[PATCH v3 5/5] iotests: fortify compare_images() against crashes

[John Snow <jsnow@redhat.com>]

Fortify compare_images() to be more discerning about the status codes it
receives. If qemu_img() returns an exit code that implies it didn't
actually perform the comparison, treat that as an exceptional
circumstance and force the caller to be aware of the peril.

If a negative test is desired (perhaps to test how qemu_img compare
behaves on malformed images, for instance), it is still possible to
catch the exception in the test and deal with that circumstance
manually.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 tests/qemu-iotests/iotests.py | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/tests/qemu-iotests/iotests.py b/tests/qemu-iotests/iotests.py
index ec4568b24a..7057db0686 100644
--- a/tests/qemu-iotests/iotests.py
+++ b/tests/qemu-iotests/iotests.py
@@ -506,11 +506,22 @@ def qemu_nbd_popen(*args):
             p.kill()
             p.wait()
 
-def compare_images(img1, img2, fmt1=imgfmt, fmt2=imgfmt):
-    '''Return True if two image files are identical'''
-    res = qemu_img('compare', '-f', fmt1,
-                   '-F', fmt2, img1, img2, check=False)
-    return res.returncode == 0
+def compare_images(img1: str, img2: str,
+                   fmt1: str = imgfmt, fmt2: str = imgfmt) -> bool:
+    """
+    Compare two images with QEMU_IMG; return True if they are identical.
+
+    :raise CalledProcessError:
+        when qemu-img crashes or returns a status code of anything other
+        than 0 (identical) or 1 (different).
+    """
+    try:
+        qemu_img('compare', '-f', fmt1, '-F', fmt2, img1, img2)
+        return True
+    except subprocess.CalledProcessError as exc:
+        if exc.returncode == 1:
+            return False
+        raise
 
 def create_image(name, size):
     '''Create a fully-allocated raw image with sector markers'''
-- 
2.34.1