AsyncResolver: remove dependency on SignalThread.

This change removes dependency on SignalThread which is a
heavy user of re-entered mutexes, and is a step to being
able to delete SignalThread.

The new AsyncResolver is based on task queue instead.

Bug: webrtc:11567, webrtc:7723
Change-Id: Iab125ccbc0fb9d72af44341e58f89b3868c952cb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175910
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31361}
diff --git a/rtc_base/net_helpers.h b/rtc_base/net_helpers.h
index 1e06940..c6aa4be 100644
--- a/rtc_base/net_helpers.h
+++ b/rtc_base/net_helpers.h
@@ -21,16 +21,23 @@
 
 #include "rtc_base/async_resolver_interface.h"
 #include "rtc_base/ip_address.h"
-#include "rtc_base/signal_thread.h"
 #include "rtc_base/socket_address.h"
+#include "rtc_base/synchronization/sequence_checker.h"
 #include "rtc_base/system/rtc_export.h"
+#include "rtc_base/task_utils/pending_task_safety_flag.h"
+#include "rtc_base/thread.h"
+#include "rtc_base/thread_annotations.h"
 
 namespace rtc {
 
 // AsyncResolver will perform async DNS resolution, signaling the result on
 // the SignalDone from AsyncResolverInterface when the operation completes.
-class RTC_EXPORT AsyncResolver : public SignalThread,
-                                 public AsyncResolverInterface {
+//
+// This class is thread-compatible, and all methods and destruction needs to
+// happen from the same rtc::Thread, except for Destroy which is allowed to
+// happen on another context provided it's not happening concurrently to another
+// public API call, and is the last access to the object.
+class RTC_EXPORT AsyncResolver : public AsyncResolverInterface {
  public:
   AsyncResolver();
   ~AsyncResolver() override;
@@ -40,17 +47,22 @@
   int GetError() const override;
   void Destroy(bool wait) override;
 
-  const std::vector<IPAddress>& addresses() const { return addresses_; }
-  void set_error(int error) { error_ = error; }
-
- protected:
-  void DoWork() override;
-  void OnWorkDone() override;
+  const std::vector<IPAddress>& addresses() const;
 
  private:
-  SocketAddress addr_;
-  std::vector<IPAddress> addresses_;
-  int error_;
+  void ResolveDone(std::vector<IPAddress> addresses, int error)
+      RTC_EXCLUSIVE_LOCKS_REQUIRED(sequence_checker_);
+  void MaybeSelfDestruct();
+
+  SocketAddress addr_ RTC_GUARDED_BY(sequence_checker_);
+  std::vector<IPAddress> addresses_ RTC_GUARDED_BY(sequence_checker_);
+  int error_ RTC_GUARDED_BY(sequence_checker_);
+  webrtc::ScopedTaskSafety safety_ RTC_GUARDED_BY(sequence_checker_);
+  std::unique_ptr<Thread> popup_thread_ RTC_GUARDED_BY(sequence_checker_);
+  bool recursion_check_ =
+      false;  // Protects against SignalDone calling into Destroy.
+  bool destroy_called_ = false;
+  webrtc::SequenceChecker sequence_checker_;
 };
 
 // rtc namespaced wrappers for inet_ntop and inet_pton so we can avoid