From f4dd4402108cc005d45acd4ca83c8530c36a93ca Mon Sep 17 00:00:00 2001
From: Sergey B Kirpichev <skirpichev@gmail.com>
Date: Tue, 17 Sep 2024 20:16:15 +0300
Subject: [PATCH] gh-123836: workaround fmod(x, y) bug on Windows (#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 result
should have the same sign as x.

This amends commit 28aea5d07d.
---
 .../2024-09-17-18-06-42.gh-issue-124171.PHCvRJ.rst       | 3 +++
 Modules/mathmodule.c                                     | 9 +++++++++
 2 files changed, 12 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2024-09-17-18-06-42.gh-issue-124171.PHCvRJ.rst

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
new file mode 100644
index 00000000000..c2f0bb14f55
--- /dev/null
+++ 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
index b7eb745177f..baf2dc439b8 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -2348,6 +2348,15 @@ 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 (isnan(r)) {
         if (!isnan(x) && !isnan(y))
             errno = EDOM;