tst: time: Provide Y2038 tests for mktime (tst-mktime4.c)

This change adds new test to assess mktime's functionality.

To be more specific - following use cases are checked:
- Pass struct tm as epoch time
- Pass struct tm as value just before Y2038 threshold (returned
  value shall be 0x7FFFFFFF)
- Pass struct tm as the first value after Y2038 threshold
  (expected value - 0x80000000)

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

time/Makefile

@@ -48,7 +48,7 @@ tests	:= test_time clocktest tst-posixtz tst-strptime tst_wcsftime \
 	   tst-strptime3 bug-getdate1 tst-strptime-whitespace tst-ftime \
 	   tst-tzname tst-y2039 bug-mktime4 tst-strftime2 tst-strftime3 \
 	   tst-clock tst-clock2 tst-clock_nanosleep tst-cpuclock1 \
-	   tst-adjtime tst-ctime tst-difftime
+	   tst-adjtime tst-ctime tst-difftime tst-mktime4
 
 include ../Rules
 

time/tst-mktime4.c

@@ -0,0 +1,109 @@
+/* Test for mktime (4)
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <time.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+const struct tm tm0 =
+  {
+    .tm_year = 70,
+    .tm_mon = 0,
+    .tm_mday = 1,
+    .tm_hour = 0,
+    .tm_min = 0,
+    .tm_sec = 0,
+    .tm_wday = 4,
+    .tm_yday = 0,
+  };
+
+const struct tm tmY2038 =
+  {
+    .tm_year = 138,
+    .tm_mon = 0,
+    .tm_mday = 19,
+    .tm_hour = 3,
+    .tm_min = 14,
+    .tm_sec = 7,
+  };
+
+const struct tm tm32bitmax =
+  {
+    .tm_year = 206,
+    .tm_mon = 1,
+    .tm_mday = 7,
+    .tm_hour = 6,
+    .tm_min = 28,
+    .tm_sec = 15,
+  };
+
+static
+int test_mktime_helper (struct tm *tm, long long int exp_val, int line)
+{
+  time_t result, t;
+
+  /* Check if we run on port with 32 bit time_t size.  */
+  if (__builtin_add_overflow (exp_val, 0, &t))
+    return 0;
+
+  result = mktime (tm);
+  if (result == (time_t) -1)
+    FAIL_RET ("*** mktime failed: %m in line: %d", line);
+
+  if ((long long int) result != exp_val)
+    FAIL_RET ("*** Result different than expected (%lld != %lld) in %d\n",
+              (long long int) result, exp_val, line);
+
+  return 0;
+}
+
+static int
+do_test (void)
+{
+  struct tm t;
+  /* Use glibc time zone extension "TZ=:" to to guarantee that UTC
+     without leap seconds is used for the test.  */
+  TEST_VERIFY_EXIT (setenv ("TZ", ":", 1) == 0);
+
+  /* Check that mktime (1970-01-01 00:00:00) returns 0.  */
+  t = tm0;
+  test_mktime_helper (&t, 0, __LINE__);
+
+  /* Check that mktime (2038-01-19 03:14:07) returns 0x7FFFFFFF.  */
+  t = tmY2038;
+  test_mktime_helper (&t, 0x7fffffff, __LINE__);
+
+  /* Check that mktime (2038-01-19 03:14:08) returns 0x80000000
+     (time_t overflow).  */
+  t = tmY2038;
+  t.tm_sec++;
+  test_mktime_helper (&t, 0x80000000, __LINE__);
+
+  /* Check that mktime (2106-02-07 06:28:15) returns 0xFFFFFFFF.  */
+  t = tm32bitmax;
+  test_mktime_helper (&t, 0xFFFFFFFF, __LINE__);
+
+  /* Check that mktime (2106-02-07 06:28:16) returns 0x100000000.  */
+  t = tm32bitmax;
+  t.tm_sec++;
+  test_mktime_helper (&t, 0x100000000, __LINE__);
+
+  return 0;
+}
+
+#include <support/test-driver.c>