Making candidate pool size behave as decided in JSEP.

webrtc/p2p/base/portallocator.h

Index: webrtc/p2p/base/portallocator.h
diff --git a/webrtc/p2p/base/portallocator.h b/webrtc/p2p/base/portallocator.h
index 016a7dc996b41acca85987323274637b1951a730..87285925db9e213ac550a0a594e84fa7562a70ce 100644
--- a/webrtc/p2p/base/portallocator.h
+++ b/webrtc/p2p/base/portallocator.h
@@ -306,6 +306,7 @@ class PortAllocator : public sigslot::has_slots<> {
       allow_tcp_listen_(true),
       candidate_filter_(CF_ALL) {
   }
+
   virtual ~PortAllocator() {}
 
   // This should be called on the PortAllocator's thread before the
@@ -315,11 +316,13 @@ class PortAllocator : public sigslot::has_slots<> {
   // Set STUN and TURN servers to be used in future sessions, and set
   // candidate pool size, as described in JSEP.
   //
-  // If the servers are changing and the candidate pool size is nonzero,
-  // existing pooled sessions will be destroyed and new ones created.
+  // If the servers are changing, and the candidate pool size is nonzero, and
+  // FreezeCandidatePool hasn't been called, existing pooled sessions will be
+  // destroyed and new ones created.
   //
-  // If the servers are not changing but the candidate pool size is,
-  // pooled sessions will be either created or destroyed as necessary.
+  // If the servers are not changing but the candidate pool size is, and
+  // FreezeCandidatePool hasn't been called, pooled sessions will be either
+  // created or destroyed as necessary.
   //
   // Returns true if the configuration could successfully be changed.
   bool SetConfiguration(const ServerAddresses& stun_servers,
@@ -362,6 +365,18 @@ class PortAllocator : public sigslot::has_slots<> {
   // Returns the next session that would be returned by TakePooledSession.
   const PortAllocatorSession* GetPooledSession() const;
 
+  // After FreezeCandidatePool is called, changing the candidate pool size will
+  // no longer be allowed, and changing ICE servers will not cause pooled
+  // sessions to be recreated.
+  //
+  // Expected to be called when SetLocalDescription is called on a
+  // PeerConnection. Can be called safely on any thread as long as not
+  // simultaneously with SetConfiguration.
+  void FreezeCandidatePool();
+
+  // Discard any remaining pooled sessions.
+  void DiscardCandidatePool();
+
   uint32_t flags() const { return flags_; }
   void set_flags(uint32_t flags) { flags_ = flags; }
 
@@ -429,6 +444,7 @@ class PortAllocator : public sigslot::has_slots<> {
   std::vector<RelayServerConfig> turn_servers_;
   int candidate_pool_size_ = 0;  // Last value passed into SetConfiguration.
   std::deque<std::unique_ptr<PortAllocatorSession>> pooled_sessions_;
+  bool candidate_pool_frozen_ = false;
   bool prune_turn_ports_ = false;
 
   webrtc::MetricsObserverInterface* metrics_observer_ = nullptr;