fix test_time failure in non-DST time zones

Fixes: emscripten-core#5617

Author: hujiajie

AUTHORS

@@ -312,3 +312,4 @@ a license to everyone to use it as detailed in LICENSE.)
 * Zhiming Wang <[email protected]>
 * Jameson Ernst <[email protected]>
 * Yoan Lecoq <[email protected]>
+* Jiajie Hu <[email protected]> (copyright owned by Intel Corporation)

src/library.js

@@ -1913,7 +1913,8 @@ LibraryManager.library = {
     var winterOffset = start.getTimezoneOffset();
     var dstOffset = Math.min(winterOffset, summerOffset); // DST is in December in South
     if (dst < 0) {
-      {{{ makeSetValue('tmPtr', C_STRUCTS.tm.tm_isdst, 'Number(dstOffset == guessedOffset)', 'i32') }}};
+      // Attention: some regions don't have DST at all.
+      {{{ makeSetValue('tmPtr', C_STRUCTS.tm.tm_isdst, 'Number(summerOffset != winterOffset && dstOffset == guessedOffset)', 'i32') }}};
     } else if ((dst > 0) != (dstOffset == guessedOffset)) {
       var nonDstOffset = Math.max(winterOffset, summerOffset);
       var trueOffset = dst > 0 ? dstOffset : nonDstOffset;
@@ -1995,10 +1996,10 @@ LibraryManager.library = {
     {{{ makeSetValue('tmPtr', C_STRUCTS.tm.tm_yday, 'yday', 'i32') }}};
     {{{ makeSetValue('tmPtr', C_STRUCTS.tm.tm_gmtoff, '-(date.getTimezoneOffset() * 60)', 'i32') }}};
 
-    // DST is in December in South
+    // Attention: DST is in December in South, and some regions don't have DST at all.
     var summerOffset = new Date(2000, 6, 1).getTimezoneOffset();
     var winterOffset = start.getTimezoneOffset();
-    var dst = (date.getTimezoneOffset() == Math.min(winterOffset, summerOffset))|0;
+    var dst = (summerOffset != winterOffset && date.getTimezoneOffset() == Math.min(winterOffset, summerOffset))|0;
     {{{ makeSetValue('tmPtr', C_STRUCTS.tm.tm_isdst, 'dst', 'i32') }}};
 
     var zonePtr = {{{ makeGetValue(makeGlobalUse('_tzname'), 'dst ? Runtime.QUANTUM_SIZE : 0', 'i32') }}};

tests/test_core.py

@@ -2083,7 +2083,17 @@ def test_tcgetattr(self):
   def test_time(self):
     src = open(path_from_root('tests', 'time', 'src.cpp'), 'r').read()
     expected = open(path_from_root('tests', 'time', 'output.txt'), 'r').read()
-    self.do_run(src, expected);
+    self.do_run(src, expected)
+    try:
+      for tz in ['EST+05EDT', 'UTC+0']:
+        # Run the test with different time zone settings if
+        # possible. It seems that the TZ environment variable does not
+        # work all the time (at least it's not well respected by
+        # Node.js on Windows), but it does no harm either.
+        os.environ['TZ'] = tz
+        self.do_run(src, expected)
+    finally:
+      del os.environ['TZ']
 
   def test_timeb(self):
     # Confirms they are called in reverse order