1 files changed, 38 insertions, 1 deletions
diff --git a/synapse/util/stringutils.py b/synapse/util/stringutils.py
index 6f318c6a29..fdcb375f95 100644
--- a/synapse/util/stringutils.py
+++ b/synapse/util/stringutils.py
@@ -16,7 +16,8 @@
 import random
 import string
 
-from six import PY3
+import six
+from six import PY2, PY3
 from six.moves import range
 
 _string_with_symbols = (
@@ -71,3 +72,39 @@ def to_ascii(s):
         return s.encode("ascii")
     except UnicodeEncodeError:
         return s
+
+
+def exception_to_unicode(e):
+    """Helper function to extract the text of an exception as a unicode string
+
+    Args:
+        e (Exception): exception to be stringified
+
+    Returns:
+        unicode
+    """
+    # urgh, this is a mess. The basic problem here is that psycopg2 constructs its
+    # exceptions with PyErr_SetString, with a (possibly non-ascii) argument. str() will
+    # then produce the raw byte sequence. Under Python 2, this will then cause another
+    # error if it gets mixed with a `unicode` object, as per
+    # https://github.com/matrix-org/synapse/issues/4252
+
+    # First of all, if we're under python3, everything is fine because it will sort this
+    # nonsense out for us.
+    if not PY2:
+        return str(e)
+
+    # otherwise let's have a stab at decoding the exception message. We'll circumvent
+    # Exception.__str__(), which would explode if someone raised Exception(u'non-ascii')
+    # and instead look at what is in the args member.
+
+    if len(e.args) == 0:
+        return u""
+    elif len(e.args) > 1:
+        return six.text_type(repr(e.args))
+
+    msg = e.args[0]
+    if isinstance(msg, bytes):
+        return msg.decode('utf-8', errors='replace')
+    else:
+        return msg