From ea3cadffc728f5a8262243f6f9cafdc331b12b2b Mon Sep 17 00:00:00 2001
From: Rob Browning <rlb@defaultvalue.org>
Date: Sat, 12 Sep 2020 17:14:21 -0500
Subject: [PATCH] compat: add "with pending_raise(ex)" to simplify nested
 exceptions

Thanks to Johannes Berg for the suggesting the approach.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>
---
 CODINGSTYLE          | 23 ++++++++++++++++++-----
 Makefile             |  1 +
 lib/bup/compat.py    | 34 ++++++++++++++++++++++++++++++++++
 lib/bup/t/tcompat.py | 32 ++++++++++++++++++++++++++++++++
 4 files changed, 85 insertions(+), 5 deletions(-)
 create mode 100644 lib/bup/t/tcompat.py

diff --git a/CODINGSTYLE b/CODINGSTYLE
index 69c2cfb..348a2f5 100644
--- a/CODINGSTYLE
+++ b/CODINGSTYLE
@@ -43,7 +43,6 @@ explicitly add stack traces to any exceptions that are going to be
 re-raised by anything other than a no-argument raise (otherwise the
 stack trace will be lost)::
 
-
   try:
       ...
   except ... as ex:
@@ -52,11 +51,24 @@ stack trace will be lost)::
   ...
   raise pending_ex
 
-If an exception is thrown from an exception handler, the pending
+When an exception is thrown from an exception handler, the pending
 exception should be the `"context"
 <https://docs.python.org/3/reference/simple_stmts.html#the-raise-statement>`_
-of the new exception This can be accomplished via
-``add_ex_ctx()``::
+of the new exception, which can be accomplished (portably) via
+``pending_raise()``::
+
+  try:
+      ...
+  except ... as ex:
+      with pending_raise(ex):
+          clean_up()
+
+This should do roughly the same thing in Python 2 and Python 3,
+throwing any exception from ``clean_up()`` after adding ex as the
+``__context__`` if clean_up() throws, and throwing ``ex`` otherwise.
+
+If for some reason, you need more control, you can use
+``add_ex_ctx()`` directly::
 
   try:
       ...
@@ -69,4 +81,5 @@ of the new exception This can be accomplished via
           raise add_ex_ctx(ex2, ex)
       raise
 
-See the end of ``lib/bup/compat.py`` for a functional example.
+See the end of ``lib/bup/compat.py`` for a functional example, and all
+of this can be removed once we drop support for Python 2.
diff --git a/Makefile b/Makefile
index 4d0784b..5e31562 100644
--- a/Makefile
+++ b/Makefile
@@ -163,6 +163,7 @@ runtests: runtests-python runtests-cmdline
 python_tests := \
   lib/bup/t/tbloom.py \
   lib/bup/t/tclient.py \
+  lib/bup/t/tcompat.py \
   lib/bup/t/tgit.py \
   lib/bup/t/thashsplit.py \
   lib/bup/t/thelpers.py \
diff --git a/lib/bup/compat.py b/lib/bup/compat.py
index 39dd810..2cd6fba 100644
--- a/lib/bup/compat.py
+++ b/lib/bup/compat.py
@@ -37,6 +37,20 @@ if py3:
         """Do nothing (already handled by Python 3 infrastructure)."""
         return ex
 
+    class pending_raise:
+        """Rethrow either the provided ex, or any exception raised by the with
+        statement body.  (Supports Python 2 compatibility.)
+
+        """
+        def __init__(self, ex):
+            self.ex = ex
+        def __enter__(self):
+            return None
+        def __exit__(self, exc_type, exc_value, traceback):
+            if not exc_type:
+                raise self.ex
+            return None
+
     def items(x):
         return x.items()
 
@@ -103,6 +117,26 @@ else:  # Python 2
                 ex.__context__ = context_ex
         return ex
 
+    class pending_raise:
+        """Rethrow either the provided ex, or any exception raised by the with
+        statement body, after making ex the __context__ of the newer
+        exception (assuming there's no existing __context__).  Ensure
+        the exceptions have __tracebacks__.  (Supports Python 2
+        compatibility.)
+
+        """
+        def __init__(self, ex):
+            self.ex = ex
+        def __enter__(self):
+            add_ex_tb(self.ex)
+            return None
+        def __exit__(self, exc_type, exc_value, traceback):
+            if not exc_type:
+                raise self.ex
+            add_ex_tb(exc_value)
+            add_ex_ctx(exc_value, self.ex)
+            return None
+
     def dump_traceback(ex):
         stack = [ex]
         next_ex = getattr(ex, '__context__', None)
diff --git a/lib/bup/t/tcompat.py b/lib/bup/t/tcompat.py
new file mode 100644
index 0000000..039eb12
--- /dev/null
+++ b/lib/bup/t/tcompat.py
@@ -0,0 +1,32 @@
+
+from __future__ import absolute_import, print_function
+
+from wvtest import *
+
+from bup.compat import pending_raise
+
+@wvtest
+def test_pending_raise():
+    outer = Exception('outer')
+    inner = Exception('inner')
+
+    try:
+        try:
+            raise outer
+        except Exception as ex:
+            with pending_raise(ex):
+                pass
+    except Exception as ex:
+        WVPASSEQ(outer, ex)
+        WVPASSEQ(None, getattr(outer, '__context__', None))
+
+    try:
+        try:
+            raise outer
+        except Exception as ex:
+            with pending_raise(ex):
+                raise inner
+    except Exception as ex:
+        WVPASSEQ(inner, ex)
+        WVPASSEQ(None, getattr(outer, '__context__', None))
+        WVPASSEQ(outer, getattr(inner, '__context__', None))
-- 
2.39.2