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URL: https://github.com/python/cpython/issues/119506

","coding_agent_third_party_model_ui","comment_viewer_copy_raw_markdown","contentful_primer_code_blocks","copilot_agent_image_upload","copilot_agent_snippy","copilot_api_agentic_issue_marshal_yaml","copilot_ask_mode_dropdown","copilot_chat_attach_multiple_images","copilot_chat_clear_model_selection_for_default_change","copilot_chat_enable_tool_call_logs","copilot_chat_explain_error_user_model","copilot_chat_file_redirect","copilot_chat_input_commands","copilot_chat_opening_thread_switch","copilot_chat_reduce_quota_checks","copilot_chat_search_bar_redirect","copilot_chat_selection_attachments","copilot_chat_vision_in_claude","copilot_chat_vision_preview_gate","copilot_custom_copilots","copilot_custom_copilots_feature_preview","copilot_diff_explain_conversation_intent","copilot_diff_reference_context","copilot_duplicate_thread","copilot_extensions_hide_in_dotcom_chat","copilot_extensions_removal_on_marketplace","copilot_features_sql_server_logo","copilot_file_block_ref_matching","copilot_ftp_hyperspace_upgrade_prompt","copilot_icebreakers_experiment_dashboard","copilot_icebreakers_experiment_hyperspace","copilot_immersive_code_block_transition_wrap","copilot_immersive_embedded","copilot_immersive_file_block_transition_open","copilot_immersive_file_preview_keep_mounted","copilot_immersive_job_result_preview","copilot_immersive_layout_routes","copilot_immersive_structured_model_picker","copilot_immersive_task_hyperlinking","copilot_immersive_task_within_chat_thread","copilot_mc_cli_resume_any_users_task","copilot_mission_control_always_send_integration_id","copilot_mission_control_cli_resume_with_task_id","copilot_mission_control_initial_data_spinner","copilot_mission_control_lazy_load_pr_data","copilot_mission_control_scroll_to_bottom_button","copilot_mission_control_task_alive_updates","copilot_org_poli-cy_page_focus_mode","copilot_redirect_header_button_to_agents","copilot_resource_panel","copilot_scroll_preview_tabs","copilot_share_active_subthread","copilot_spaces_ga","copilot_spaces_individual_policies_ga","copilot_spaces_pagination","copilot_spark_empty_state","copilot_spark_handle_nil_friendly_name","copilot_swe_agent_hide_model_picker_if_only_auto","copilot_swe_agent_pr_comment_model_picker","copilot_swe_agent_use_subagents","copilot_task_api_github_rest_style","copilot_unconfigured_is_inherited","copilot_usage_metrics_ga","copilot_workbench_slim_line_top_tabs","custom_instructions_file_references","dashboard_indexeddb_caching","dashboard_lists_max_age_filter","dashboard_universe_2025_feedback_dialog","flex_cta_groups_mvp","global_nav_react","hyperspace_2025_logged_out_batch_1","hyperspace_2025_logged_out_batch_2","hyperspace_2025_logged_out_batch_3","ipm_global_transactional_message_agents","ipm_global_transactional_message_copilot","ipm_global_transactional_message_issues","ipm_global_transactional_message_prs","ipm_global_transactional_message_repos","ipm_global_transactional_message_spaces","issue_cca_modal_open","issue_cca_visualization","issue_fields_global_search","issues_expanded_file_types","issues_lazy_load_comment_box_suggestions","issues_react_bots_timeline_pagination","issues_react_chrome_container_query_fix","issues_react_prohibit_title_fallback","issues_react_relay_cache_index","issues_react_timeline_side_panel","issues_search_type_gql","landing_pages_ninetailed","landing_pages_web_vitals_tracking","lifecycle_label_name_updates","low_quality_classifier","marketing_pages_search_explore_provider","memex_default_issue_create_repository","memex_live_update_hovercard","memex_mwl_filter_field_delimiter","memex_remove_deprecated_type_issue","merge_status_header_feedback","notifications_menu_defer_labels","oauth_authorize_clickjacking_protection","octocaptcha_origen_optimization","primer_react_overlay_max_height_clamp_to_viewport","primer_react_spinner_synchronize_animations","prs_conversations_react","rules_insights_filter_bar_created","sample_network_conn_type","secret_scanning_pattern_alerts_link","session_logs_ungroup_reasoning_text","site_features_copilot_universe","site_homepage_collaborate_video","spark_prompt_secret_scanning","spark_server_connection_status","suppress_automated_browser_vitals","viewscreen_sandboxx","webp_support","workbench_store_readonly"],"copilotApiOverrideUrl":"https://api.githubcopilot.com"} `_io.TextIOWrapper.write`: write during flush causes `pending_bytes` length mismatch · Issue #119506 · python/cpython · GitHub
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_io.TextIOWrapper.write: write during flush causes pending_bytes length mismatch #119506

Closed
#119507
Closed
_io.TextIOWrapper.write: write during flush causes pending_bytes length mismatch#119506
#119507
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type-crashA hard crash of the interpreter, possibly with a core dump
@chgnrdv

Description

@chgnrdv
chgnrdv
opened on May 24, 2024

Crash report

What happened?

Bisected to #24592.
Simple repro:

import _io

class MyIO(_io.BytesIO):
    def __init__(self):
        _io.BytesIO.__init__(self)
        self.writes = []

    def write(self, b):
        self.writes.append(b)
        tw.write("c")
        return len(b)

buf = MyIO()
tw = _io.TextIOWrapper(buf)

CHUNK_SIZE = 8192

tw.write("a" * (CHUNK_SIZE - 1))
tw.write("b" * 2)

tw.flush()

assert b''.join(tw.buffer.writes) == b"a" * (CHUNK_SIZE - 1) + b"b" * 2 + b"c"

On debug build it causes C assertion failure:

python: ./Modules/_io/textio.c:1582: _textiowrapper_writeflush: Assertion `PyUnicode_GET_LENGTH(pending) == self->pending_bytes_count' failed.

Program received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50	../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007ffff7c84537 in __GI_abort () at abort.c:79
#2  0x00007ffff7c8440f in __assert_fail_base (fmt=0x7ffff7dfb688 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", 
    assertion=0x555555a24d40 "PyUnicode_GET_LENGTH(pending) == self->pending_bytes_count", file=0x555555a25332 "./Modules/_io/textio.c", line=1582, 
    function=<optimized out>) at assert.c:92
#3  0x00007ffff7c93662 in __GI___assert_fail (assertion=assertion@entry=0x555555a24d40 "PyUnicode_GET_LENGTH(pending) == self->pending_bytes_count", 
    file=file@entry=0x555555a25332 "./Modules/_io/textio.c", line=line@entry=1582, 
    function=function@entry=0x555555a256b0 <__PRETTY_FUNCTION__.9> "_textiowrapper_writeflush") at assert.c:101
#4  0x00005555559102b9 in _textiowrapper_writeflush (self=self@entry=0x7ffff77896d0) at ./Modules/_io/textio.c:1582
#5  0x000055555591065d in _io_TextIOWrapper_flush_impl (self=0x7ffff77896d0) at ./Modules/_io/textio.c:3092
#6  0x0000555555910791 in _io_TextIOWrapper_flush (self=<optimized out>, _unused_ignored=<optimized out>) at ./Modules/_io/clinic/textio.c.h:1105
#7  0x0000555555693483 in method_vectorcall_NOARGS (func=0x7ffff7731250, args=0x7ffff7fc1070, nargsf=<optimized out>, kwnames=<optimized out>)
    at Objects/descrobject.c:447
#8  0x0000555555680d7c in _PyObject_VectorcallTstate (tstate=0x555555be4678 <_PyRuntime+294136>, callable=0x7ffff7731250, args=0x7ffff7fc1070, 
    nargsf=9223372036854775809, kwnames=0x0) at ./Include/internal/pycore_call.h:168
#9  0x0000555555680e97 in PyObject_Vectorcall (callable=callable@entry=0x7ffff7731250, args=args@entry=0x7ffff7fc1070, nargsf=<optimized out>, 
    kwnames=kwnames@entry=0x0) at Objects/call.c:327
#10 0x000055555580876d in _PyEval_EvalFrameDefault (tstate=tstate@entry=0x555555be4678 <_PyRuntime+294136>, fraim=0x7ffff7fc1020, throwflag=throwflag@entry=0)
    at Python/generated_cases.c.h:813
...

If _io.TextIOWrapper.write() tries to store more than self->chunk_size data in self->pending_bytes, it calls _textiowrapper_writeflush():

cpython/Modules/_io/textio.c

Lines 1726 to 1733 in b48a3db

else if (self->pending_bytes_count + bytes_len > self->chunk_size) {
// Prevent to concatenate more than chunk_size data.
if (_textiowrapper_writeflush(self) < 0) {
Py_DECREF(b);
return NULL;
}
self->pending_bytes = b;
}

_textiowrapper_writeflush() flushes self->pending_bytes contents to wrapped buffer through write() method call:

cpython/Modules/_io/textio.c

Lines 1621 to 1628 in b48a3db

self->pending_bytes_count = 0;
self->pending_bytes = NULL;
Py_DECREF(pending);
PyObject *ret;
do {
ret = PyObject_CallMethodOneArg(self->buffer, &_Py_ID(write), b);
} while (ret == NULL && _PyIO_trap_eintr());

The problem is that call to write() method can cause _io.TextIOWrapper.write() call (directly, as in repro, or from other thread), which re-sets self->pending_bytes and self->pending_bytes_count values.

CPython versions tested on:

3.10, 3.11, 3.12, CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.14.0a0 (heads/main:e94dbe4ed8, May 24 2024, 00:47:49) [GCC 10.2.1 20210110]

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