[1.7.x] Fixed #21553 -- Ensured unusable database connections get closed.

Backport of 5f2f47f from master

4 files changed, 9 insertions, 4 deletions

diff --git a/django/db/backends/__init__.py b/django/db/backends/__init__.py
index 98dab13ba8..40261e096b 100644
--- a/django/db/backends/__init__.py
+++ b/django/db/backends/__init__.py
@@ -445,9 +445,14 @@ class BaseDatabaseWrapper(object):
     def is_usable(self):
         """
         Tests if the database connection is usable.
+
         This function may assume that self.connection is not None.
+
+        Actual implementations should take care not to raise exceptions
+        as that may prevent Django from recycling unusable connections.
         """
-        raise NotImplementedError('subclasses of BaseDatabaseWrapper may require an is_usable() method')
+        raise NotImplementedError(
+            "subclasses of BaseDatabaseWrapper may require an is_usable() method")
 
     def close_if_unusable_or_obsolete(self):
         """
diff --git a/django/db/backends/mysql/base.py b/django/db/backends/mysql/base.py
index 61a8ab72fc..cbe37fc84c 100644
--- a/django/db/backends/mysql/base.py
+++ b/django/db/backends/mysql/base.py
@@ -552,7 +552,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
     def is_usable(self):
         try:
             self.connection.ping()
-        except DatabaseError:
+        except Database.Error:
             return False
         else:
             return True
diff --git a/django/db/backends/oracle/base.py b/django/db/backends/oracle/base.py
index 830a8a9862..da0fdb121f 100644
--- a/django/db/backends/oracle/base.py
+++ b/django/db/backends/oracle/base.py
@@ -704,7 +704,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
             else:
                 # Use a cx_Oracle cursor directly, bypassing Django's utilities.
                 self.connection.cursor().execute("SELECT 1 FROM DUAL")
-        except DatabaseError:
+        except Database.Error:
             return False
         else:
             return True
diff --git a/django/db/backends/postgresql_psycopg2/base.py b/django/db/backends/postgresql_psycopg2/base.py
index e89a4e604a..6ea17cf543 100644
--- a/django/db/backends/postgresql_psycopg2/base.py
+++ b/django/db/backends/postgresql_psycopg2/base.py
@@ -213,7 +213,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         try:
             # Use a psycopg cursor directly, bypassing Django's utilities.
             self.connection.cursor().execute("SELECT 1")
-        except DatabaseError:
+        except Database.Error:
             return False
         else:
             return True