iotests: add qemu_img_json()

qemu_img_json() is a new helper built on top of qemu_img() that tries to
pull a valid JSON document out of the stdout stream.

In the event that the return code is negative (the program crashed), or
the code is greater than zero and did not produce valid JSON output, the
VerboseProcessError raised by qemu_img() is re-raised.

In the event that the return code is zero but we can't parse valid JSON,
allow the JSON deserialization error to be raised.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20220321201618.903471-7-jsnow@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

tests/qemu-iotests/iotests.py

@@ -277,6 +277,38 @@ def ordered_qmp(qmsg, conv_keys=True):
 def qemu_img_create(*args: str) -> 'subprocess.CompletedProcess[str]':
     return qemu_img('create', *args)
 
+def qemu_img_json(*args: str) -> Any:
+    """
+    Run qemu-img and return its output as deserialized JSON.
+
+    :raise CalledProcessError:
+        When qemu-img crashes, or returns a non-zero exit code without
+        producing a valid JSON document to stdout.
+    :raise JSONDecoderError:
+        When qemu-img returns 0, but failed to produce a valid JSON document.
+
+    :return: A deserialized JSON object; probably a dict[str, Any].
+    """
+    try:
+        res = qemu_img(*args, combine_stdio=False)
+    except subprocess.CalledProcessError as exc:
+        # Terminated due to signal. Don't bother.
+        if exc.returncode < 0:
+            raise
+
+        # Commands like 'check' can return failure (exit codes 2 and 3)
+        # to indicate command completion, but with errors found. For
+        # multi-command flexibility, ignore the exact error codes and
+        # *try* to load JSON.
+        try:
+            return json.loads(exc.stdout)
+        except json.JSONDecodeError:
+            # Nope. This thing is toast. Raise the /process/ error.
+            pass
+        raise
+
+    return json.loads(res.stdout)
+
 def qemu_img_measure(*args):
     return json.loads(qemu_img_pipe("measure", "--output", "json", *args))