Use res_ninit for thread safety and a timer for reloading resolv.conf on Linux.

net/base/host_resolver.cc

Index: net/base/host_resolver.cc
diff --git a/net/base/host_resolver.cc b/net/base/host_resolver.cc
index 23a1c00940e75c1c9a5fcf36ad386789eb48b3ad..ec693eedf74a807ee7a9c79164130620d685e6d9 100644
--- a/net/base/host_resolver.cc
+++ b/net/base/host_resolver.cc
@@ -21,6 +21,12 @@
 #include "net/base/address_list.h"
 #include "net/base/net_errors.h"
 
+#if defined(OS_LINUX)
+#include "base/singleton.h"
+#include "base/thread_local_storage.h"
+#include "base/time.h"
+#endif
+
 #if defined(OS_WIN)
 #include "net/base/winsock_init.h"
 #endif
@@ -40,6 +46,70 @@ HostMapper* SetHostMapper(HostMapper* value) {
   return value;
 }
 
+#if defined(OS_LINUX)
+// On Linux 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.
+
+// Keep a timer per calling thread to rate limit the calling of res_ninit.
+class DnsReloadTimer {
+ public:
+  DnsReloadTimer() {
+    tls_index_.Initialize(SlotReturnFunction);
+  }
+
+  ~DnsReloadTimer() { }
+
+  // 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:
+  // We use thread local storage to identify which base::TimeTicks to
+  // interact with.
+  static ThreadLocalStorage::Slot tls_index_ ;
+
+  DISALLOW_COPY_AND_ASSIGN(DnsReloadTimer);
+};
+
+// A TLS slot to the TimeTicks for the current thread.
+// static
+ThreadLocalStorage::Slot DnsReloadTimer::tls_index_(base::LINKER_INITIALIZED);
+
+#endif  // defined(OS_LINUX)
+
 static int HostResolverProc(
     const std::string& host, const std::string& port, struct addrinfo** out) {
   struct addrinfo hints = {0};
@@ -76,10 +146,14 @@ static int HostResolverProc(
 
   int err = getaddrinfo(host.c_str(), port.c_str(), &hints, out);
 #if defined(OS_LINUX)
+  net::DnsReloadTimer* dns_timer = Singleton<net::DnsReloadTimer>::get();
   // If we fail, re-initialise the resolver just in case there have been any
   // changes to /etc/resolv.conf and retry. See http://crbug.com/11380 for info.
-  if (err && !res_init())
-    err = getaddrinfo(host.c_str(), port.c_str(), &hints, out);
+  if (err && dns_timer->Expired()) {
+    res_nclose(&_res);
+    if (!res_ninit(&_res))
+      err = getaddrinfo(host.c_str(), port.c_str(), &hints, out);
+  }
 #endif
 
   return err ? ERR_NAME_NOT_RESOLVED : OK;