chain_ex: don't opportunistically add tracebacks to context_ex

Don't add a traceback to the context_ex while chaining.  This could be
wrong, depending on the context, and in the common case, the traceback
more likely belonged on the result exception, not the context_ex.,
i.e.

  except ... as ex:
      raise chain_ex(ex, pending_ex)

Instead, just combine chain_ex with add_ex_tb when appropriate:

  except ... as ex:
      raise chain_ex(add_tb_ex(ex), pending_ex)

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>

diff --git a/lib/bup/compat.py b/lib/bup/compat.py
index 707ccbafea529cb8dfeaf5de0195306929b32913..73e89f8212ad528bda6c6505557cbb56eef12e60 100644 (file)
--- a/lib/bup/compat.py
+++ b/lib/bup/compat.py
@@ -15,6 +15,7 @@ if py3:
         return ex
 
     def chain_ex(ex, context_ex):
+        """Do nothing (already handled by Python 3 infrastructure)."""
         return ex
 
 else:  # Python 2
@@ -30,8 +31,11 @@ else:  # Python 2
         return ex
 
     def chain_ex(ex, context_ex):
+        """Chain context_ex to ex as the __context__ (unless it already has
+        one).  Return ex.
+
+        """
         if context_ex:
-            add_ex_tb(context_ex)
             if not getattr(ex, '__context__', None):
                 ex.__context__ = context_ex
         return ex
@@ -72,11 +76,14 @@ def wrap_main(main):
         sys.exit(1)
 
 
-# Excepting wrap_main() in the traceback, these should produce the same output:
+# Excepting wrap_main() in the traceback, these should produce similar output:
 #   python2 lib/bup/compat.py
 #   python3 lib/bup/compat.py
 # i.e.:
 #   diff -u <(python2 lib/bup/compat.py 2>&1) <(python3 lib/bup/compat.py 2>&1)
+#
+# Though the python3 output for 'second' will include a stacktrace
+# starting from wrap_main, rather than from outer().
 
 if __name__ == '__main__':
 
@@ -87,6 +94,10 @@ if __name__ == '__main__':
         try:
             inner()
         except Exception as ex:
-            raise chain_ex(Exception('second'), ex)
+            add_ex_tb(ex)
+            try:
+                raise Exception('second')
+            except Exception as ex2:
+                raise chain_ex(add_ex_tb(ex2), ex)
 
     wrap_main(outer)