gh-127146: Some expected failures in Emscripten time tests (#127843)

Disables two tests in the test_time suite, and adjusts test_os to reflect 
precision limits in Emscripten.

1 files changed, 21 insertions, 7 deletions

diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index f3d2ceb..8aac929 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -805,14 +805,28 @@ class UtimeTests(unittest.TestCase):
         set_time(filename, (atime_ns, mtime_ns))
         st = os.stat(filename)
 
-        if support_subsecond:
-            self.assertAlmostEqual(st.st_atime, atime_ns * 1e-9, delta=1e-6)
-            self.assertAlmostEqual(st.st_mtime, mtime_ns * 1e-9, delta=1e-6)
+        if support.is_emscripten:
+            # Emscripten timestamps are roundtripped through a 53 bit integer of
+            # nanoseconds. If we want to represent ~50 years which is an 11
+            # digits number of seconds:
+            # 2*log10(60) + log10(24) + log10(365) + log10(60) + log10(50)
+            # is about 11. Because 53 * log10(2) is about 16, we only have 5
+            # digits worth of sub-second precision.
+            # Some day it would be good to fix this upstream.
+            delta=1e-5
+            self.assertAlmostEqual(st.st_atime, atime_ns * 1e-9, delta=1e-5)
+            self.assertAlmostEqual(st.st_mtime, mtime_ns * 1e-9, delta=1e-5)
+            self.assertAlmostEqual(st.st_atime_ns, atime_ns, delta=1e9 * 1e-5)
+            self.assertAlmostEqual(st.st_mtime_ns, mtime_ns, delta=1e9 * 1e-5)
         else:
-            self.assertEqual(st.st_atime, atime_ns * 1e-9)
-            self.assertEqual(st.st_mtime, mtime_ns * 1e-9)
-        self.assertEqual(st.st_atime_ns, atime_ns)
-        self.assertEqual(st.st_mtime_ns, mtime_ns)
+            if support_subsecond:
+                self.assertAlmostEqual(st.st_atime, atime_ns * 1e-9, delta=1e-6)
+                self.assertAlmostEqual(st.st_mtime, mtime_ns * 1e-9, delta=1e-6)
+            else:
+                self.assertEqual(st.st_atime, atime_ns * 1e-9)
+                self.assertEqual(st.st_mtime, mtime_ns * 1e-9)
+            self.assertEqual(st.st_atime_ns, atime_ns)
+            self.assertEqual(st.st_mtime_ns, mtime_ns)
 
     def test_utime(self):
         def set_time(filename, ns):