[PATCH v3 4/9] nbd/client-connection: nbd_co_establish_connection(): return real error

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

The only user of errp is call to nbd_do_establish_connection() in
nbd_open(). The only way to cancel this call is through open_timer
timeout. And for this case, user will be more interested in description
of last failed connect rather than in
"Connection attempt cancelled by other operation".

So, let's change behavior on cancel to return previous failure error if
available.

Do the same for non-blocking failure case. In this case we still don't
have a caller that is interested in errp. But let's be consistent.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 nbd/client-connection.c | 50 ++++++++++++++++++++++++++++-------------
 1 file changed, 34 insertions(+), 16 deletions(-)

diff --git a/nbd/client-connection.c b/nbd/client-connection.c
index 695f855754..722998c985 100644
--- a/nbd/client-connection.c
+++ b/nbd/client-connection.c
@@ -39,16 +39,18 @@ struct NBDClientConnection {
 
     QemuMutex mutex;
 
+    NBDExportInfo updated_info;
     /*
-     * @sioc and @err represent a connection attempt.  While running
-     * is true, they are only used by the connection thread, and mutex
-     * locking is not needed.  Once the thread finishes,
-     * nbd_co_establish_connection then steals these pointers while
-     * under the mutex.
+     * @sioc represents a successful result. While thread is running, @sioc is
+     * used only by thread and not protected by mutex. When thread is not
+     * running, @sioc is stolen by nbd_co_establish_connection() under mutex.
      */
-    NBDExportInfo updated_info;
     QIOChannelSocket *sioc;
     QIOChannel *ioc;
+    /*
+     * @err represents previous attempt. It may be copied by
+     * nbd_co_establish_connection() when it reports failure.
+     */
     Error *err;
 
     /* All further fields are accessed only under mutex */
@@ -170,18 +172,18 @@ static void *connect_thread_func(void *opaque)
 
     qemu_mutex_lock(&conn->mutex);
     while (!conn->detached) {
+        Error *local_err = NULL;
+
         assert(!conn->sioc);
         conn->sioc = qio_channel_socket_new();
 
         qemu_mutex_unlock(&conn->mutex);
 
-        error_free(conn->err);
-        conn->err = NULL;
         conn->updated_info = conn->initial_info;
 
         ret = nbd_connect(conn->sioc, conn->saddr,
                           conn->do_negotiation ? &conn->updated_info : NULL,
-                          conn->tlscreds, &conn->ioc, &conn->err);
+                          conn->tlscreds, &conn->ioc, &local_err);
 
         /*
          * conn->updated_info will finally be returned to the user. Clear the
@@ -194,6 +196,10 @@ static void *connect_thread_func(void *opaque)
 
         qemu_mutex_lock(&conn->mutex);
 
+        error_free(conn->err);
+        conn->err = NULL;
+        error_propagate(&conn->err, local_err);
+
         if (ret < 0) {
             object_unref(OBJECT(conn->sioc));
             conn->sioc = NULL;
@@ -311,14 +317,17 @@ nbd_co_establish_connection(NBDClientConnection *conn, NBDExportInfo *info,
             }
 
             conn->running = true;
-            error_free(conn->err);
-            conn->err = NULL;
             qemu_thread_create(&thread, "nbd-connect",
                                connect_thread_func, conn, QEMU_THREAD_DETACHED);
         }
 
         if (!blocking) {
-            error_setg(errp, "No connection at the moment");
+            if (conn->err) {
+                error_propagate(errp, error_copy(conn->err));
+            } else {
+                error_setg(errp, "No connection at the moment");
+            }
+
             return NULL;
         }
 
@@ -339,14 +348,23 @@ nbd_co_establish_connection(NBDClientConnection *conn, NBDExportInfo *info,
              * attempt as failed, but leave the connection thread running,
              * to reuse it for the next connection attempt.
              */
-            error_setg(errp, "Connection attempt cancelled by other operation");
+            if (conn->err) {
+                error_propagate(errp, error_copy(conn->err));
+            } else {
+                error_setg(errp,
+                           "Connection attempt cancelled by other operation");
+            }
+
             return NULL;
         } else {
-            error_propagate(errp, conn->err);
-            conn->err = NULL;
-            if (!conn->sioc) {
+            /* Thread finished. There must be either error or sioc */
+            assert(!conn->err != !conn->sioc);
+
+            if (conn->err) {
+                error_propagate(errp, error_copy(conn->err));
                 return NULL;
             }
+
             if (conn->do_negotiation) {
                 memcpy(info, &conn->updated_info, sizeof(*info));
                 if (conn->ioc) {
-- 
2.29.2