8000 gh-129275: avoid temporary buffer in dec_as_long() by skirpichev · Pull Request #129630 · python/cpython · GitHub
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gh-129275: avoid temporary buffer in dec_as_long() #129630

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gh-129275: avoid temporary buffer in dec_as_long() #129630

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gh-129275: avoid temporary buffer in dec_as_long() 
According to the documentation: "If rdata is non-NULL, it MUST be
allocated by one of libmpdec’s allocation functions and rlen MUST be
correct.  If necessary, the function will resize rdata.  Resizing is
slow and should not occur if rlen has been obtained by a call to
mpd_sizeinbase."

So, possible resizing in mpd_qexport_u32/16() is for guarding against
broken log10() implementations (log10 is used in the mpd_sizeinbase()).
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@skirpichev
skirpichev committed Feb 4, 2025
commit 311f1cef7281d44d717b886565647034382b33be
36 changes: 17 additions & 19 deletions Modules/_decimal/_decimal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3698,39 +3698,37 @@ dec_as_long(PyObject *dec, PyObject *context, int round)
assert(layout->digit_size == 2 || layout->digit_size == 4);

uint32_t base = (uint32_t)1 << layout->bits_per_digit;
/* We use a temporary buffer for digits for now, as for nonzero rdata
mpd_qexport_u32/u16() require either space "allocated by one of
libmpdec’s allocation functions" or "rlen MUST be correct" (to avoid
reallocation). This can be further optimized by using rlen from
mpd_sizeinbase(). See gh-127925. */
void *tmp_digits = NULL;
size_t n;
size_t n, len = mpd_sizeinbase(x, base);
void *digits;
PyLongWriter *writer = PyLongWriter_Create(mpd_isnegative(x), len, &digits);

if (writer == NULL) {
mpd_del(x);
return NULL;
}

status = 0;
/* The mpd_qexport_*() functions used here assume that no resizing occurs,
that is, 'len' was obtained via mpd_sizeinbase(). We also fill 'digits'
with zeros first since 'len' can be overestimated. */
if (layout->digit_size == 4) {
n = mpd_qexport_u32((uint32_t **)&tmp_digits, 0, base, x, &status);
memset(digits, 0, 4*len);
n = mpd_qexport_u32((uint32_t **)&digits, len, base, x, &status);
}
else {
n = mpd_qexport_u16((uint16_t **)&tmp_digits, 0, base, x, &status);
memset(digits, 0, 2*len);
n = mpd_qexport_u16((uint16_t **)&digits, len, base, x, &status);
}

if (n == SIZE_MAX) {
PyErr_NoMemory();
PyLongWriter_Discard(writer);
mpd_del(x);
mpd_free(tmp_digits);
return NULL;
}

void *digits;
PyLongWriter *writer = PyLongWriter_Create(mpd_isnegative(x), n, &digits);

assert(n <= len);
mpd_del(x);
if (writer == NULL) {
mpd_free(tmp_digits);
return NULL;
}
memcpy(digits, tmp_digits, layout->digit_size*n);
mpd_free(tmp 5137 _digits);
return PyLongWriter_Finish(writer);
}

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