[PATCH v2 14/17] iotests: Conditionally silence certain AQMP errors

[John Snow]

AQMP likes to be very chatty about errors it encounters. In general,
this is good because it allows us to get good diagnostic information for
otherwise complex async failures.

For example, during a failed QMP connection attempt, we might see:

+ERROR:qemu.aqmp.qmp_client.qemub-2536319:Negotiation failed: EOFError
+ERROR:qemu.aqmp.qmp_client.qemub-2536319:Failed to establish session: EOFError

This might be nice in iotests output, because failure scenarios
involving the new QMP library will be spelled out plainly in the output
diffs.

For tests that are intentionally causing this scenario though, filtering
that log output could be a hassle. For now, add a context manager that
simply lets us toggle this output off during a critical region.

(Additionally, a forthcoming patch allows the use of either legacy or
async QMP to be toggled with an environment variable. In this
circumstance, we can't amend the iotest output to just always expect the
error message, either. Just suppress it for now. More rigorous log
filtering can be investigated later if/when it is deemed safe to
permanently replace the legacy QMP library.)

Signed-off-by: John Snow <jsnow@redhat.com>
---
 tests/qemu-iotests/iotests.py             | 20 +++++++++++++++++++-
 tests/qemu-iotests/tests/mirror-top-perms |  9 +++++----
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/tests/qemu-iotests/iotests.py b/tests/qemu-iotests/iotests.py
index 9afa258a405..4d39b86ed85 100644
--- a/tests/qemu-iotests/iotests.py
+++ b/tests/qemu-iotests/iotests.py
@@ -30,7 +30,7 @@
 import subprocess
 import sys
 import time
-from typing import (Any, Callable, Dict, Iterable,
+from typing import (Any, Callable, Dict, Iterable, Iterator,
                     List, Optional, Sequence, TextIO, Tuple, Type, TypeVar)
 import unittest
 
@@ -116,6 +116,24 @@
 sample_img_dir = os.environ['SAMPLE_IMG_DIR']
 
 
+@contextmanager
+def change_log_level(
+        logger_name: str, level: int = logging.CRITICAL) -> Iterator[None]:
+    """
+    Utility function for temporarily changing the log level of a logger.
+
+    This can be used to silence errors that are expected or uninteresting.
+    """
+    _logger = logging.getLogger(logger_name)
+    current_level = _logger.level
+    _logger.setLevel(level)
+
+    try:
+        yield
+    finally:
+        _logger.setLevel(current_level)
+
+
 def unarchive_sample_image(sample, fname):
     sample_fname = os.path.join(sample_img_dir, sample + '.bz2')
     with bz2.open(sample_fname) as f_in, open(fname, 'wb') as f_out:
diff --git a/tests/qemu-iotests/tests/mirror-top-perms 
b/tests/qemu-iotests/tests/mirror-top-perms
index 9fe315e3b01..5a34ec655e2 100755
--- a/tests/qemu-iotests/tests/mirror-top-perms
+++ b/tests/qemu-iotests/tests/mirror-top-perms
@@ -21,7 +21,7 @@
 
 import os
 import iotests
-from iotests import qemu_img
+from iotests import change_log_level, qemu_img
 
 # Import qemu after iotests.py has amended sys.path
 # pylint: disable=wrong-import-order
@@ -100,9 +100,10 @@ class TestMirrorTopPerms(iotests.QMPTestCase):
         self.vm_b.add_blockdev(f'file,node-name=drive0,filename={source}')
         self.vm_b.add_device('virtio-blk,drive=drive0,share-rw=on')
         try:
-            self.vm_b.launch()
-            print('ERROR: VM B launched successfully, this should not have '
-                  'happened')
+            with change_log_level('qemu.aqmp'):
+                self.vm_b.launch()
+                print('ERROR: VM B launched successfully, '
+                      'this should not have happened')
         except (qemu.qmp.QMPConnectError, ConnectError):
             assert 'Is another process using the image' in self.vm_b.get_log()
 
-- 
2.31.1