PySide6 6.11.1: QWebEngineUrlRequestJob.requestBody() causes SIGSEGV for POST requests to custom URL schemes

PyXiion

PySide6 QWebEngineUrlRequestJob.requestBody() causes SIGSEGV on POST requests to custom URL schemes

The crash happens immediately inside requestBody(), before accessing the returned object.

MRE

from __future__ import annotations

import sys

from PySide6 import __version__ as pyside6_ver
from PySide6.QtCore import QByteArray, QBuffer, QIODevice, QTimer, QUrl, qVersion
from PySide6.QtWebEngineCore import (
    QWebEngineUrlScheme,
    QWebEngineUrlSchemeHandler,
    QWebEngineUrlRequestJob,
    QWebEngineProfile,
)

print(f"Python: {sys.version}", flush=True)
print(f"PySide6: {pyside6_ver}", flush=True)
print(f"Qt: {qVersion()}", flush=True)

scheme = QWebEngineUrlScheme(b"mre")
scheme.setFlags(
    QWebEngineUrlScheme.Flag.SecureScheme
    | QWebEngineUrlScheme.Flag.ContentSecurityPolicyIgnored
    | QWebEngineUrlScheme.Flag.FetchApiAllowed
    | QWebEngineUrlScheme.Flag.CorsEnabled
)
scheme.setSyntax(QWebEngineUrlScheme.Syntax.Host)
QWebEngineUrlScheme.registerScheme(scheme)

from PySide6.QtWidgets import QApplication
from PySide6.QtWebEngineWidgets import QWebEngineView


class Handler(QWebEngineUrlSchemeHandler):
    def requestStarted(self, job: QWebEngineUrlRequestJob) -> None:
        print(
            f"{job.requestMethod().data().decode()} {job.requestUrl().toString()}"
        )

        print("before requestBody()", flush=True)
        job.requestBody()
        print("after requestBody()", flush=True)

        buf = QBuffer()
        buf.setData(b'{"ok":true}')
        buf.open(QIODevice.OpenModeFlag.ReadOnly)
        buf.setParent(job)
        job.reply(b"application/json", buf)


app = QApplication(sys.argv)

handler = Handler()
QWebEngineProfile.defaultProfile().installUrlSchemeHandler(
    QByteArray(b"mre"), handler
)

view = QWebEngineView()
view.setHtml(
    """<!DOCTYPE html>
<html><body>
<p id="status">waiting...</p>
<script>
async function run() {
  try {

    document.getElementById('status').innerText = 'POST...';
    r = await fetch('mre://test', {method: 'POST', body: '123'});
    console.log('POST OK', await r.json());
    
    //document.getElementById('status').innerText = 'GET...';
    //let r = await fetch('mre://test', {method: 'GET'});
    //console.log('GET OK', await r.json());

    document.getElementById('status').innerText = 'Both OK';
  } catch(e) {
    document.getElementById('status').innerText = e.toString();
  }
}
setTimeout(run, 1500);
</script>
</body></html>""",
    baseUrl=QUrl("https://example.com/"),
)
view.resize(600, 400)
view.show()

print("ready", flush=True)
QTimer.singleShot(10000, app.quit)
sys.exit(app.exec())

/usr/bin/uv run /home/pyxiion/Projects/PxModRim/.venv/bin/python /home/pyxiion/Projects/PxModRim/mre_post_crash.py 
Python: 3.12.13 (main, Jun 11 2026, 04:03:26) [Clang 22.1.3 ]
PySide6: 6.11.1
Qt: 6.11.1
ready
POST mre://test/
before requestBody()

Process finished with exit code 139 (interrupted by signal 11:SIGSEGV)

PyQT

"""MRE: PyQt6 QWebEngineUrlSchemeHandler POST requestBody() crash"""
from __future__ import annotations

import sys

from PyQt6.QtCore import QByteArray, QBuffer, QIODevice, QTimer, QUrl, qVersion
from PyQt6.QtWebEngineCore import (
    QWebEngineUrlScheme,
    QWebEngineUrlSchemeHandler,
    QWebEngineUrlRequestJob,
    QWebEngineProfile,
)
from PyQt6 import QtCore

print(f"PyQt6: {QtCore.PYQT_VERSION_STR}", flush=True)
print(f"Qt: {qVersion()}", flush=True)

# Register scheme before QApplication
scheme = QWebEngineUrlScheme(b"mre")
scheme.setFlags(
    QWebEngineUrlScheme.Flag.SecureScheme
    | QWebEngineUrlScheme.Flag.ContentSecurityPolicyIgnored
    | QWebEngineUrlScheme.Flag.FetchApiAllowed
    | QWebEngineUrlScheme.Flag.CorsEnabled
)
scheme.setSyntax(QWebEngineUrlScheme.Syntax.Host)
QWebEngineUrlScheme.registerScheme(scheme)

from PyQt6.QtWidgets import QApplication
from PyQt6.QtWebEngineWidgets import QWebEngineView


class Handler(QWebEngineUrlSchemeHandler):
    def requestStarted(self, job: QWebEngineUrlRequestJob) -> None:
        print(
            f"{job.requestMethod().data().decode()} {job.requestUrl().toString()}",
            file=sys.stderr, flush=True,
        )
        print("before requestBody()", file=sys.stderr, flush=True)
        body = job.requestBody()
        print("after requestBody()", file=sys.stderr, flush=True)

        if body is not None:
            print(f"  body obj: {body}", file=sys.stderr, flush=True)
            print(f"  isOpen: {body.isOpen()}", file=sys.stderr, flush=True)
            if not body.isOpen():
                body.open(QIODevice.OpenModeFlag.ReadOnly)
            raw = body.readAll().data()
            print(f"  body ({len(raw)} bytes): {raw[:200]}", file=sys.stderr, flush=True)

        buf = QBuffer()
        buf.setData(b'{"ok":true}')
        buf.open(QIODevice.OpenModeFlag.ReadOnly)
        buf.setParent(job)
        job.reply(b"application/json", buf)


app = QApplication(sys.argv)

handler = Handler()
QWebEngineProfile.defaultProfile().installUrlSchemeHandler(
    QByteArray(b"mre"), handler
)

view = QWebEngineView()
view.setHtml(
    """<!DOCTYPE html>
<html><body>
<p id="status">waiting...</p>
<script>
async function run() {
  try {
    document.getElementById('status').innerText = 'GET...';
    let r = await fetch('mre://test', {method: 'GET'});
    console.log('GET OK', await r.json());

    document.getElementById('status').innerText = 'POST (empty body)...';
    r = await fetch('mre://test', {method: 'POST'});
    console.log('POST OK', await r.json());

    document.getElementById('status').innerText = 'POST (with body)...';
    r = await fetch('mre://test', {method: 'POST', body: JSON.stringify({hello:'world'})});
    console.log('POST body OK', await r.json());

    document.getElementById('status').innerText = 'All OK ✓';
  } catch(e) {
    document.getElementById('status').innerText = e.toString();
  }
}
setTimeout(run, 1500);
</script>
</body></html>""",
    baseUrl=QUrl("https://example.com/"),
)
view.resize(600, 400)
view.show()

print("ready", file=sys.stderr, flush=True)
QTimer.singleShot(10000, app.quit)
sys.exit(app.exec())

/usr/bin/uv run /home/pyxiion/Projects/RimSort/.venv/bin/python /home/pyxiion/Projects/RimSort/mre_post_crash_pyqt6.py 
PyQt6: 6.11.0
Qt: 6.11.1
ready
GET mre://test/
before requestBody()
after requestBody()
  body obj: <PyQt6.QtCore.QIODevice object at 0x7f251b2b6490>
  isOpen: False
  body (0 bytes): b''
POST mre://test/
before requestBody()
after requestBody()
  body obj: <PyQt6.QtCore.QIODevice object at 0x7f251b2b6530>
  isOpen: False
  body (0 bytes): b''
POST mre://test/
before requestBody()
after requestBody()
  body obj: <PyQt6.QtCore.QIODevice object at 0x7f251b2b6490>
  isOpen: False
  body (17 bytes): b'{"hello":"world"}'

Process finished with exit code 0

GDB bt

(gdb) bt
#0  0x00007ffff73723ac in ?? () from /home/pyxiion/Projects/PxModRim/.venv/lib/python3.12/site-packages/PySide6/libpyside6.abi3.so.6.11
#1  0x00007ffff73730b8 in PySide::getWrapperForQObject(QObject*, _typeobject*) () from /home/pyxiion/Projects/PxModRim/.venv/lib/python3.12/site-packages/PySide6/libpyside6.abi3.so.6.11
#2  0x00007ffff618f22c in ?? () from /home/pyxiion/Projects/PxModRim/.venv/lib/python3.12/site-packages/PySide6/QtWebEngineCore.abi3.so
#3  0x00000000018106c3 in cfunction_vectorcall_NOARGS.llvm.10372914493467373164 ()
#4  0x00000000018154f3 in _PyEval_EvalFrameDefault ()
#5  0x000000000181199a in method_vectorcall.llvm ()
#6  0x00007ffff6194c50 in ?? () from /home/pyxiion/Projects/PxModRim/.venv/lib/python3.12/site-packages/PySide6/QtWebEngineCore.abi3.so
#7  0x00007ffff6194d0a in ?? () from /home/pyxiion/Projects/PxModRim/.venv/lib/python3.12/site-packages/PySide6/QtWebEngineCore.abi3.so
#8  0x00007fffe8087dae in ?? () from /home/pyxiion/Projects/PxModRim/.venv/lib/python3.12/site-packages/PySide6/Qt/lib/libQt6WebEngineCore.so.6

PyXiion

Same crash for 6.10.1, 6.10.3, 6.11.0