[PATCH v2 06/18] iotests: add qemu_img_json()

[John Snow <jsnow@redhat.com>]

A little 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 instead.

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>
---
 tests/qemu-iotests/iotests.py | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/tests/qemu-iotests/iotests.py b/tests/qemu-iotests/iotests.py
index 24765de2e27..1e6947b893b 100644
--- a/tests/qemu-iotests/iotests.py
+++ b/tests/qemu-iotests/iotests.py
@@ -309,6 +309,41 @@ 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.
+    """
+    json_data = ...  # json.loads can legitimately return 'None'.
+
+    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:
+            json_data = json.loads(exc.stdout)
+        except json.JSONDecodeError:
+            pass  # Nope. This thing is toast.
+
+        if json_data is ...:
+            raise
+
+    if json_data is ...:
+        json_data = json.loads(res.stdout)
+    return json_data
+
 def qemu_img_measure(*args):
     return json.loads(qemu_img_pipe("measure", "--output", "json", *args))
 
-- 
2.34.1