fix(dialect): make extend_sqlglot idempotent

_override saved whatever was currently installed under _<name>, so a second
extend_sqlglot() call saved the override itself as the original. The wrapper
then called itself and every subsequent type/CAST parse raised RecursionError,
with sqlglot's original method lost.

_override now returns early when the override is already installed.

Adds tests/core/test_dialect.py::test_extend_sqlglot_is_idempotent, which fails
with RecursionError without the fix.

Signed-off-by: Sanjay Santhanam <51058514+Sanjays2402@users.noreply.github.com>

Author: Sanjays2402

sqlmesh/core/dialect.py

@@ -782,6 +782,10 @@ def _parse_interval_span(self: Parser, this: exp.Expr) -> exp.Interval:
 
 def _override(klass: t.Type[Tokenizer | Parser], func: t.Callable) -> None:
     name = func.__name__
+    if getattr(klass, name, None) is func:
+        # Already overridden. Re-applying would save the override itself as the
+        # "original", making the wrapper call itself and recurse infinitely.
+        return
     setattr(klass, f"_{name}", getattr(klass, name))
     setattr(klass, name, func)
 

tests/core/test_dialect.py

@@ -1185,3 +1185,11 @@ def test_pipe_syntax():
         ast.sql("bigquery")
         == "SELECT * FROM (WITH __tmp1 AS (SELECT id FROM t2) SELECT * FROM __tmp1)"
     )
+
+
+def test_extend_sqlglot_is_idempotent():
+    # extend_sqlglot() runs at import time; calling it again must not re-wrap the
+    # already-installed overrides, otherwise they call themselves (RecursionError).
+    d.extend_sqlglot()
+
+    assert parse_one("SELECT CAST(1 AS INT)").sql() == "SELECT CAST(1 AS INT)"