sync: Gracefully handle very early shutdown

sync/engine/net/server_connection_manager.h

Index: sync/engine/net/server_connection_manager.h
diff --git a/sync/engine/net/server_connection_manager.h b/sync/engine/net/server_connection_manager.h
index 3bd533e78fdd07d555e54d296e9b70465fd5bbf6..a00c0e873e6425132a37a8dc4e631d72cec8208f 100644
--- a/sync/engine/net/server_connection_manager.h
+++ b/sync/engine/net/server_connection_manager.h
@@ -8,14 +8,13 @@
 #include <iosfwd>
 #include <string>
 
-#include "base/atomicops.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list.h"
 #include "base/strings/string_util.h"
-#include "base/synchronization/lock.h"
 #include "base/threading/non_thread_safe.h"
 #include "base/threading/thread_checker.h"
 #include "sync/base/sync_export.h"
+#include "sync/internal_api/public/base/cancelation_observer.h"
 #include "sync/syncable/syncable_id.h"
 
 namespace sync_pb {
@@ -24,6 +23,8 @@ class ClientToServerMessage;
 
 namespace syncer {
 
+class CancelationSignal;
+
 namespace syncable {
 class Directory;
 }
@@ -183,7 +184,8 @@ class SYNC_EXPORT_PRIVATE ServerConnectionManager {
   ServerConnectionManager(const std::string& server,
                           int port,
                           bool use_ssl,
-                          bool use_oauth2_token);
+                          bool use_oauth2_token,
+                          CancelationSignal* const cancelation_signal);

[tim (not reviewing), 2013/09/06 21:47:34]

I think you can omit the *const here and keep it in the impl only?  It's weird
to see here since it's useless to the caller, right? 

[rlarocque, 2013/09/06 22:42:53]

Yep, this is the result of a regex gone wrong.  I thought I had fixed all
instances, but I guess I missed this one.

 
   virtual ~ServerConnectionManager();
 
@@ -213,13 +215,7 @@ class SYNC_EXPORT_PRIVATE ServerConnectionManager {
 
   // Factory method to create an Connection object we can use for
   // communication with the server.
-  virtual Connection* MakeConnection();
-
-  // Aborts any active HTTP POST request.
-  // We expect this to get called on a different thread than the valid
-  // ThreadChecker thread, as we want to kill any pending http traffic without
-  // having to wait for the request to complete.
-  void TerminateAllIO();
+  virtual scoped_ptr<Connection> MakeConnection();
 
   void set_client_id(const std::string& client_id) {
     DCHECK(thread_checker_.CalledOnValidThread());
@@ -272,10 +268,6 @@ class SYNC_EXPORT_PRIVATE ServerConnectionManager {
   // terminated, this will return NULL.
   Connection* MakeActiveConnection();
 
-  // Called by Connection objects as they are destroyed to allow the
-  // ServerConnectionManager to cleanup active connections.
-  void OnConnectionDestroyed(Connection* connection);
-
   // The sync_server_ is the server that requests will be made to.
   std::string sync_server_;
 
@@ -308,35 +300,34 @@ class SYNC_EXPORT_PRIVATE ServerConnectionManager {
 
   base::ThreadChecker thread_checker_;
 
-  // Protects all variables below to allow bailing out of active connections.
-  base::Lock terminate_connection_lock_;
-
-  // If true, we've been told to terminate IO and expect to be destroyed
-  // shortly.  No future network requests will be made.
-  bool terminated_;
-
-  // A non-owning pointer to any active http connection, so that we can abort
-  // it if necessary.
-  Connection* active_connection_;
+  CancelationSignal* const cancelation_signal_;
 
  private:
   friend class Connection;
   friend class ScopedServerStatusWatcher;
 
-  // A class to help deal with cleaning up active Connection objects when (for
-  // ex) multiple early-exits are present in some scope. ScopedConnectionHelper
-  // informs the ServerConnectionManager before the Connection object it takes
-  // ownership of is destroyed.
-  class ScopedConnectionHelper {
+  // A class to help manage the active connection.  It handles the registration
+  // and unregistration with the CancelationSignal.  It also takes ownership of
+  // the connection and will delete it if the abort signal was sent early or
+  // when this class goes out of scope.
+  class ScopedConnectionHelper : public CancelationObserver {
    public:
-    // |manager| must outlive this. Takes ownership of |connection|.
-    ScopedConnectionHelper(ServerConnectionManager* manager,
-                           Connection* connection);
-    ~ScopedConnectionHelper();
+    ScopedConnectionHelper(CancelationSignal* cancelation_signal,
+                           scoped_ptr<Connection> connection);
+    virtual ~ScopedConnectionHelper();
     Connection* get();
+
+    // Called from a different thread when the CancelationObserver's
+    // RequestStop() is called and this class has been registered as a handler.
+    //
+    // Marked final because there's no way to safely override it.  See comment
+    // in this class' destructor.
+    virtual void OnStopRequested() OVERRIDE FINAL;
+
    private:
-    ServerConnectionManager* manager_;
+    CancelationSignal* const cancelation_signal_;
     scoped_ptr<Connection> connection_;
+
     DISALLOW_COPY_AND_ASSIGN(ScopedConnectionHelper);
   };