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, 11 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/dml.py b/lib/sqlalchemy/dialects/postgresql/dml.py
index 76dfafd04..bff61e173 100644
--- a/lib/sqlalchemy/dialects/postgresql/dml.py
+++ b/lib/sqlalchemy/dialects/postgresql/dml.py
@@ -10,6 +10,7 @@ from ... import util
 from ...sql import coercions
 from ...sql import roles
 from ...sql import schema
+from ...sql.base import _exclusive_against
 from ...sql.base import _generative
 from ...sql.dml import Insert as StandardInsert
 from ...sql.elements import ClauseElement
@@ -50,7 +51,16 @@ class Insert(StandardInsert):
         """
         return alias(self.table, name="excluded").columns
 
+    _on_conflict_exclusive = _exclusive_against(
+        "_post_values_clause",
+        msgs={
+            "_post_values_clause": "This Insert construct already has "
+            "an ON CONFLICT clause established"
+        },
+    )
+
     @_generative
+    @_on_conflict_exclusive
     def on_conflict_do_update(
         self,
         constraint=None,
@@ -117,6 +127,7 @@ class Insert(StandardInsert):
         )
 
     @_generative
+    @_on_conflict_exclusive
     def on_conflict_do_nothing(
         self, constraint=None, index_elements=None, index_where=None
     ):