ovsdb-idl: Only process successful txn in ovsdb_idl_loop_run.

Otherwise we hide the transaction result from the user.  This may cause
problems as the user will not detect error cases.  For example, if the
server refuses a transaction with "constraint violation", the user
should be notified because the transaction might need to be retried.

For clients that process database changes incrementally (using change
tracking) this lack of failure notification creates a problem if it
occurs while no other database changes happen.  In that case:
- ovsdb_idl_loop_run() silently consumes the failure, initializes a
  new transaction.
- no other table update was received from the server so the user will
  not add anything to the new transaction.
- ovsdb_idl_loop_commit_and_wait() will "succeed" as nothing changed
  from the client's perspective.
In reality, the first transaction failed and the client wasn't given
the chance to handle the failure.

Commit 0401cf5f9e ("ovsdb idl: Try committing the pending txn in
ovsdb_idl_loop_run.") tried to optimize for the common, successful
case.  Maintain the same approach and optimize for transactions that
succeeded but fall back to the old mechanism of processing failures
within ovsdb_idl_loop_commit_and_wait() instead.

Fixes: 0401cf5f9e ("ovsdb idl: Try committing the pending txn in ovsdb_idl_loop_run.")
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/ovsdb-idl.c

@@ -4255,8 +4255,8 @@ ovsdb_idl_loop_run(struct ovsdb_idl_loop *loop)
 {
     ovsdb_idl_run(loop->idl);
 
-    /* See if we can commit the loop->committing_txn. */
-    if (loop->committing_txn) {
+    /* See if the 'committing_txn' succeeded in the meantime. */
+    if (loop->committing_txn && loop->committing_txn->status == TXN_SUCCESS) {
         ovsdb_idl_try_commit_loop_txn(loop, NULL);
     }