[PULL 4/5] iotests.py: QemuIoInteractive: print output on failure

[Eric Blake <eblake@redhat.com>]

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Make it simpler to debug when qemu-io fails due to wrong arguments or
environment.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200701105331.121670-3-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 tests/qemu-iotests/iotests.py | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tests/qemu-iotests/iotests.py b/tests/qemu-iotests/iotests.py
index 109fb3884a26..2a08fea3c9ec 100644
--- a/tests/qemu-iotests/iotests.py
+++ b/tests/qemu-iotests/iotests.py
@@ -217,7 +217,13 @@ class QemuIoInteractive:
                                    stdout=subprocess.PIPE,
                                    stderr=subprocess.STDOUT,
                                    universal_newlines=True)
-        assert self._p.stdout.read(9) == 'qemu-io> '
+        out = self._p.stdout.read(9)
+        if out != 'qemu-io> ':
+            # Most probably qemu-io just failed to start.
+            # Let's collect the whole output and exit.
+            out += self._p.stdout.read()
+            self._p.wait(timeout=1)
+            raise ValueError(out)

     def close(self):
         self._p.communicate('q\n')
-- 
2.27.0