Provide a common exception caused by base class

Both taskflow and tooz (and others of `raise_with_cause`
should be able to use this as there base exception class
to ensure that there usage of `raise_with_cause` works
as expected).

This adds the class that taskflow was/is using that provides
a 'good enough' version exception chaining (and a decently useful
`pformat` and `__str__` overrides that approximates the exception
chaining message on py2.x).

When used correctly this produces the following:

http://paste.openstack.org/show/326021/

Which is a 'good enough' approximation of exception chaining
on py2.x (and just uses the native exception chaining on py3.x)

This also adds some basic tests to ensure the functionality
provided continues to work as expected.

Change-Id: If814816dec91f7da3c90f4b9b8e2d3cf7228b928

oslo_utils/excutils.py

@@ -18,6 +18,7 @@ Exception related utilities.
 """
 
 import logging
+import os
 import sys
 import time
 import traceback
@@ -25,6 +26,80 @@ import traceback
 import six
 
 from oslo_utils._i18n import _LE
+from oslo_utils import reflection
+
+
+class CausedByException(Exception):
+    """Base class for exceptions which have associated causes.
+
+    NOTE(harlowja): in later versions of python we can likely remove the need
+    to have a ``cause`` here as PY3+ have implemented :pep:`3134` which
+    handles chaining in a much more elegant manner.
+
+    :param message: the exception message, typically some string that is
+                    useful for consumers to view when debugging or analyzing
+                    failures.
+    :param cause: the cause of the exception being raised, when provided this
+                  should itself be an exception instance, this is useful for
+                  creating a chain of exceptions for versions of python where
+                  this is not yet implemented/supported natively.
+    """
+    def __init__(self, message, cause=None):
+        super(CausedByException, self).__init__(message)
+        self.cause = cause
+
+    def __bytes__(self):
+        return self.pformat().encode("utf8")
+
+    def __str__(self):
+        return self.pformat()
+
+    def _get_message(self):
+        # We must *not* call into the ``__str__`` method as that will
+        # reactivate the pformat method, which will end up badly (and doesn't
+        # look pretty at all); so be careful...
+        return self.args[0]
+
+    def pformat(self, indent=2, indent_text=" ", show_root_class=False):
+        """Pretty formats a caused exception + any connected causes."""
+        if indent < 0:
+            raise ValueError("Provided 'indent' must be greater than"
+                             " or equal to zero instead of %s" % indent)
+        buf = six.StringIO()
+        if show_root_class:
+            buf.write(reflection.get_class_name(self, fully_qualified=False))
+            buf.write(": ")
+        buf.write(self._get_message())
+        active_indent = indent
+        next_up = self.cause
+        seen = []
+        while next_up is not None and next_up not in seen:
+            seen.append(next_up)
+            buf.write(os.linesep)
+            if isinstance(next_up, CausedByException):
+                buf.write(indent_text * active_indent)
+                buf.write(reflection.get_class_name(next_up,
+                                                    fully_qualified=False))
+                buf.write(": ")
+                buf.write(next_up._get_message())
+            else:
+                lines = traceback.format_exception_only(type(next_up), next_up)
+                for i, line in enumerate(lines):
+                    buf.write(indent_text * active_indent)
+                    if line.endswith("\n"):
+                        # We'll add our own newlines on...
+                        line = line[0:-1]
+                    buf.write(line)
+                    if i + 1 != len(lines):
+                        buf.write(os.linesep)
+            if not isinstance(next_up, CausedByException):
+                # Don't go deeper into non-caused-by exceptions... as we
+                # don't know if there exception 'cause' attributes are even
+                # useable objects...
+                break
+            active_indent += indent
+            next_up = getattr(next_up, 'cause', None)
+        return buf.getvalue()
 
 
 def raise_with_cause(exc_cls, message, *args, **kwargs):
@@ -40,7 +115,8 @@ def raise_with_cause(exc_cls, message, *args, **kwargs):
     inspected/retained on py2.x to get *similar* information as would be
     automatically included/obtainable in py3.x.
 
-    :param exc_cls: the exception class to raise.
+    :param exc_cls: the exception class to raise (typically one derived
+                    from :py:class:`.CausedByException` or equivalent).
     :param message: the text/str message that will be passed to
                     the exceptions constructor as its first positional
                     argument.

oslo_utils/tests/test_excutils.py

@@ -25,6 +25,41 @@ from oslo_utils import excutils
 mox = moxstubout.mox
 
 
+class Fail1(excutils.CausedByException):
+    pass
+
+
+class Fail2(excutils.CausedByException):
+    pass
+
+
+class CausedByTest(test_base.BaseTestCase):
+
+    def test_caused_by_explicit(self):
+        e = self.assertRaises(Fail1,
+                              excutils.raise_with_cause,
+                              Fail1, "I was broken",
+                              cause=Fail2("I have been broken"))
+        self.assertIsInstance(e.cause, Fail2)
+        e_p = e.pformat()
+        self.assertIn("I have been broken", e_p)
+        self.assertIn("Fail2", e_p)
+
+    def test_caused_by_implicit(self):
+
+        def raises_chained():
+            try:
+                raise Fail2("I have been broken")
+            except Fail2:
+                excutils.raise_with_cause(Fail1, "I was broken")
+
+        e = self.assertRaises(Fail1, raises_chained)
+        self.assertIsInstance(e.cause, Fail2)
+        e_p = e.pformat()
+        self.assertIn("I have been broken", e_p)
+        self.assertIn("Fail2", e_p)
+
+
 class SaveAndReraiseTest(test_base.BaseTestCase):
 
     def test_save_and_reraise_exception(self):