Fix ruffus exception output

I found this issue in ruffus 2.6.3
https://github.com/bunbun/ruffus/issues/65
also discussed here
https://github.com/bunbun/ruffus/pull/67

ruffus 2.6.3 RethrownJobError don't follow the normal conventions and
so its exception causes problems when they cross process boundaries.
This change carefully examines the various forms of ruffus exception
objects that can appear in 2.6.3 and parses them more carefully. It
also removes any direct posting of the exception to the logger because
this triggers another serializing of the exception object, mutating it
further.

ocrmypdf/main.py

@@ -1033,51 +1033,82 @@ def cleanup_ruffus_error_message(msg):
     return msg
 
 
+def do_ruffus_exception(ruffus_five_tuple):
+    """Replace the elaborate ruffus stack trace with a user friendly
+    description of the error message that occurred."""
+
+    task_name, job_name, exc_name, exc_value, exc_stack = ruffus_five_tuple
+    if exc_name == 'builtins.SystemExit':
+        match = re.search(r"\.(.+?)\)", exc_value)
+        exit_code_name = match.groups()[0]
+        exit_code = getattr(ExitCode, exit_code_name, 'other_error')
+        return exit_code
+    elif exc_name == 'ruffus.ruffus_exceptions.MissingInputFileError':
+        _log.error(cleanup_ruffus_error_message(exc_value))
+        return ExitCode.input_file
+    elif exc_name == 'builtins.TypeError':
+        # Even though repair_pdf will fail, ruffus will still try
+        # to call split_pages with no input files, likely due to a bug
+        if task_name == 'split_pages':
+            _log.error("Input file '{0}' is not a valid PDF".format(
+                options.input_file))
+            return ExitCode.input_file
+    elif exc_name == 'subprocess.CalledProcessError':
+        # It's up to the subprocess handler to report something useful
+        msg = "Error occurred while running this command:"
+        _log.error(msg + '\n' + exc_value)
+        return ExitCode.child_process_error
+    elif not options.verbose:
+        _log.error(exc_stack)
+        return ExitCode.other_error
+
+
+def traverse_ruffus_exception(e):
+    """Walk through a RethrownJobError and find the first exception.
+
+    The exit code will be based on this, even if multiple exceptions occurred
+    at the same time."""
+
+    if isinstance(e[0], str) and len(e) == 5:
+        return do_ruffus_exception(e)
+    elif hasattr(e, '__iter__'):
+        for exc in e:
+            return traverse_ruffus_exception(exc)
+
+
 def run_pipeline():
     if not options.jobs:
         options.jobs = available_cpu_count()
     try:
-        options.history_file = os.path.join(work_folder, 'ruffus_history.sqlite')
+        options.history_file = os.path.join(
+            work_folder, 'ruffus_history.sqlite')
         cmdline.run(options)
     except ruffus_exceptions.RethrownJobError as e:
         if options.verbose:
-            _log.debug(e)
+            _log.debug(str(e))  # stringify exception so logger doesn't have to
 
-        # Yuck. Hunt through the ruffus exception to find out what the
-        # return code is supposed to be.
-        # Ruffus flattens the exception to a string, throwing away all kinds
-        # of helpful details
-        # task_name, job_name - ruffus status
-        # exc_name - class name of exception
-        # exc_value - irritating string that makes impossible to recover
-        #   exception object
-        # exc_stack - string that contains traceback of exception
-        for exc in e.args:
-            task_name, job_name, exc_name, exc_value, exc_stack = exc
-            if exc_name == 'builtins.SystemExit':
-                match = re.search(r"\.(.+?)\)", exc_value)
-                exit_code_name = match.groups()[0]
-                exit_code = getattr(ExitCode, exit_code_name, 'other_error')
-                return exit_code
-            elif exc_name == 'ruffus.ruffus_exceptions.MissingInputFileError':
-                _log.error(cleanup_ruffus_error_message(exc_value))
-                return ExitCode.input_file
-            elif exc_name == 'builtins.TypeError':
-                # Even though repair_pdf will fail, ruffus will still try
-                # to call split_pages with no input files, likely due to a bug
-                if task_name == 'split_pages':
-                    _log.error("Input file '{0}' is not a valid PDF".format(
-                        options.input_file))
-                    return ExitCode.input_file
-            elif exc_name == 'subprocess.CalledProcessError':
-                # It's up to the subprocess handler to report something useful
-                msg = "Error occurred while running this command:"
-                _log.error(msg + '\n' + exc_value)
-                return ExitCode.child_process_error
-            elif not options.verbose:
-                _log.error(e)
+        # Ruffus flattens exception to 5 element tuples. Because of a bug
+        # in <= 2.6.3 it may present either the single:
+        #   (task, job, exc, value, stack)
+        # or something like:
+        #   [[(task, job, exc, value, stack)]]
+        #
+        # Generally cross-process exception marshalling doesn't work well
+        # and ruffus doesn't support because BaseException has its own
+        # implementation of __reduce__ that attempts to reconstruct the
+        # exception based on e.__init__(e.args).
+        #
+        # Attempting to log the exception directly marshalls it to the logger
+        # which is probably in another process, so it's better to log only
+        # data from the exception at this point.
 
-        return ExitCode.other_error
+        exitcode = traverse_ruffus_exception(e.args)
+        if exitcode is None:
+            _log.error("Unexpected ruffus exception: " + str(e))
+            _log.error(repr(e))
+            return ExitCode.other_error
+        else:
+            return exitcode
     except Exception as e:
         _log.error(e)
         return ExitCode.other_error

setup.py

@@ -217,7 +217,7 @@ setup(
         'ocrmypdf/lib/compile_leptonica.py:ffi'
     ],
     install_requires=[
-        'ruffus>=2.6.3',
+        'ruffus==2.6.3',
         'Pillow>=3.0.0',
         'reportlab>=3.1.44',
         'PyPDF2>=1.25.1',