Fix a parser bug when parsing "num/denom" strings with numbers close to 'sys.maxsize'.

When the parser detected that the numerator is getting too large to fit safely into a C integer,, it continues parsing digits for the denominator without clearing the digit collection buffer, thus effectively contatenating numerator and denominator into the parsed denominator.

Author: scoder

CHANGES.rst

@@ -1,6 +1,14 @@
 ChangeLog
 =========
 
+1.21 (2025-06-13)
+-----------------
+
+* A serious parser bug could accidentally concatenate numerator and denominator
+  as final denominator when parsing "x/y" where x or y are close to ``sys.maxsize``,
+  thus returning a ``Fraction("x/xy")``.
+
+
 1.20 (2025-06-13)
 -----------------
 

src/quicktions.pyx

@@ -1790,6 +1790,8 @@ cdef tuple _parse_fraction(AnyString s, Py_ssize_t s_len, orig_str):
                 break
             pos += 1
 
+        # Start fresh digits collection.
+        c_digits = c_digits_start
         while pos < s_len:
             c = _char_at(s_data, s_kind, pos) if AnyString is unicode else cdata[pos]
             pos += 1

src/test_fractions.py

@@ -1975,7 +1975,11 @@ def test_large_values(self):
             (63, 64),
             (64, 70),
             (128, 511),
-        ]]
+        ]] + ["%s/%s" % (a, b) for a, b in itertools.permutations([
+            sys.maxsize // n + i
+            for i in range(-3, +3)
+            for n in (1, 2, 4, 10, 100)
+        ], 2)]
         values = [
             v for value in values
             for v in ([value, value.replace('/', '.'), value.replace('/', '.') + "E" + value[:4]]