gh-120608: Make reversed thread-safe by eendebakpt · Pull Request #120609 · python/cpython · GitHub
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eendebakpt wants to merge 4 commits into from
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gh-120608: Make reversed thread-safe #120609

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f5d81ba
Make reversed thread-safe
eendebakpt Jun 16, 2024
40018a1
Add Py_EXIT_CRITICAL_SECTION
eendebakpt Jun 16, 2024
726f6ca
📜🤖 Added by blurb_it.
blurb-it[bot] Jun 16, 2024
8c82d5e
Update Misc/NEWS.d/next/Core and Builtins/2024-06-16-22-42-47.gh-issu…
eendebakpt Jun 17, 2024
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3 changes: 3 additions & 0 deletions Include/internal/pycore_critical_section.h
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2024-06-16-22-42-47.gh-issue-120608._-YtWX.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Make :func:`reversed` thread-safe.
5 changes: 5 additions & 0 deletions Objects/enumobject.c
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Expand Up @@ -2,6 +2,7 @@

#include "Python.h"
#include "pycore_call.h" // _PyObject_CallNoArgs()
#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION()
#include "pycore_long.h" // _PyLong_GetOne()
#include "pycore_modsupport.h" // _PyArg_NoKwnames()
#include "pycore_object.h" // _PyObject_GC_TRACK()
Expand Down Expand Up @@ -431,16 +432,20 @@ reversed_next(reversedobject *ro)
PyObject *item;
Py_ssize_t index = ro->index;

Py_BEGIN_CRITICAL_SECTION(ro);
if (index >= 0) {
item = PySequence_GetItem(ro->seq, index);
if (item != NULL) {
ro->index--;

@corona10 corona10 Jun 18, 2024

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We don't declare the critical section for the list_next

cpython/Objects/listobject.c

Lines 3890 to 3911 in dacc5ac

Why do we have to declare the critical section here?
And what about just handling as the out-of-bound,
and if the ro->index needs to be decremented, why not just use atomic operation in here?

cc @colesbury

@eendebakpt eendebakpt Jun 18, 2024

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The check index >= 0 and decrement ro->index-- need to happen atomically. We could use the atomic _Py_atomic_compare_exchange for this, but it would increase code complexity. If performance is a concern, I can update the PR with this approach. The PySequence_GetItem is not inside the critical section, but I assumed it to be (or to be made later) thread safe

Py_EXIT_CRITICAL_SECTION();
return item;
}
if (PyErr_ExceptionMatches(PyExc_IndexError) ||
PyErr_ExceptionMatches(PyExc_StopIteration))
PyErr_Clear();
}
Py_END_CRITICAL_SECTION();

ro->index = -1;
Py_CLEAR(ro->seq);
return NULL;
Expand Down