
    but describe what will happen during transmission */
 #define NBD_FLAG_HAS_FLAGS      (1 << 0)        /* Flags are there */

nbd/server.c

@@ -902,26 +902,6 @@ static int nbd_receive_request(QIOChannel *ioc, NBDRequest *request,
     return 0;
 }
 
-static int nbd_send_reply(QIOChannel *ioc, NBDReply *reply, Error **errp)
-{
-    uint8_t buf[NBD_REPLY_SIZE];
-
-    reply->error = system_errno_to_nbd_errno(reply->error);
-
-    trace_nbd_send_reply(reply->error, reply->handle);
-
-    /* Reply
-       [ 0 ..  3]    magic   (NBD_SIMPLE_REPLY_MAGIC)
-       [ 4 ..  7]    error   (0 == no error)
-       [ 7 .. 15]    handle
-     */
-    stl_be_p(buf, NBD_SIMPLE_REPLY_MAGIC);
-    stl_be_p(buf + 4, reply->error);
-    stq_be_p(buf + 8, reply->handle);
-
-    return nbd_write(ioc, buf, sizeof(buf), errp);
-}
-
 #define MAX_NBD_REQUESTS 16
 
 void nbd_client_get(NBDClient *client)
@@ -1208,24 +1188,35 @@ void nbd_export_close_all(void)
     }
 }
 
+static inline void set_be_simple_reply(NBDSimpleReply *reply, uint64_t error,
+                                       uint64_t handle)
+{
+    stl_be_p(&reply->magic, NBD_SIMPLE_REPLY_MAGIC);
+    stl_be_p(&reply->error, error);
+    stq_be_p(&reply->handle, handle);
+}
+
 static int nbd_co_send_simple_reply(NBDRequestData *req, NBDReply *reply,
                                     int len, Error **errp)
 {
     NBDClient *client = req->client;
+    NBDSimpleReply simple_reply;
+    int nbd_err = system_errno_to_nbd_errno(reply->error);
     int ret;
 
     g_assert(qemu_in_coroutine());
 
-    trace_nbd_co_send_simple_reply(reply->handle, reply->error, len);
+    trace_nbd_co_send_simple_reply(reply->handle, nbd_err, len);
+    set_be_simple_reply(&simple_reply, nbd_err, reply->handle);
 
     qemu_co_mutex_lock(&client->send_lock);
     client->send_coroutine = qemu_coroutine_self();
 
     if (!len) {
-        ret = nbd_send_reply(client->ioc, reply, errp);
+        ret = nbd_write(client->ioc, &simple_reply, sizeof(simple_reply), errp);
     } else {
         qio_channel_set_cork(client->ioc, true);
-        ret = nbd_send_reply(client->ioc, reply, errp);
+        ret = nbd_write(client->ioc, &simple_reply, sizeof(simple_reply), errp);
         if (ret == 0) {
             ret = nbd_write(client->ioc, req->data, len, errp);
             if (ret < 0) {

nbd/trace-events

@@ -51,7 +51,6 @@ nbd_negotiate_old_style(uint64_t size, unsigned flags) "advertising size %" PRIu
 nbd_negotiate_new_style_size_flags(uint64_t size, unsigned flags) "advertising size %" PRIu64 " and flags 0x%x"
 nbd_negotiate_success(void) "Negotiation succeeded"
 nbd_receive_request(uint32_t magic, uint16_t flags, uint16_t type, uint64_t from, uint32_t len) "Got request: { magic = 0x%" PRIx32 ", .flags = 0x%" PRIx16 ", .type = 0x%" PRIx16 ", from = %" PRIu64 ", len = %" PRIu32 " }"
-nbd_send_reply(int32_t error, uint64_t handle) "Sending response to client: { .error = %" PRId32 ", handle = %" PRIu64 " }"
 nbd_blk_aio_attached(const char *name, void *ctx) "Export %s: Attaching clients to AIO context %p\n"
 nbd_blk_aio_detach(const char *name, void *ctx) "Export %s: Detaching clients from AIO context %p\n"
 nbd_co_send_simple_reply(uint64_t handle, uint32_t error, int len) "Send simple reply: handle = %" PRIu64 ", error = %" PRIu32 ", len = %d"