[3.10] gh-97786: Fix compiler warnings in pytime.c (GH-101826) (#102150)

* [3.10] gh-97786: Fix compiler warnings in pytime.c (GH-101826)

Fixes compiler warnings in pytime.c..
(cherry picked from commit b1b375e)

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>

* Add comment about the casts

---------

Co-authored-by: Gregory P. Smith <greg@krypto.org>

Author: mdickinson

Diff for: Misc/NEWS.d/next/Library/2023-02-11-13-23-29.gh-issue-97786.QjvQ1B.rst

@@ -0,0 +1,2 @@
+Fix potential undefined behaviour in corner cases of floating-point-to-time
+conversions.

Diff for: Python/pytime.c

@@ -34,6 +34,25 @@
 #define NS_TO_MS (1000 * 1000)
 #define NS_TO_US (1000)
 
+#if SIZEOF_TIME_T == SIZEOF_LONG_LONG
+#  define PY_TIME_T_MAX LLONG_MAX
+#  define PY_TIME_T_MIN LLONG_MIN
+#elif SIZEOF_TIME_T == SIZEOF_LONG
+#  define PY_TIME_T_MAX LONG_MAX
+#  define PY_TIME_T_MIN LONG_MIN
+#else
+#  error "unsupported time_t size"
+#endif
+
+#if PY_TIME_T_MAX + PY_TIME_T_MIN != -1
+#  error "time_t is not a two's complement integer type"
+#endif
+
+#if _PyTime_MIN + _PyTime_MAX != -1
+#  error "_PyTime_t is not a two's complement integer type"
+#endif
+
+
 static void
 error_time_t_overflow(void)
 {
@@ -157,7 +176,21 @@ _PyTime_DoubleToDenominator(double d, time_t *sec, long *numerator,
     }
     assert(0.0 <= floatpart && floatpart < denominator);
 
-    if (!_Py_InIntegralTypeRange(time_t, intpart)) {
+    /*
+       Conversion of an out-of-range value to time_t gives undefined behaviour
+       (C99 §6.3.1.4p1), so we must guard against it. However, checking that
+       `intpart` is in range is delicate: the obvious expression `intpart <=
+       PY_TIME_T_MAX` will first convert the value `PY_TIME_T_MAX` to a double,
+       potentially changing its value and leading to us failing to catch some
+       UB-inducing values. The code below works correctly under the mild
+       assumption that time_t is a two's complement integer type with no trap
+       representation, and that `PY_TIME_T_MIN` is within the representable
+       range of a C double.
+
+       Note: we want the `if` condition below to be true for NaNs; therefore,
+       resist any temptation to simplify by applying De Morgan's laws.
+    */
+    if (!((double)PY_TIME_T_MIN <= intpart && intpart < -(double)PY_TIME_T_MIN)) {
         error_time_t_overflow();
         return -1;
     }
@@ -210,7 +243,8 @@ _PyTime_ObjectToTime_t(PyObject *obj, time_t *sec, _PyTime_round_t round)
         d = _PyTime_Round(d, round);
         (void)modf(d, &intpart);
 
-        if (!_Py_InIntegralTypeRange(time_t, intpart)) {
+        /* See comments in _PyTime_DoubleToDenominator */
+        if (!((double)PY_TIME_T_MIN <= intpart && intpart < -(double)PY_TIME_T_MIN)) {
             error_time_t_overflow();
             return -1;
         }
@@ -395,7 +429,8 @@ _PyTime_FromDouble(_PyTime_t *t, double value, _PyTime_round_t round,
     d *= (double)unit_to_ns;
     d = _PyTime_Round(d, round);
 
-    if (!_Py_InIntegralTypeRange(_PyTime_t, d)) {
+    /* See comments in _PyTime_DoubleToDenominator */
+    if (!((double)_PyTime_MIN <= d && d < -(double)_PyTime_MIN)) {
         _PyTime_overflow();
         return -1;
     }
@@ -722,7 +757,9 @@ py_get_system_clock(_PyTime_t *tp, _Py_clock_info_t *info, int raise)
         info->monotonic = 0;
         info->adjustable = 1;
         if (clock_getres(CLOCK_REALTIME, &res) == 0) {
-            info->resolution = res.tv_sec + res.tv_nsec * 1e-9;
+            /* the explicit (double) casts silence loss-of-precision warnings
+               on some platforms */
+            info->resolution = (double)res.tv_sec + (double)res.tv_nsec * 1e-9;
         }
         else {
             info->resolution = 1e-9;