bpo-44954: Fix wrong result in float.fromhex corner case (GH-27834)

(cherry picked from commit 60b93d9)

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>

Author: miss-islington

Lib/test/test_float.py

@@ -1468,6 +1468,20 @@ def test_from_hex(self):
         self.identical(fromHex('0X1.0000000000001fp0'), 1.0+2*EPS)
         self.identical(fromHex('0x1.00000000000020p0'), 1.0+2*EPS)
 
+        # Regression test for a corner-case bug reported in b.p.o. 44954
+        self.identical(fromHex('0x.8p-1074'), 0.0)
+        self.identical(fromHex('0x.80p-1074'), 0.0)
+        self.identical(fromHex('0x.81p-1074'), TINY)
+        self.identical(fromHex('0x8p-1078'), 0.0)
+        self.identical(fromHex('0x8.0p-1078'), 0.0)
+        self.identical(fromHex('0x8.1p-1078'), TINY)
+        self.identical(fromHex('0x80p-1082'), 0.0)
+        self.identical(fromHex('0x81p-1082'), TINY)
+        self.identical(fromHex('.8p-1074'), 0.0)
+        self.identical(fromHex('8p-1078'), 0.0)
+        self.identical(fromHex('-.8p-1074'), -0.0)
+        self.identical(fromHex('+8p-1078'), 0.0)
+
     def test_roundtrip(self):
         def roundtrip(x):
             return fromHex(toHex(x))

Misc/NEWS.d/next/Core and Builtins/2021-08-19-14-43-24.bpo-44954.dLn3lg.rst

@@ -0,0 +1,2 @@
+Fixed a corner case bug where the result of ``float.fromhex('0x.8p-1074')``
+was rounded the wrong way.

Objects/floatobject.c

@@ -1463,8 +1463,8 @@ float_fromhex(PyTypeObject *type, PyObject *string)
        bits lsb, lsb-2, lsb-3, lsb-4, ... is 1. */
     if ((digit & half_eps) != 0) {
         round_up = 0;
-        if ((digit & (3*half_eps-1)) != 0 ||
-            (half_eps == 8 && (HEX_DIGIT(key_digit+1) & 1) != 0))
+        if ((digit & (3*half_eps-1)) != 0 || (half_eps == 8 &&
+                key_digit+1 < ndigits && (HEX_DIGIT(key_digit+1) & 1) != 0))
             round_up = 1;
         else
             for (i = key_digit-1; i >= 0; i--)