[PATCH v2 02/18] iotests: add VerboseProcessError

[John Snow <jsnow@redhat.com>]

This adds an Exception that extends the Python built-in
subprocess.CalledProcessError. When this exception is raised, it will
still be caught when selecting for the stdlib variant.

The difference is that the str() method of this Exception also adds the
stdout/stderr logs. In effect, if this exception goes unhandled, Python
will print the output in a nice, highlighted box to the terminal so that
it's easy to spot.

This should save some headache from having to re-run test suites with
debugging enabled by augmenting the exceptions to print more information
in the default case.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 tests/qemu-iotests/iotests.py | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/tests/qemu-iotests/iotests.py b/tests/qemu-iotests/iotests.py
index 6ba65eb1ffe..5617f991da7 100644
--- a/tests/qemu-iotests/iotests.py
+++ b/tests/qemu-iotests/iotests.py
@@ -30,6 +30,7 @@
 import struct
 import subprocess
 import sys
+import textwrap
 import time
 from typing import (Any, Callable, Dict, Iterable, Iterator,
                     List, Optional, Sequence, TextIO, Tuple, Type, TypeVar)
@@ -39,6 +40,7 @@
 
 from qemu.machine import qtest
 from qemu.qmp import QMPMessage
+from qemu.utils import enboxify
 
 # Use this logger for logging messages directly from the iotests module
 logger = logging.getLogger('qemu.iotests')
@@ -117,6 +119,38 @@
 sample_img_dir = os.environ['SAMPLE_IMG_DIR']
 
 
+class VerboseProcessError(subprocess.CalledProcessError):
+    """
+    The same as CalledProcessError, but more verbose.
+
+    This is useful for debugging failed calls during test executions.
+    The return code, signal (if any), and terminal output will be displayed
+    on unhandled exceptions.
+    """
+    def summary(self) -> str:
+        return super().__str__()
+
+    def __str__(self) -> str:
+        lmargin = '  '
+        width = shutil.get_terminal_size()[0] - len(lmargin)
+        sections = []
+
+        name = 'output' if self.stderr is None else 'stdout'
+        if self.stdout:
+            sections.append(enboxify(self.stdout, width, name))
+        else:
+            sections.append(f"{name}: N/A")
+
+        if self.stderr:
+            sections.append(enboxify(self.stderr, width, 'stderr'))
+        elif self.stderr is not None:
+            sections.append("stderr: N/A")
+
+        return os.linesep.join((
+            self.summary(),
+            textwrap.indent(os.linesep.join(sections), prefix=lmargin),
+        ))
+
 @contextmanager
 def change_log_level(
         logger_name: str, level: int = logging.CRITICAL) -> Iterator[None]:
-- 
2.34.1