Cleanup dead code in DnsSocketPool

Remove everything related to the "default" socket pool that is
completely unused.  Also simplify the DnsSocketPool interface for the
remaining much simpler usage of the class.

Also delete dns_session_unittest.cc since it was really only testing
that allocated sockets were properly freed in the pool, but that no
longer matters and it wasn't worth maintaining the mockability and hooks
in the pool needed for the test.

Bug: 1116579
Change-Id: Ib322bffb653e85a9c33a4b3835f4067fd8d29530
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357615
Commit-Queue: Eric Orth <ericorth@chromium.org>
Reviewed-by: Dan McArdle <dmcardle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800248}

Author: Eric Orth

net/dns/BUILD.gn

@@ -391,7 +391,6 @@ source_set("tests") {
     "dns_hosts_unittest.cc",
     "dns_query_unittest.cc",
     "dns_response_unittest.cc",
-    "dns_session_unittest.cc",
     "dns_socket_pool_unittest.cc",
     "dns_transaction_unittest.cc",
     "dns_udp_tracker_unittest.cc",

net/dns/dns_client.cc

@@ -245,8 +245,8 @@ class DnsClientImpl : public DnsClient {
     if (new_effective_config) {
       DCHECK(new_effective_config.value().IsValid());
 
-      std::unique_ptr<DnsSocketPool> socket_pool(
-          DnsSocketPool::CreateNull(socket_factory_, rand_int_callback_));
+      auto socket_pool = std::make_unique<DnsSocketPool>(
+          socket_factory_, new_effective_config.value().nameservers, net_log_);
       session_ =
           new DnsSession(std::move(new_effective_config).value(),
                          std::move(socket_pool), rand_int_callback_, net_log_);

net/dns/dns_session.cc

@@ -48,7 +48,6 @@ DnsSession::DnsSession(const DnsConfig& config,
                                          0,
                                          std::numeric_limits<uint16_t>::max())),
       net_log_(net_log) {
-  socket_pool_->Initialize(&config_.nameservers, net_log);
   UMA_HISTOGRAM_CUSTOM_COUNTS("AsyncDNS.ServerCount",
                               config_.nameservers.size(), 1, 10, 11);
 }
@@ -65,7 +64,7 @@ std::unique_ptr<DnsSession::SocketLease> DnsSession::AllocateSocket(
     const NetLogSource& source) {
   std::unique_ptr<DatagramClientSocket> socket;
 
-  socket = socket_pool_->AllocateSocket(server_index);
+  socket = socket_pool_->CreateConnectedUdpSocket(server_index);
   if (!socket.get())
     return std::unique_ptr<SocketLease>();
 
@@ -79,7 +78,7 @@ std::unique_ptr<DnsSession::SocketLease> DnsSession::AllocateSocket(
 std::unique_ptr<StreamSocket> DnsSession::CreateTCPSocket(
     size_t server_index,
     const NetLogSource& source) {
-  return socket_pool_->CreateTCPSocket(server_index, source);
+  return socket_pool_->CreateTcpSocket(server_index, source);
 }
 
 void DnsSession::InvalidateWeakPtrsForTesting() {
@@ -92,8 +91,6 @@ void DnsSession::FreeSocket(size_t server_index,
   DCHECK(socket.get());
 
   socket->NetLog().EndEvent(NetLogEventType::SOCKET_IN_USE);
-
-  socket_pool_->FreeSocket(server_index, std::move(socket));
 }
 
 }  // namespace net

net/dns/dns_session.h

@@ -38,6 +38,8 @@ class NET_EXPORT_PRIVATE DnsSession : public base::RefCounted<DnsSession> {
  public:
   typedef base::RepeatingCallback<int()> RandCallback;
 
+  // TODO(crbug.com/1116579): Remove this unnecessary abstraction now that DNS
+  // sockets are not pooled and do not need a special release signal.
   class NET_EXPORT_PRIVATE SocketLease {
    public:
     SocketLease(scoped_refptr<DnsSession> session,
@@ -71,11 +73,13 @@ class NET_EXPORT_PRIVATE DnsSession : public base::RefCounted<DnsSession> {
 
   // Allocate a socket, already connected to the server address.
   // When the SocketLease is destroyed, the socket will be freed.
+  // TODO(crbug.com/1116579): Remove this and directly call into DnsSocketPool.
   std::unique_ptr<SocketLease> AllocateSocket(size_t server_index,
                                               const NetLogSource& source);
 
   // Creates a StreamSocket from the factory for a transaction over TCP. These
   // sockets are not pooled.
+  // TODO(crbug.com/1116579): Remove this and directly call into DnsSocketPool.
   std::unique_ptr<StreamSocket> CreateTCPSocket(size_t server_index,
                                                 const NetLogSource& source);