[net/dns] Fix how DnsTransaction computes next server index after fallback.

BUG=239657
TEST=net_unittests --gtest_filter=DnsTransactionTest.ServerFallbackAndRotate

Review URL: https://chromiumcodereview.appspot.com/14676012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@199402 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 4 insertions, 3 deletions

diff --git a/net/dns/dns_transaction.cc b/net/dns/dns_transaction.cc
index 6d4bf01..fdd2eb2 100644
--- a/net/dns/dns_transaction.cc
+++ b/net/dns/dns_transaction.cc
@@ -651,8 +651,8 @@ class DnsTransactionImpl : public DnsTransaction,
 
     const DnsConfig& config = session_->config();
 
-    unsigned server_index = first_server_index_ +
-        (attempt_number % config.nameservers.size());
+    unsigned server_index =
+        (first_server_index_ + attempt_number) % config.nameservers.size();
 
     scoped_ptr<DnsSession::SocketLease> lease =
         session_->AllocateSocket(server_index, net_log_.source());
diff --git a/net/dns/dns_transaction_unittest.cc b/net/dns/dns_transaction_unittest.cc
index c3c4c8f..3266b9c 100644
--- a/net/dns/dns_transaction_unittest.cc
+++ b/net/dns/dns_transaction_unittest.cc
@@ -645,6 +645,7 @@ TEST_F(DnsTransactionTest, ServerFallbackAndRotate) {
   AddAsyncQueryAndRcode(kT0HostName, kT0Qtype, dns_protocol::kRcodeNXDOMAIN);
   // Responses for second request.
   AddAsyncQueryAndRcode(kT1HostName, kT1Qtype, dns_protocol::kRcodeSERVFAIL);
+  AddAsyncQueryAndRcode(kT1HostName, kT1Qtype, dns_protocol::kRcodeSERVFAIL);
   AddAsyncQueryAndRcode(kT1HostName, kT1Qtype, dns_protocol::kRcodeNXDOMAIN);
 
   TransactionHelper helper0(kT0HostName, kT0Qtype, ERR_NAME_NOT_RESOLVED);
@@ -655,7 +656,7 @@ TEST_F(DnsTransactionTest, ServerFallbackAndRotate) {
 
   unsigned kOrder[] = {
       0, 1, 2, 0, 1,    // The first transaction.
-      1, 2,             // The second transaction starts from the next server.
+      1, 2, 0,          // The second transaction starts from the next server.
   };
   CheckServerOrder(kOrder, arraysize(kOrder));
 }