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.

PR-URL: #20026
Fixes: #19974
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node.cc

@@ -80,6 +80,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>
+#include <time.h>  // tzset(), _tzset()
 
 #include <string>
 #include <vector>
@@ -135,6 +136,7 @@ using v8::Array;
 using v8::ArrayBuffer;
 using v8::Boolean;
 using v8::Context;
+using v8::Date;
 using v8::EscapableHandleScope;
 using v8::Exception;
 using v8::Float64Array;
@@ -2679,6 +2681,10 @@ static void EnvSetter(Local<Name> property,
   node::Utf8Value key(info.GetIsolate(), property);
   node::Utf8Value val(info.GetIsolate(), value);
   setenv(*key, *val, 1);
+  if (key.length() == 2 && key[0] == 'T' && key[1] == 'Z') {
+    tzset();
+    Date::DateTimeConfigurationChangeNotification(info.GetIsolate());
+  }
 #else  // _WIN32
   node::TwoByteValue key(info.GetIsolate(), property);
   node::TwoByteValue val(info.GetIsolate(), value);
@@ -2687,6 +2693,10 @@ static void EnvSetter(Local<Name> property,
   if (key_ptr[0] != L'=') {
     SetEnvironmentVariableW(key_ptr, reinterpret_cast<WCHAR*>(*val));
   }
+  if (key.length() == 2 && key[0] == L'T' && key[1] == L'Z') {
+    _tzset();
+    Date::DateTimeConfigurationChangeNotification(info.GetIsolate());
+  }
 #endif
   // Whether it worked or not, always return value.
   info.GetReturnValue().Set(value);

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

@@ -0,0 +1,23 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+
+if (common.isWindows)  // Using a different TZ format.
+  common.skip('todo: test on Windows');
+
+if (common.isAIX || common.isSunOS)  // Reports 2018 CEST as CET.
+  common.skip('tzdata too old');
+
+const date = new Date('2018-04-14T12:34:56.789Z');
+
+process.env.TZ = 'Europe/Amsterdam';
+if (/\(Europe\)/.test(date.toString()))
+  common.skip('not using bundled ICU');  // Shared library or --with-intl=none.
+assert.strictEqual(date.toString(), 'Sat Apr 14 2018 14:34:56 GMT+0200 (CEST)');
+
+process.env.TZ = 'Europe/London';
+assert.strictEqual(date.toString(), 'Sat Apr 14 2018 13:34:56 GMT+0100 (BST)');
+
+process.env.TZ = 'Etc/UTC';
+assert.strictEqual(date.toString(), 'Sat Apr 14 2018 12:34:56 GMT+0000 (UTC)');