[IMP] fields: use a "lazy" cursor to compute attribute digits

Borrowing a cursor each time you access `field.digits` may be costly, because
of the connection reset.  Moreover, in most cases, the cursor is not used at
all, since the decimal precision are kept in cache.

For the installation of module `product` with its demo data, the number of
cursor allocations was reduced from ~1500 to about 50!

openerp/fields.py

@@ -30,6 +30,7 @@ import logging
 import pytz
 import xmlrpclib
 
+from openerp.sql_db import LazyCursor
 from openerp.tools import float_round, frozendict, html_sanitize, ustr, OrderedSet
 from openerp.tools import DEFAULT_SERVER_DATE_FORMAT as DATE_FORMAT
 from openerp.tools import DEFAULT_SERVER_DATETIME_FORMAT as DATETIME_FORMAT
@@ -1070,7 +1071,7 @@ class Float(Field):
     @property
     def digits(self):
         if callable(self._digits):
-            with fields._get_cursor() as cr:
+            with LazyCursor() as cr:
                 return self._digits(cr)
         else:
             return self._digits
@@ -1882,7 +1883,7 @@ class Id(Field):
         raise TypeError("field 'id' cannot be assigned")
 
 # imported here to avoid dependency cycle issues
-from openerp import SUPERUSER_ID, registry
+from openerp import SUPERUSER_ID
 from .exceptions import Warning, AccessError, MissingError
 from .models import BaseModel, MAGIC_COLUMNS
 from .osv import fields

openerp/osv/fields.py

@@ -43,21 +43,18 @@ import pytz
 import re
 import xmlrpclib
 from operator import itemgetter
-from contextlib import contextmanager
 from psycopg2 import Binary
 
 import openerp
 import openerp.tools as tools
+from openerp.sql_db import LazyCursor
 from openerp.tools.translate import _
 from openerp.tools import float_repr, float_round, frozendict, html_sanitize
 import simplejson
-from openerp import SUPERUSER_ID, registry
+from openerp import SUPERUSER_ID
 
-@contextmanager
-def _get_cursor():
-    # yield a valid cursor from any environment or create a new one if none found
-    with registry().cursor() as cr:
-        yield cr
+# deprecated; kept for backward compatibility only
+_get_cursor = LazyCursor
 
 EMPTY_DICT = frozendict()
 
@@ -396,7 +393,7 @@ class float(_column):
     @property
     def digits(self):
         if self._digits_compute:
-            with _get_cursor() as cr:
+            with LazyCursor() as cr:
                 return self._digits_compute(cr)
         else:
             return self._digits
@@ -1323,7 +1320,7 @@ class function(_column):
     @property
     def digits(self):
         if self._digits_compute:
-            with _get_cursor() as cr:
+            with LazyCursor() as cr:
                 return self._digits_compute(cr)
         else:
             return self._digits

openerp/sql_db.py

@@ -443,6 +443,40 @@ class TestCursor(Cursor):
         self.execute("ROLLBACK TO SAVEPOINT test_cursor")
         self.execute("SAVEPOINT test_cursor")
 
+class LazyCursor(object):
+    """ A proxy object to a cursor. The cursor itself is allocated only if it is
+        needed. This class is useful for cached methods, that use the cursor
+        only in the case of a cache miss.
+    """
+    def __init__(self, dbname=None):
+        self._dbname = dbname
+        self._cursor = None
+        self._depth = 0
+
+    @property
+    def dbname(self):
+        return self._dbname or threading.currentThread().dbname
+
+    def __getattr__(self, name):
+        cr = self._cursor
+        if cr is None:
+            from openerp import registry
+            cr = self._cursor = registry(self.dbname).cursor()
+            for _ in xrange(self._depth):
+                cr.__enter__()
+        return getattr(cr, name)
+
+    def __enter__(self):
+        self._depth += 1
+        if self._cursor is not None:
+            self._cursor.__enter__()
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        self._depth -= 1
+        if self._cursor is not None:
+            self._cursor.__exit__(exc_type, exc_value, traceback)
+
 class PsycoConnection(psycopg2.extensions.connection):
     pass