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()).

Modules/_decimal/_decimal.c

@@ -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_digits);
     return PyLongWriter_Finish(writer);
 }