src: make process.env.TZ setter clear tz cache

Since the presence of the libc and V8 timezone caches seem to be
a perennial source of confusion to users ("why doesn't it work?!"),
let's try to support that pattern by intercepting assignments to
the TZ environment variable and reset the caches as a side effect.

Fixes: #19974
PR-URL: #20026
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Steven R Loomis <srloomis@us.ibm.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>

src/node_env_var.cc

@@ -2,6 +2,8 @@
 #include "node_errors.h"
 #include "node_process.h"
 
+#include <time.h>  // tzset(), _tzset()
+
 #ifdef __APPLE__
 #include <crt_externs.h>
 #define environ (*_NSGetEnviron())
@@ -64,6 +66,19 @@ Mutex env_var_mutex;
 std::shared_ptr<KVStore> system_environment = std::make_shared<RealEnvStore>();
 }  // namespace per_process
 
+template <typename T>
+void DateTimeConfigurationChangeNotification(Isolate* isolate, const T& key) {
+  if (key.length() == 2 && key[0] == 'T' && key[1] == 'Z') {
+#ifdef __POSIX__
+    tzset();
+#else
+    _tzset();
+#endif
+    auto constexpr time_zone_detection = Isolate::TimeZoneDetection::kRedetect;
+    isolate->DateTimeConfigurationChangeNotification(time_zone_detection);
+  }
+}
+
 Local<String> RealEnvStore::Get(Isolate* isolate,
                                 Local<String> property) const {
   Mutex::ScopedLock lock(per_process::env_var_mutex);
@@ -115,6 +130,7 @@ void RealEnvStore::Set(Isolate* isolate,
     SetEnvironmentVariableW(key_ptr, reinterpret_cast<WCHAR*>(*val));
   }
 #endif
+  DateTimeConfigurationChangeNotification(isolate, key);
 }
 
 int32_t RealEnvStore::Query(Isolate* isolate, Local<String> property) const {
@@ -150,6 +166,7 @@ void RealEnvStore::Delete(Isolate* isolate, Local<String> property) {
   WCHAR* key_ptr = reinterpret_cast<WCHAR*>(*key);
   SetEnvironmentVariableW(key_ptr, nullptr);
 #endif
+  DateTimeConfigurationChangeNotification(isolate, key);
 }
 
 Local<Array> RealEnvStore::Enumerate(Isolate* isolate) const {

test/parallel/test-process-env-tz.js

@@ -0,0 +1,47 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+
+if (!common.isMainThread)
+  common.skip('process.env.TZ is not intercepted in Workers');
+
+if (common.isWindows)  // Using a different TZ format.
+  common.skip('todo: test on Windows');
+
+const date = new Date('2018-04-14T12:34:56.789Z');
+
+process.env.TZ = 'Europe/Amsterdam';
+
+if (date.toString().includes('(Europe)'))
+  common.skip('not using bundled ICU');  // Shared library or --with-intl=none.
+
+if ('Sat Apr 14 2018 12:34:56 GMT+0000 (GMT)' === date.toString())
+  common.skip('missing tzdata');  // Alpine buildbots lack Europe/Amsterdam.
+
+if (date.toString().includes('(Central European Time)') ||
+    date.toString().includes('(CET)')) {
+  // The AIX and SmartOS buildbots report 2018 CEST as CET
+  // because apparently for them that's still the deep future.
+  common.skip('tzdata too old');
+}
+
+assert.strictEqual(
+  date.toString().replace('Central European Summer Time', 'CEST'),
+  'Sat Apr 14 2018 14:34:56 GMT+0200 (CEST)');
+
+process.env.TZ = 'Europe/London';
+assert.strictEqual(
+  date.toString().replace('British Summer Time', 'BST'),
+  'Sat Apr 14 2018 13:34:56 GMT+0100 (BST)');
+
+process.env.TZ = 'Etc/UTC';
+assert.strictEqual(
+  date.toString().replace('Coordinated Universal Time', 'UTC'),
+  'Sat Apr 14 2018 12:34:56 GMT+0000 (UTC)');
+
+// Just check that deleting the environment variable doesn't crash the process.
+// We can't really check the result of date.toString() because we don't know
+// the default time zone.
+delete process.env.TZ;
+date.toString();