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gh-85393: Handle partial and blocking .write in TextIOWrapper#134467

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,3 @@
Improve behavior of :meth:`!io.TextIOWrapper.write` when there is a partial
write caused by underlying buffer being :class:`io.RawIOBase` or if there is a
:exc:`BlockingIOError`
193 changes: 180 additions & 13 deletionsModules/_io/textio.c
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Original file line numberDiff line numberDiff line change
Expand Up@@ -1573,17 +1573,72 @@ _io_TextIOWrapper_detach_impl(textio *self)
return buffer;
}

/* Prepend bytes to pending_bytes.

At the front because other bytes were added via TextIOWrapper.write
during buffer.write (see: gh-119506, gh-118138). */
static int
_textiowrapper_prepend(textio *self, PyObject *unwritten)
{
assert(PyBytes_CheckExact(unwritten));

/* Ensure pending_bytes and pending_bytes_count are protected. */
_Py_AssertHoldsTstate();

if (self->pending_bytes == NULL) {
self->pending_bytes = unwritten;
assert(self->pending_bytes_count == 0);
self->pending_bytes_count += PyBytes_GET_SIZE(unwritten);
}
else if (!PyList_CheckExact(self->pending_bytes)) {
PyObject *list = PyList_New(2);
if (list == NULL) {
Py_DECREF(unwritten);
return -1;
}
// Since Python 3.12, allocating GC object won't trigger GC and release
// GIL. See https://github.com/python/cpython/issues/97922
assert(!PyList_CheckExact(self->pending_bytes));
PyList_SET_ITEM(list, 0, unwritten);
PyList_SET_ITEM(list, 1, self->pending_bytes);
self->pending_bytes = list;
self->pending_bytes_count += PyBytes_GET_SIZE(unwritten);
}
else {
PyList_Insert(self->pending_bytes, 0, unwritten);
self->pending_bytes_count += PyBytes_GET_SIZE(unwritten);
Py_DECREF(unwritten);
}
return 0;
}

/* Flush the internal write buffer. This doesn't explicitly flush the
underlying buffered object, though. */
static int
_textiowrapper_writeflush(textio *self)
{
/* Validate have exclusive access to members.

NOTE: this code relies on GIL / @critical_section. It needs to ensure
state is "safe" before/after calls to other Python code.

In particular during calls to `buffer.write` self->pending_bytes may be
modified by other threads.

At the end of this function either pending_bytes needs to be NULL _or_
there needs to be an exception.

For more details see: gh-119506, gh-118138. */
_Py_AssertHoldsTstate();

if (self->pending_bytes == NULL)
return 0;

PyObject *pending = self->pending_bytes;
Py_ssize_t bytes_to_write = self->pending_bytes_count;
PyObject *b;

/* Make pending_bytes into a contiguous bytes. */
if (PyBytes_Check(pending)) {
b = Py_NewRef(pending);
}
Expand DownExpand Up@@ -1628,20 +1683,133 @@ _textiowrapper_writeflush(textio *self)
assert(pos == self->pending_bytes_count);
}

/* pending_bytes is now owned by this function. */
self->pending_bytes_count = 0;
self->pending_bytes = NULL;
Py_DECREF(pending);

PyObject *ret;
do {
/* Continue until all data written or an error occurs. */
while (1) {
/* note: gh-119506 self->pending_bytes and self->pending_bytes_count
may be altered during this call. */
ret = PyObject_CallMethodOneArg(self->buffer, &_Py_ID(write), b);
} while (ret == NULL && _PyIO_trap_eintr());

/* Validate have exclusive access to members. */
_Py_AssertHoldsTstate();

if (ret == NULL) {
/* PEP 475 Retry on EINTR */
if (_PyIO_trap_eintr()) {
continue;
}

/* Try to find out how many bytes were written before exception. */
PyObject *exc = PyErr_GetRaisedException();
/* If it's a BlockingIOError, use the bytes written. */
if (PyErr_GivenExceptionMatches(exc, PyExc_BlockingIOError)) {
PyOSErrorObject *err = (PyOSErrorObject *)exc;
bytes_to_write -= err->written;
if (bytes_to_write) {
pending = PyBytes_FromStringAndSize(
PyBytes_AS_STRING(b) + err->written,
bytes_to_write);
if (pending) {
/* _textiowrapper can raise an exception if it fails to
allocate a list, that just adds to active error
list. Already exiting as fast as possible. */
_textiowrapper_prepend(self, pending);
}
else {
PyErr_SetString(PyExc_MemoryError,
"lost unwritten bytes during partial write");

}
}
}
/* XXX: For other exceptions this assumes all data was written. */
/* Re-raise exceptions. */
PyErr_SetRaisedException(exc);
Py_DECREF(b); /* note: ret is NULL / no need to decref */
return -1;
}

/* Try to determine bytes written from return value

XXX: Unexpected return: match previous behavior assume all data was
written. */
/* OPEN QUESTION: None is common in CPython, should this warn? */
if (ret == Py_None) {
Py_DECREF(b);
Py_DECREF(ret);
return 0;
}

/* Should be an integer. */
if (!_PyIndex_Check(ret)) {
PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
"write returned '%s' not specified by BufferedIOBase or"
" TextIOBase, typically count of bytes written",
ret);
Py_DECREF(b);
Py_DECREF(ret);
return 0;
}
Py_ssize_t size = PyNumber_AsSsize_t(ret, PyExc_OverflowError);

/* Check for unexpected return values. */
/* Can't get out size. */
if (size == -1 && PyErr_Occurred()) {
/* Warn about the value, but do not error. */
PyObject *exc = PyErr_GetRaisedException();
PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
"write returned value '%s' not specified by"
" BufferedIOBase or TextIOBase (%s)", ret, exc);
Py_DECREF(exc);
Py_DECREF(b);
Py_DECREF(ret);
return 0;
}
/* Negative count of bytes doesn't make sense. */
else if (size < 0) {
PyErr_WarnFormat(PyExc_RuntimeWarning, 1,
"write returned negative count of bytes '%s'", ret);
Py_DECREF(b);
Py_DECREF(ret);
return 0;
}
/* More written than passed in call. */
else if (size > bytes_to_write) {
PyErr_WarnFormat(PyExc_RuntimeWarning, 1,
"write returned '%s' bytes written but was only called"
" with '%s' bytes to write", ret, bytes_to_write);
Py_DECREF(b);
Py_DECREF(ret);
return 0;
}

/* Wrote data! */
bytes_to_write -= size;
assert(bytes_to_write >= 0);

/* Avoid a new byte allocation if fully written. */
if (bytes_to_write == 0) {
Py_DECREF(ret);
break;
}

/* Make a new PyBytes to keep type for next call to write. */
pending = PyBytes_FromStringAndSize(
PyBytes_AS_STRING(b) + size,
bytes_to_write);
Py_DECREF(b);
b = pending;
pending = NULL;
}

/* All data owned by this function is written. Other bytes could have shown
up while running. */
Py_DECREF(b);
// NOTE: We cleared buffer but we don't know how many bytes are actually written
// when an error occurred.
if (ret == NULL)
return -1;
Py_DECREF(ret);
return 0;
}

Expand DownExpand Up@@ -1733,12 +1901,11 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
// Flush the buffer before adding b to the buffer if b is not small.
// https://github.com/python/cpython/issues/87426
if (bytes_len >= self->chunk_size) {
// _textiowrapper_writeflush() calls buffer.write().
// self->pending_bytes can be appended during buffer->write()
// or other thread.
// We need to loop until buffer becomes empty.
// https://github.com/python/cpython/issues/118138
// https://github.com/python/cpython/issues/119506
/* writeflush ensures one pending_bytes is written.

Additional data may be written to the TextIO when the lock is
released while calling buffer.write (and show up in pending_bytes).
When that happens, flush again to avoid copying the current bytes. */
while (self->pending_bytes != NULL) {
if (_textiowrapper_writeflush(self) < 0) {
Py_DECREF(b);
Expand Down
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