tests: Work around corrupted UBI volume on partial write.

Corrupting the volume when the write doesn't write everything it's
supposed to, is an intentional security feature of UBI, and it can
trigger when we do checksum mismatch tests, which suddenly stop
writing when they discover that the checksum is wrong. This apparently
is checked more rigorously in recent kernels, so accept a non-readable
volume when performing UBI tests.

Changelog: None

Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>

tests/acceptance/test_update.py

@@ -568,9 +568,25 @@ def test_signed_updates(self, sig_case, bitbake_path, bitbake_variables):
             else:
                 expected_content = old_content
 
-            content = run("dd if=%s bs=%d count=1"
-                          % (passive, len(expected_content)))
-            assert content == expected_content, "Case: %s" % sig_case.label
+            try:
+                content = run("dd if=%s bs=%d count=1"
+                              % (passive, len(expected_content)))
+                assert content == expected_content, "Case: %s" % sig_case.label
+
+            # In Fabric context, SystemExit means CalledProcessError. We should
+            # not catch all exceptions, because we want to leave assertions
+            # alone.
+            except SystemExit:
+                if "mender-ubi" in bitbake_variables['DISTRO_FEATURES'].split():
+                    # For UBI volumes specifically: The UBI_IOCVOLUP call which
+                    # Mender uses prior to writing the data, takes a size
+                    # argument, and if you don't write that amount of bytes, the
+                    # volume is marked corrupted as a security measure. This
+                    # sometimes triggers in our checksum mismatch tests, so
+                    # accept the volume being unreadable in that case.
+                    pass
+                else:
+                    raise
 
         finally:
             # Reset environment to what it was.