[PATCH v2] tst: Provide Y2038 tests for mktime (tst-mktime4.c)

[Lukasz Majewski]

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)

---
Changes for v2:
- Remove FAIL_UNSUPPORTED() when sizeof (time_t) <= 4
- Remove tzset () as it is already done in mktime ()
- Use TEST_COMPARE to check if correct result is received
- Use "TZ=:" glibc extension to guarantee UTC time zone
- Add two more checks - prepare struct tm in a way that mktime will
  return 0xFFFFFFFF and 0x100000000
---
 time/Makefile      |   4 +-
 time/tst-mktime4.c | 100 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 103 insertions(+), 1 deletion(-)
 create mode 100644 time/tst-mktime4.c

diff --git a/time/Makefile b/time/Makefile
index 7de2ce0196..1c8678e46b 100644
--- a/time/Makefile
+++ b/time/Makefile
@@ -51,7 +51,7 @@ tests	:= test_time clocktest tst-posixtz tst-strptime tst_wcsftime \
 	   tst-clock tst-clock2 tst-clock_nanosleep tst-cpuclock1 \
 	   tst-adjtime tst-clock-y2038 tst-clock2-y2038 \
 	   tst-cpuclock1-y2038 tst-clock_nanosleep-y2038 tst-clock_settime \
-	   tst-clock_adjtime tst-ctime tst-difftime
+	   tst-clock_adjtime tst-ctime tst-difftime tst-mktime4
 
 include ../Rules
 
@@ -88,4 +88,6 @@ test_time-ARGS= EST5EDT CST
 tst-tzname-ENV = TZDIR=${common-objpfx}timezone/testdata
 CPPFLAGS-tst-tzname.c += -DTZDEFRULES='"$(posixrules-file)"'
 
+CFLAGS-tst-mktime4.c += -Wno-overflow
+
 bug-getdate1-ARGS = ${objpfx}bug-getdate1-fmt
diff --git a/time/tst-mktime4.c b/time/tst-mktime4.c
new file mode 100644
index 0000000000..811120721e
--- /dev/null
+++ b/time/tst-mktime4.c
@@ -0,0 +1,100 @@
+/* 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>
+
+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
+};
+
+struct tm tmY2038 = {
+                     .tm_year = 138,
+                     .tm_mon = 0,
+                     .tm_mday = 19,
+                     .tm_hour = 3,
+                     .tm_min = 14,
+                     .tm_sec = 7,
+                     .tm_wday = 2,
+                     .tm_yday = 18
+};
+
+struct tm tm1 = {
+                     .tm_year = 206,
+                     .tm_mon = 1,
+                     .tm_mday = 7,
+                     .tm_hour = 6,
+                     .tm_min = 28,
+                     .tm_sec = 15,
+                     .tm_wday = 0,
+                     .tm_yday = 0
+};
+
+static
+void test_mktime_helper (struct tm *tm, time_t exp_val)
+{
+  time_t result = mktime (tm);
+  if (result == -1)
+    FAIL_EXIT1 ("*** mktime failed: %m");
+
+  TEST_COMPARE (result, exp_val);
+}
+
+static int
+do_test (void)
+{
+  /* 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.  */
+  test_mktime_helper (&tm0, 0);
+
+  /* Check that mktime(2038-01-19 03:14:07) returns 0x7FFFFFFF.  */
+  test_mktime_helper (&tmY2038, 0x7fffffff);
+
+  if (sizeof (time_t) > 4)
+    {
+      /* Check that mktime(2038-01-19 03:14:08) returns 0x80000000
+         (time_t overflow).  */
+      tmY2038.tm_sec++;
+      test_mktime_helper (&tmY2038, 0x80000000);
+
+      /* Check that mktime(2106-02-07 06:28:15) returns 0xFFFFFFFF.  */
+      test_mktime_helper (&tm1, 0xFFFFFFFF);
+
+      /* Check that mktime(2106-02-07 06:28:16) returns 0x100000000.
+
+         This test is executed with -Wno-overflow to prevent build
+         breaks when it is compiled on ports with 32 bit time_t.  */
+      tm1.tm_sec++;
+      test_mktime_helper (&tm1, 0x100000000);
+    }
+
+  return 0;
+}
+
+#include <support/test-driver.c>
-- 
2.20.1