Discard disconnected connection from the pool

When a connection in the pool is disconnected, it stays in the pool and it may
still be served.
Using such connection raise an OperationalError and so the request is retried
but before that the connection is put back to the pool. So we may get it back
again.
It is not possible to distinct between an OperationalError due to a
disconnection or due to a isolation error for example.
So with the current design of Transaction API, an OperationalError always put
back the connection to the pool.
This main that we can only ensure that the connections served are still
connected by executing a simple query. And close connections that are failing.

Closes #12452
(grafted from f8aaf62a0c7508e3f3398e47dc2b86e6f8f06cc0)

--HG--
branch : 6.0

trytond/trytond/backend/postgresql/database.py

@@ -257,10 +257,10 @@ class Database(DatabaseInterface):
         return self
 
     def get_connection(self, autocommit=False, readonly=False):
-        for count in range(config.getint('database', 'retry'), -1, -1):
+        retry = max(config.getint('database', 'retry'), _maxconn)
+        for count in range(retry, -1, -1):
             try:
                 conn = self._connpool.getconn()
-                break
             except PoolError:
                 if count and not self._connpool.closed:
                     logger.info('waiting a connection')
@@ -271,19 +271,26 @@ class Database(DatabaseInterface):
                 logger.error(
                     'connection to "%s" failed', self.name, exc_info=True)
                 raise
-        # We do not use set_session because psycopg2 < 2.7 and psycopg2cffi
-        # change the default_transaction_* attributes which breaks external
-        # pooling at the transaction level.
-        if autocommit:
-            conn.set_isolation_level(ISOLATION_LEVEL_AUTOCOMMIT)
-        else:
-            conn.set_isolation_level(ISOLATION_LEVEL_REPEATABLE_READ)
-        # psycopg2cffi does not have the readonly property
-        if hasattr(conn, 'readonly'):
-            conn.readonly = readonly
-        elif not autocommit and readonly:
-            cursor = conn.cursor()
-            cursor.execute('SET TRANSACTION READ ONLY')
+            try:
+                # We do not use set_session because psycopg2 < 2.7 and
+                # psycopg2cffi change the default_transaction_* attributes
+                # which breaks external pooling at the transaction level.
+                if autocommit:
+                    conn.set_isolation_level(ISOLATION_LEVEL_AUTOCOMMIT)
+                else:
+                    conn.set_isolation_level(ISOLATION_LEVEL_REPEATABLE_READ)
+                # psycopg2cffi does not have the readonly property
+                if hasattr(conn, 'readonly'):
+                    conn.readonly = readonly
+                elif not autocommit and readonly:
+                    cursor = conn.cursor()
+                    cursor.execute('SET TRANSACTION READ ONLY')
+                # Detect disconnection
+                conn.cursor().execute('SELECT 1')
+            except DatabaseOperationalError:
+                self._connpool.putconn(conn, close=True)
+                continue
+            break
         return conn
 
     def put_connection(self, connection, close=False):