diff: fix timezone bug on Solaris

Problem reported by Vladimir Marek (bug#51228).
* NEWS: Mention this.
* src/context.c (print_context_label): Pass localtz to nstrftime,
instead of always passing 0.
* src/diff.c (main) [!HAVE_TM_GMTOFF]:
Initialize localtz if time_format uses %z.
* src/diff.h (localtz): New decl.
* tests/Makefile.am (TESTS): Add timezone.
* tests/timezone: New test.

3 files changed, 11 insertions, 1 deletions

diff --git a/src/context.c b/src/context.c
index b3cfa7d..adcfdb3 100644
--- a/src/context.c
+++ b/src/context.c
@@ -52,7 +52,7 @@ print_context_label (char const *mark,
                     INT_STRLEN_BOUND (time_t) + 11)];
       struct tm const *tm = localtime (&inf->stat.st_mtime);
       int nsec = get_stat_mtime_ns (&inf->stat);
-      if (! (tm && nstrftime (buf, sizeof buf, time_format, tm, 0, nsec)))
+      if (! (tm && nstrftime (buf, sizeof buf, time_format, tm, localtz, nsec)))
         {
           verify (TYPE_IS_INTEGER (time_t));
           if (LONG_MIN <= TYPE_MINIMUM (time_t)
diff --git a/src/diff.c b/src/diff.c
index fedc7df..0961afb 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -729,6 +729,9 @@ main (int argc, char **argv)
 #else
       time_format = "%Y-%m-%d %H:%M:%S %z";
 #endif
+#if !HAVE_TM_GMTOFF
+      localtz = tzalloc (getenv ("TZ"));
+#endif
     }
   else
     {
diff --git a/src/diff.h b/src/diff.h
index d65c1da..34caa37 100644
--- a/src/diff.h
+++ b/src/diff.h
@@ -155,6 +155,13 @@ XTERN bool ignore_file_name_case;
    (--no-dereference).  */
 XTERN bool no_dereference_symlinks;
 
+/* Local timezone for 'c' output headers, if needed.  */
+#if HAVE_TM_GMTOFF
+# define localtz 0 /* Placeholder since localtz is never needed.  */
+#else
+XTERN timezone_t localtz;
+#endif
+
 /* File labels for '-c' output headers (--label).  */
 XTERN char *file_label[2];