nbd/server: Include human-readable message in structured errors

The NBD spec permits including a human-readable error string if
structured replies are in force, so we might as well send the
client the message that we logged on any error.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20171027104037.8319-9-eblake@redhat.com>

nbd/server.c

@@ -1296,24 +1296,25 @@ static int coroutine_fn nbd_co_send_structured_read(NBDClient *client,
 static int coroutine_fn nbd_co_send_structured_error(NBDClient *client,
                                                      uint64_t handle,
                                                      uint32_t error,
+                                                     const char *msg,
                                                      Error **errp)
 {
     NBDStructuredError chunk;
     int nbd_err = system_errno_to_nbd_errno(error);
     struct iovec iov[] = {
         {.iov_base = &chunk, .iov_len = sizeof(chunk)},
-        /* FIXME: Support human-readable error message */
+        {.iov_base = (char *)msg, .iov_len = msg ? strlen(msg) : 0},
     };
 
     assert(nbd_err);
     trace_nbd_co_send_structured_error(handle, nbd_err,
-                                       nbd_err_lookup(nbd_err));
+                                       nbd_err_lookup(nbd_err), msg ? msg : "");
     set_be_chunk(&chunk.h, NBD_REPLY_FLAG_DONE, NBD_REPLY_TYPE_ERROR, handle,
-                 sizeof(chunk) - sizeof(chunk.h));
+                 sizeof(chunk) - sizeof(chunk.h) + iov[1].iov_len);
     stl_be_p(&chunk.error, nbd_err);
-    stw_be_p(&chunk.message_length, 0);
+    stw_be_p(&chunk.message_length, iov[1].iov_len);
 
-    return nbd_co_send_iov(client, iov, 1, errp);
+    return nbd_co_send_iov(client, iov, 1 + !!iov[1].iov_len, errp);
 }
 
 /* nbd_co_receive_request
@@ -1414,6 +1415,7 @@ static coroutine_fn void nbd_trip(void *opaque)
     int flags;
     int reply_data_len = 0;
     Error *local_err = NULL;
+    char *msg = NULL;
 
     trace_nbd_trip();
     if (client->closing) {
@@ -1530,14 +1532,17 @@ reply:
     if (local_err) {
         /* If we get here, local_err was not a fatal error, and should be sent
          * to the client. */
+        assert(ret < 0);
+        msg = g_strdup(error_get_pretty(local_err));
         error_report_err(local_err);
         local_err = NULL;
     }
 
-    if (client->structured_reply && request.type == NBD_CMD_READ) {
+    if (client->structured_reply &&
+        (ret < 0 || request.type == NBD_CMD_READ)) {
         if (ret < 0) {
             ret = nbd_co_send_structured_error(req->client, request.handle,
-                                               -ret, &local_err);
+                                               -ret, msg, &local_err);
         } else {
             ret = nbd_co_send_structured_read(req->client, request.handle,
                                               request.from, req->data,
@@ -1548,6 +1553,7 @@ reply:
                                        ret < 0 ? -ret : 0,
                                        req->data, reply_data_len, &local_err);
     }
+    g_free(msg);
     if (ret < 0) {
         error_prepend(&local_err, "Failed to send reply: ");
         goto disconnect;

nbd/trace-events

@@ -57,7 +57,7 @@ nbd_blk_aio_attached(const char *name, void *ctx) "Export %s: Attaching clients
 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, const char *errname, int len) "Send simple reply: handle = %" PRIu64 ", error = %" PRIu32 " (%s), len = %d"
 nbd_co_send_structured_read(uint64_t handle, uint64_t offset, void *data, size_t size) "Send structured read data reply: handle = %" PRIu64 ", offset = %" PRIu64 ", data = %p, len = %zu"
-nbd_co_send_structured_error(uint64_t handle, int err, const char *errname) "Send structured error reply: handle = %" PRIu64 ", error = %d (%s)"
+nbd_co_send_structured_error(uint64_t handle, int err, const char *errname, const char *msg) "Send structured error reply: handle = %" PRIu64 ", error = %d (%s), msg = '%s'"
 nbd_co_receive_request_decode_type(uint64_t handle, uint16_t type, const char *name) "Decoding type: handle = %" PRIu64 ", type = %" PRIu16 " (%s)"
 nbd_co_receive_request_payload_received(uint64_t handle, uint32_t len) "Payload received: handle = %" PRIu64 ", len = %" PRIu32
 nbd_co_receive_request_cmd_write(uint32_t len) "Reading %" PRIu32 " byte(s)"