Don't do recovery operations under Empty/AttributeError

Made some adjustments to :class:`.Pool` and :class:`.Connection` such
that recovery logic is not run underneath exception catches for
``pool.Empty``, ``AttributeError``, since when the recovery operation
itself fails, Python 3 creates a misleading stack trace referring to the
``Empty`` / ``AttributeError`` as the cause, when in fact these exception
catches are part of control flow.

Change-Id: Id3ed9a8f96ce4ccb4009c94af30ddc2ddb9818b9
Fixes: #4028

1 files changed, 15 insertions, 4 deletions

diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index 91f4493c2..b5c95cb17 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -345,10 +345,13 @@ class Connection(Connectable):
         try:
             return self.__connection
         except AttributeError:
-            try:
-                return self._revalidate_connection()
-            except BaseException as e:
-                self._handle_dbapi_exception(e, None, None, None, None)
+            # escape "except AttributeError" before revalidating
+            # to prevent misleading stacktraces in Py3K
+            pass
+        try:
+            return self._revalidate_connection()
+        except BaseException as e:
+            self._handle_dbapi_exception(e, None, None, None, None)
 
     def get_isolation_level(self):
         """Return the current isolation level assigned to this
@@ -962,6 +965,10 @@ class Connection(Connectable):
             try:
                 conn = self.__connection
             except AttributeError:
+                # escape "except AttributeError" before revalidating
+                # to prevent misleading stacktraces in Py3K
+                conn = None
+            if conn is None:
                 conn = self._revalidate_connection()
 
             dialect = self.dialect
@@ -1111,6 +1118,10 @@ class Connection(Connectable):
             try:
                 conn = self.__connection
             except AttributeError:
+                # escape "except AttributeError" before revalidating
+                # to prevent misleading stacktraces in Py3K
+                conn = None
+            if conn is None:
                 conn = self._revalidate_connection()
 
             context = constructor(dialect, self, conn, *args)