[3.13] gh-123836: workaround fmod(x, y) bug on Windows (GH-124171)

Buildbot failure on Windows 10 with MSC v.1916 64 bit (AMD64): FAIL: testFmod (test.test_math.MathTests.testFmod) ---------------------------------------------------------------------- Traceback (most recent call last): File "D:\buildarea\3.x.bolen-windows10\build\Lib\test\test_math.py", line 605, in testFmod self.ftest('fmod(-10, 1)', math.fmod(-10, 1), -0.0) ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "D:\buildarea\3.x.bolen-windows10\build\Lib\test\test_math.py", line 258, in ftest self.fail("{}: {}".format(name, failure)) ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ AssertionError: fmod(-10, 1): expected -0.0, got 0.0 (zero has wrong sign) Here Windows loose sign of the result; if y is nonzero, the 8000 result should have the same sign as x. This amends commit 28aea5d. (cherry picked from commit f4dd440) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
python · Yhg1s · Sep 30, 2024 · Sep 17, 2024 · Sep 17, 2024 · e5f6a2be7a35c57139e2c4145d9425bffd725cec
commit e5f6a2be7a35c57139e2c4145d9425bffd725cec
diff --git a/Misc/NEWS.d/next/Library/2024-09-17-18-06-42.gh-issue-124171.PHCvRJ.rst b/Misc/NEWS.d/next/Library/2024-09-17-18-06-42.gh-issue-124171.PHCvRJ.rst
@@ -0,0 +1,3 @@
+Add workaround for broken :c:func:`!fmod()` implementations on Windows, that
+loose zero sign (e.g. ``fmod(-10, 1)`` returns ``0.0``).  Patch by Sergey B
+Kirpichev.
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
@@ -2385,6 +2385,15 @@ math_fmod_impl(PyObject *module, double x, double y)
         return PyFloat_FromDouble(x);
     errno = 0;
     r = fmod(x, y);
+#ifdef _MSC_VER
+    /* Windows (e.g. Windows 10 with MSC v.1916) loose sign
+       for zero result.  But C99+ says: "if y is nonzero, the result
+       has the same sign as x".
+     */
+    if (r == 0.0 && y != 0.0) {
+        r = copysign(r, x);
+    }
+#endif
     if (Py_IS_NAN(r)) {
         if (!Py_IS_NAN(x) && !Py_IS_NAN(y))
             errno = EDOM;