vim-patch:8.1.1567: localtime_r() does not respond to $TZ changes

Problem:    Localtime_r() does not respond to $TZ changes.
Solution:   If $TZ changes then call tzset(). (Tom Ryder)
db51730df1

src/nvim/os/time.c

@@ -2,9 +2,6 @@
 // it. PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
 
 #include <assert.h>
-#include <stdint.h>
-#include <stdbool.h>
-#include <time.h>
 #include <limits.h>
 
 #include <uv.h>
@@ -13,7 +10,7 @@
 #include "nvim/os/time.h"
 #include "nvim/os/input.h"
 #include "nvim/event/loop.h"
-#include "nvim/vim.h"
+#include "nvim/os/os.h"
 #include "nvim/main.h"
 
 static uv_mutex_t delay_mutex;
@@ -112,6 +109,10 @@ void os_microdelay(uint64_t us, bool ignoreinput)
   uv_mutex_unlock(&delay_mutex);
 }
 
+// Cache of the current timezone name as retrieved from TZ, or an empty string
+// where unset, up to 64 octets long including trailing null byte.
+static char tz_cache[64];
+
 /// Portable version of POSIX localtime_r()
 ///
 /// @return NULL in case of error
@@ -120,6 +121,19 @@ struct tm *os_localtime_r(const time_t *restrict clock,
 {
 #ifdef UNIX
   // POSIX provides localtime_r() as a thread-safe version of localtime().
+  //
+  // Check to see if the environment variable TZ has changed since the last run.
+  // Call tzset(3) to update the global timezone variables if it has.
+  // POSIX standard doesn't require localtime_r() implementations to do that
+  // as it does with localtime(), and we don't want to call tzset() every time.
+  const char *tz = os_getenv("TZ");
+  if (tz == NULL) {
+    tz = "";
+  }
+  if (strncmp(tz_cache, tz, sizeof(tz_cache) - 1) != 0) {
+    tzset();
+    xstrlcpy(tz_cache, tz, sizeof(tz_cache));
+  }
   return localtime_r(clock, result);  // NOLINT(runtime/threadsafe_fn)
 #else
   // Windows version of localtime() is thread-safe.

src/nvim/testdir/test_functions.vim

@@ -186,6 +186,29 @@ func Test_strftime()
 
   call assert_fails('call strftime([])', 'E730:')
   call assert_fails('call strftime("%Y", [])', 'E745:')
+
+  " Check that the time changes after we change the timezone
+  " Save previous timezone value, if any
+  if exists('$TZ')
+    let tz = $TZ
+  endif
+
+  " Force EST and then UTC, save the current hour (24-hour clock) for each
+  let $TZ = 'EST' | let est = strftime('%H')
+  let $TZ = 'UTC' | let utc = strftime('%H')
+
+  " Those hours should be two bytes long, and should not be the same; if they
+  " are, a tzset(3) call may have failed somewhere
+  call assert_equal(strlen(est), 2)
+  call assert_equal(strlen(utc), 2)
+  call assert_notequal(est, utc)
+
+  " If we cached a timezone value, put it back, otherwise clear it
+  if exists('tz')
+    let $TZ = tz
+  else
+    unlet $TZ
+  endif
 endfunc
 
 func Test_resolve()