Linux: Monitor resolv.conf for changes and use that to reload resolver.

net/base/dns_reload_timer.cc

-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "net/base/dns_reload_timer.h"
-
-#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
-#include "base/lazy_instance.h"
-#include "base/threading/thread_local_storage.h"
-#include "base/time.h"
-
-namespace {
-
-// On Linux/BSD, changes to /etc/resolv.conf can go unnoticed thus resulting
-// in DNS queries failing either because nameservers are unknown on startup
-// or because nameserver info has changed as a result of e.g. connecting to
-// a new network. Some distributions patch glibc to stat /etc/resolv.conf
-// to try to automatically detect such changes but these patches are not
-// universal and even patched systems such as Jaunty appear to need calls
-// to res_ninit to reload the nameserver information in different threads.
-//
-// We adopt the Mozilla solution here which is to call res_ninit when
-// lookups fail and to rate limit the reloading to once per second per
-// thread.
-//
-// OpenBSD does not have thread-safe res_ninit/res_nclose so we can't do
-// the same trick there.
-
-// Keep a timer per calling thread to rate limit the calling of res_ninit.
-class DnsReloadTimer {
- public:
-  // Check if the timer for the calling thread has expired. When no
-  // timer exists for the calling thread, create one.
-  bool Expired() {
-    const base::TimeDelta kRetryTime = base::TimeDelta::FromSeconds(1);
-    base::TimeTicks now = base::TimeTicks::Now();
-    base::TimeTicks* timer_ptr =
-      static_cast<base::TimeTicks*>(tls_index_.Get());
-
-    if (!timer_ptr) {
-      timer_ptr = new base::TimeTicks();
-      *timer_ptr = base::TimeTicks::Now();
-      tls_index_.Set(timer_ptr);
-      // Return true to reload dns info on the first call for each thread.
-      return true;
-    } else if (now - *timer_ptr > kRetryTime) {
-      *timer_ptr = now;
-      return true;
-    } else {
-      return false;
-    }
-  }
-
-  // Free the allocated timer.
-  static void SlotReturnFunction(void* data) {
-    base::TimeTicks* tls_data = static_cast<base::TimeTicks*>(data);
-    delete tls_data;
-  }
-
- private:
-  friend struct base::DefaultLazyInstanceTraits<DnsReloadTimer>;
-
-  DnsReloadTimer() {
-    // During testing the DnsReloadTimer Singleton may be created and destroyed
-    // multiple times. Initialize the ThreadLocalStorage slot only once.
-    if (!tls_index_.initialized())
-      tls_index_.Initialize(SlotReturnFunction);
-  }
-
-  ~DnsReloadTimer() {
-  }
-
-  // We use thread local storage to identify which base::TimeTicks to
-  // interact with.
-  static base::ThreadLocalStorage::Slot tls_index_ ;
-
-  DISALLOW_COPY_AND_ASSIGN(DnsReloadTimer);
-};
-
-// A TLS slot to the TimeTicks for the current thread.
-// static
-base::ThreadLocalStorage::Slot DnsReloadTimer::tls_index_(
-    base::LINKER_INITIALIZED);
-
-base::LazyInstance<DnsReloadTimer,
-                   base::LeakyLazyInstanceTraits<DnsReloadTimer> >
-    g_dns_reload_timer(base::LINKER_INITIALIZED);
-
-}  // namespace
-
-namespace net {
-
-bool DnsReloadTimerHasExpired() {
-  DnsReloadTimer* dns_timer = g_dns_reload_timer.Pointer();
-  return dns_timer->Expired();
-}
-
-}  // namespace net
-
-#endif  // defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)