FIX: Implementing RecordBatchReader.Close() functionality for Arrow

[Subrata (subrata-ms)]

Work Item / Issue Reference

AB#45978

GitHub Issue: #643

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Summary

This pull request introduces a new _ArrowReader wrapper to enhance the behavior of the arrow_reader method in the Cursor class. The new wrapper ensures that server-side resources are properly released when the reader is closed, supporting robust cleanup and allowing the parent cursor to remain usable. The tests are updated and extended to verify the improved semantics, including close behavior and context manager support.

Enhancements to Arrow batch reading and resource management:

• Added the _ArrowReader class to cursor.py, which wraps a pyarrow.RecordBatchReader and implements an 8-step close sequence to properly release server-side resources, reset cursor state, and support idempotent and context-manager-based cleanup. The parent Cursor remains usable after closing the reader.
• Modified the Cursor.arrow_reader method to return an instance of _ArrowReader instead of a raw pyarrow.RecordBatchReader, ensuring that closing the reader stops fetching, releases the server-side cursor, and resets cursor state. [1] [2]

Test improvements for Arrow reader behavior:

• Updated the test_arrow_reader test to check for duck-typed compatibility with pyarrow.RecordBatchReader, reflecting the new wrapper class.
• Added new tests: test_arrow_reader_close_semantics to verify that .close() stops fetching, marks the reader as closed, is idempotent, and leaves the parent cursor usable; and test_arrow_reader_context_manager to verify that the reader is closed on context manager exit and the cursor remains usable.

[github-actions]

📊 Code Coverage Report

🔥 Diff Coverage 93%

🎯 Overall Coverage 81%

📈 Total Lines Covered: 7184 out of 8797 📁 Project: mssql-python

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Diff Coverage

Diff: main...HEAD, staged and unstaged changes

• mssql_python/cursor.py (98.9%): Missing lines 276
• mssql_python/pybind/ddbc_bindings.cpp (79.4%): Missing lines 1449,1612-1613,1623-1624,1646-1647

Summary

• Total: 125 lines
• Missing: 8 lines
• Coverage: 93%

mssql_python/cursor.py

Lines 272-280

  272         cursor = self._cursor
  273         if cursor is not None and not cursor.closed and cursor.hstmt is not None:
  274             try:
  275                 cursor.hstmt._cancel()  # pylint: disable=protected-access
! 276             except Exception as e:  # pylint: disable=broad-exception-caught
  277                 logger.debug("arrow_reader.close: SQLCancel raised: %s", e)
  278 
  279         # Close the generator — this raises GeneratorExit inside it, which
  280         # runs the try/finally cleanup block (SQLFreeStmt + diag drain +

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mssql_python/pybind/ddbc_bindings.cpp

Lines 1445-1453

  1445     SQLEndTran_ptr = GetFunctionPointer<SQLEndTranFunc>(handle, "SQLEndTran");
  1446     SQLDisconnect_ptr = GetFunctionPointer<SQLDisconnectFunc>(handle, "SQLDisconnect");
  1447     SQLFreeHandle_ptr = GetFunctionPointer<SQLFreeHandleFunc>(handle, "SQLFreeHandle");
  1448     SQLFreeStmt_ptr = GetFunctionPointer<SQLFreeStmtFunc>(handle, "SQLFreeStmt");
! 1449     SQLCancel_ptr = GetFunctionPointer<SQLCancelFunc>(handle, "SQLCancel");
  1450 
  1451     SQLGetDiagRec_ptr = GetFunctionPointer<SQLGetDiagRecFunc>(handle, "SQLGetDiagRecW");
  1452 
  1453     SQLParamData_ptr = GetFunctionPointer<SQLParamDataFunc>(handle, "SQLParamData");

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Lines 1608-1617

  1608     }
  1609 }
  1610 
  1611 void SqlHandle::cancel() {
! 1612     // SQLCancel is intentionally lenient: it is a no-op on non-STMT handles,
! 1613     // already-freed handles, or if the driver does not expose it. This lets
  1614     // _ArrowReader.close() call it unconditionally without coordinating with
  1615     // the fetch thread. The GIL is released so a blocked fetch thread can
  1616     // observe the cancel and return.
  1617     //

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Lines 1619-1628

  1619     //   The only cross-thread pattern this driver blesses is exactly the one
  1620     //   ODBC blesses: cancel() may be called from a thread *other than* the
  1621     //   fetch thread to unblock an in-flight SQLFetch/SQLExecute on the same
  1622     //   HSTMT. Per the ODBC spec, SQLCancel (with the SQLGetDiagRec/Field
! 1623     //   family) is the only entry point safe to call across threads on the
! 1624     //   same statement handle. All other operations on a Cursor/SqlHandle
  1625     //   are single-owner: per DB API 2.0 and the Cursor thread-safety note
  1626     //   in cursor.py, callers must not share a Cursor for its lifecycle
  1627     //   operations (execute/fetch/close/free) across threads. Under that
  1628     //   contract, free() / close_cursor() / SQLFreeHandle can never be in

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Lines 1642-1651

  1642     if (!SQLCancel_ptr) {
  1643         return;
  1644     }
  1645     SQLHANDLE h = _handle;
! 1646     SQLRETURN ret;
! 1647     {
  1648         py::gil_scoped_release release;
  1649         ret = SQLCancel_ptr(h);
  1650     }
  1651     // SQLCancel may return SQL_SUCCESS_WITH_INFO when there was nothing to

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📋 Files Needing Attention

📉 Files with overall lowest coverage (click to expand)

mssql_python.pybind.logger_bridge.cpp: 59.2%
mssql_python.pybind.ddbc_bindings.h: 59.9%
mssql_python.pybind.logger_bridge.hpp: 70.8%
mssql_python.pybind.ddbc_bindings.cpp: 76.2%
mssql_python.__init__.py: 77.3%
mssql_python.row.py: 77.6%
mssql_python.ddbc_bindings.py: 79.6%
mssql_python.pybind.connection.connection_pool.cpp: 82.7%
mssql_python.pybind.connection.connection.cpp: 83.7%
mssql_python.connection.py: 84.7%

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[Copilot]

Pull request overview

This PR improves Arrow streaming ergonomics by making Cursor.arrow_reader() return a RecordBatchReader-compatible wrapper whose .close() actually cancels in-flight fetches and releases server-side ODBC cursor resources, keeping the parent Cursor reusable.

Changes:

• Introduces _ArrowReader in cursor.py and updates Cursor.arrow_reader() to return it instead of a raw pyarrow.RecordBatchReader.
• Adds an ODBC SQLCancel binding and exposes it via SqlHandle._cancel() to support cross-thread cancellation during .close().
• Extends Arrow reader tests to validate close semantics, context-manager behavior, GC cleanup, and cross-thread cancellation.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
mssql_python/cursor.py	Adds _ArrowReader and reworks arrow_reader() to drive robust cleanup/cancellation and cursor state reset.
mssql_python/pybind/ddbc_bindings.h	Declares the SQLCancel function pointer type and adds SqlHandle::cancel() API surface.
mssql_python/pybind/ddbc_bindings.cpp	Loads SQLCancel, implements SqlHandle::cancel() (releasing the GIL), and exposes _cancel to Python.
tests/test_004_cursor_arrow.py	Updates/expands tests to cover new reader wrapper behavior and resource release semantics.

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[ffelixg]

Huh, I didn't know that arrow readers supported a close method! I think that _ArrowReader should be subclassing pyarrow.RecordBatchReader. That way, we won't have to worry about duck typing. It also might save on some implementation overhead. Subrata (@subrata-ms)

[Subrata (subrata-ms)]

Huh, I didn't know that arrow readers supported a close method! I think that _ArrowReader should be subclassing pyarrow.RecordBatchReader. That way, we won't have to worry about duck typing. It also might save on some implementation overhead. Subrata (@subrata-ms)

Thanks for the suggestion — I looked into it and unfortunately subclassing doesn't work cleanly here because pyarrow.RecordBatchReader is a Cython extension type, not a normal Python class. Specifically:

RecordBatchReader.from_batches(...) is a Cython factory that hard-codes the return type to the base class; Sub.from_batches(...) returns a plain RecordBatchReader, so overridden close() / read_next_batch() are never called.
class reassignment is rejected on Cython types (TypeError: class assignment only supported for mutable types or ModuleType subclasses), so downcasting a factory-produced reader isn't an option either.
A Cython instance also rejects arbitrary attributes, so self._cursor, self._generator, etc. can't live on the base instance directly.
The bare Sub.new(Sub) path leaves the underlying C stream unset — super().close() segfaults immediately (I reproduced it locally on pyarrow 24.0.0).
So there's no path where the object is simultaneously (a) an isinstance of pa.RecordBatchReader, (b) able to intercept close() for our SQLCancel + generator teardown, and (c) able to carry our wrapper state.

The current composition/duck-typing shape is intentional for that reason. Happy to add arrow_c_stream to the wrapper so consumers that follow the Arrow PyCapsule Protocol (polars, duckdb, RecordBatchReader.from_stream, etc.) can consume it without any isinstance check — that gives us the "no duck-typing worries" property without the subclass constraints.

[Gaurav Sharma (bewithgaurav)]

suggesting a better impl of the wrapper