introduce generalized decorator to prevent invalid method calls

This introduces the ``_exclusive_against()`` utility decorator
that can be used to prevent repeated invocations of methods that
typically should only be called once.

An informative error message is now raised for a selected set of DML
methods (currently all part of :class:`_dml.Insert` constructs) if they are
called a second time, which would implicitly cancel out the previous
setting.  The methods altered include:
:class:`_sqlite.Insert.on_conflict_do_update`,
:class:`_sqlite.Insert.on_conflict_do_nothing` (SQLite),
:class:`_postgresql.Insert.on_conflict_do_update`,
:class:`_postgresql.Insert.on_conflict_do_nothing` (PostgreSQL),
:class:`_mysql.Insert.on_duplicate_key_update` (MySQL)

Fixes: #5169
Change-Id: I9278fa87cd3470dcf296ff96bb0fb17a3236d49d

1 files changed, 25 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/base.py b/lib/sqlalchemy/sql/base.py
index 550111020..220bbb115 100644
--- a/lib/sqlalchemy/sql/base.py
+++ b/lib/sqlalchemy/sql/base.py
@@ -102,6 +102,31 @@ def _generative(fn):
     return decorated
 
 
+def _exclusive_against(*names, **kw):
+    msgs = kw.pop("msgs", {})
+
+    defaults = kw.pop("defaults", {})
+
+    getters = [
+        (name, operator.attrgetter(name), defaults.get(name, None))
+        for name in names
+    ]
+
+    @util.decorator
+    def check(fn, self, *args, **kw):
+        for name, getter, default_ in getters:
+            if getter(self) is not default_:
+                msg = msgs.get(
+                    name,
+                    "Method %s() has already been invoked on this %s construct"
+                    % (fn.__name__, self.__class__),
+                )
+                raise exc.InvalidRequestError(msg)
+        return fn(self, *args, **kw)
+
+    return check
+
+
 def _clone(element, **kw):
     return element._clone()