Handle __del__ of connection object more gracefully #390

--------
Co-authored-by: Christoph Pirkl <christoph.pirkl@exasol.com>

CHANGELOG.rst

@@ -3,6 +3,11 @@
 Unreleased
 ==========
 
+🐞 Fixed
+---------
+
+- Fixed `WebSocket connection isn't properly closed in case of process termination <https://github.com/exasol/pyexasol/issues/108>`_
+
 .. _changelog-4.6.0:
 
 4.6.0 — 2023-07-18

exasol/driver/websocket/_connection.py

@@ -119,10 +119,12 @@ def connection(self):
 
     def close(self):
         """See also :py:meth: `Connection.close`"""
-        if not self._connection:
+        connection_to_close = self._connection
+        self._connection = None
+        if connection_to_close is None or connection_to_close.is_closed:
             return
         try:
-            self._connection.close()
+            connection_to_close.close()
         except Exception as ex:
             raise Error() from ex
 
@@ -148,4 +150,16 @@ def cursor(self):
         return DefaultCursor(self)
 
     def __del__(self):
-        self.close()
+        if self._connection is None:
+            return
+
+        # Currently, the only way to handle this gracefully is to invoke the`__del__`
+        # method of the underlying connection rather than calling an explicit `close`.
+        #
+        # For more details, see also:
+        # * https://github.com/exasol/sqlalchemy-exasol/issues/390
+        # * https://github.com/exasol/pyexasol/issues/108
+        #
+        # If the above tickets are resolved, it should be safe to switch back to using
+        # `close` instead of `__del__`.
+        self._connection.__del__()

test/integration/regression/test_regression_bug390.py

@@ -0,0 +1,37 @@
+from inspect import cleandoc
+
+pytest_plugins = "pytester"
+
+
+def test_connection_with_block_cleans_up_properly(pytester, exasol_config):
+    config = exasol_config
+    # Because the error only occurs on process shutdown we need to run a test within a test
+    # (We require the result (stderr) of a terminated process triggering the failure.
+    pytester.makepyfile(
+        # fmt: off
+        cleandoc(
+            f"""
+        from sqlalchemy import create_engine
+        
+        def test():
+            url = "exa+websocket://{{user}}:{{pw}}@{{host}}:{{port}}/{{schema}}?SSLCertificate=SSL_VERIFY_NONE"
+            url = url.format(
+                user="{config.username}",
+                pw="{config.password}",
+                host="{config.host}",
+                port={config.port},
+                schema="TEST",
+            )
+            engine = create_engine(url)
+            query = "SELECT 42;"
+            with engine.connect() as con:
+                result = con.execute(query).fetchall()
+        """
+        ),
+        # fmt: on
+    )
+    r = pytester.runpytest_subprocess()
+    expected = ""
+    actual = str(r.stderr)
+
+    assert actual == expected