Fix calendar:system_time_to_universal_time/2 input validation and negative rounding

Validate argv[0] is an integer before calling term_maybe_unbox_int64 to
prevent interpreting arbitrary term data as an int64.

Use floor division instead of C truncation for all time-unit branches so
negative sub-second values round toward negative infinity, matching OTP
semantics. For example calendar:system_time_to_universal_time(-1,
millisecond) now correctly returns {{1969,12,31},{23,59,59}}.

Add tests for negative calendar conversions and non-integer input.

Signed-off-by: Peter M <petermm@gmail.com>

Author: petermm

src/libAtomVM/nifs.c

@@ -2055,46 +2055,39 @@ term nif_erlang_timestamp_0(Context *ctx, int argc, term argv[])
 
 term nif_calendar_system_time_to_universal_time_2(Context *ctx, int argc, term argv[])
 {
-    UNUSED(ctx);
     UNUSED(argc);
 
-    struct timespec ts;
+    if (UNLIKELY(!term_is_int64(argv[0]))) {
+        RAISE_ERROR(BADARG_ATOM);
+    }
     avm_int64_t value = term_maybe_unbox_int64(argv[0]);
 
+    avm_int64_t divisor;
     if (argv[1] == SECOND_ATOM) {
-        ts.tv_sec = (time_t) value;
-        ts.tv_nsec = 0;
-
+        divisor = 1;
     } else if (argv[1] == MILLISECOND_ATOM) {
-        ts.tv_sec = (time_t) (value / 1000);
-        ts.tv_nsec = (value % 1000) * 1000000;
-
+        divisor = 1000;
     } else if (argv[1] == MICROSECOND_ATOM) {
-        ts.tv_sec = (time_t) (value / 1000000);
-        ts.tv_nsec = (value % 1000000) * 1000;
-
+        divisor = INT64_C(1000000);
     } else if (argv[1] == NANOSECOND_ATOM || argv[1] == NATIVE_ATOM) {
-        ts.tv_sec = (time_t) (value / INT64_C(1000000000));
-        ts.tv_nsec = value % INT64_C(1000000000);
-
+        divisor = INT64_C(1000000000);
     } else if (term_is_int64(argv[1])) {
-        avm_int64_t parts_per_second = term_maybe_unbox_int64(argv[1]);
-        if (UNLIKELY(parts_per_second <= 0)) {
-            RAISE_ERROR(BADARG_ATOM);
-        }
-        if (UNLIKELY(!term_is_int64(argv[0]))) {
+        divisor = term_maybe_unbox_int64(argv[1]);
+        if (UNLIKELY(divisor <= 0)) {
             RAISE_ERROR(BADARG_ATOM);
         }
-        ts.tv_sec = (time_t) (value / parts_per_second);
-        if ((value % parts_per_second) < 0) {
-            ts.tv_sec -= 1;
-        }
-        ts.tv_nsec = 0;
-
     } else {
         RAISE_ERROR(BADARG_ATOM);
     }
 
+    // Floor division: round negative fractional seconds toward negative infinity
+    avm_int64_t quotient = value / divisor;
+    avm_int64_t remainder = value % divisor;
+    struct timespec ts = {
+        .tv_sec = (time_t) (quotient - (remainder < 0)),
+        .tv_nsec = 0
+    };
+
     struct tm broken_down_time;
     return build_datetime_from_tm(ctx, gmtime_r(&ts.tv_sec, &broken_down_time));
 }

tests/erlang_tests/test_system_time.erl

@@ -131,6 +131,13 @@ test_system_time_to_universal_time() ->
     {{2023, 7, 8}, {20, 19, 39}} = calendar:system_time_to_universal_time(1688847579, second),
 
     {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, second),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, millisecond),
+    {{1969, 12, 31}, {23, 59, 58}} = calendar:system_time_to_universal_time(-1001, millisecond),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, microsecond),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, nanosecond),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, native),
+
+    ok = expect(fun() -> calendar:system_time_to_universal_time(not_an_integer, second) end, badarg),
 
     ok = test_nanosecond_universal_time(),
     ok = test_native_universal_time(),