Restore use_threadlocal equivalent behavior to SingletonThreadPool

Fixed behavioral regression as a result of deprecating the "use_threadlocal"
flag for :class:`.Pool`, where the :class:`.SingletonThreadPool` no longer
makes use of this option which causes the "rollback on return" logic to take
place when the same :class:`.Engine` is used multiple times in the context
of a transaction to connect or implicitly execute, thereby cancelling the
transaction.   While this is not the recommended way to work with engines
and connections, it is nonetheless a confusing behavioral change as when
using :class:`.SingletonThreadPool`, the transaction should stay open
regardless of what else is done with the same engine in the same thread.
The ``use_threadlocal`` flag remains deprecated however the
:class:`.SingletonThreadPool` now implements its own version of the same
logic.

Fixes: #4585
Change-Id: I906293f2d0a5d14ed46cd9e64305a6481505a5a3

1 files changed, 21 insertions, 0 deletions

diff --git a/lib/sqlalchemy/pool/impl.py b/lib/sqlalchemy/pool/impl.py
index ada319661..e1a457bf3 100644
--- a/lib/sqlalchemy/pool/impl.py
+++ b/lib/sqlalchemy/pool/impl.py
@@ -13,6 +13,7 @@
 import traceback
 import weakref
 
+from .base import _ConnectionFairy
 from .base import _ConnectionRecord
 from .base import Pool
 from .. import exc
@@ -288,6 +289,7 @@ class SingletonThreadPool(Pool):
     def __init__(self, creator, pool_size=5, **kw):
         Pool.__init__(self, creator, **kw)
         self._conn = threading.local()
+        self._fairy = threading.local()
         self._all_conns = set()
         self.size = pool_size
 
@@ -346,6 +348,25 @@ class SingletonThreadPool(Pool):
         self._all_conns.add(c)
         return c
 
+    def connect(self):
+        # vendored from Pool to include use_threadlocal behavior
+        try:
+            rec = self._fairy.current()
+        except AttributeError:
+            pass
+        else:
+            if rec is not None:
+                return rec._checkout_existing()
+
+        return _ConnectionFairy._checkout(self, self._fairy)
+
+    def _return_conn(self, record):
+        try:
+            del self._fairy.current
+        except AttributeError:
+            pass
+        self._do_return_conn(record)
+
 
 class StaticPool(Pool):