Remove cancel message in favor of CANCELED error type.

Most code doesn't care to distinguish between an error and cancelation,
as both abort the transfer. The few places that do care (e.g., the code
that decides whether to show an error to the user) can explicitly check
the type. This also allows cancellation to be easily propagated through
the error path from the source, instead of requiring a secondary
cancellation path.

Bug: 679313
Change-Id: I3c90ff55e7a58043535244a9d92cfc9e6814a4e6
Reviewed-on: https://chromium-review.googlesource.com/c/1437657
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626294}

remoting/host/file_transfer/file_transfer_message_handler.cc

@@ -82,9 +82,10 @@ void FileTransferMessageHandler::OnIncomingMessage(
       Close();
       break;
     case protocol::FileTransfer::kError:
-      LOG(ERROR) << "File transfer error from client: " << message.error();
-      FALLTHROUGH;
-    case protocol::FileTransfer::kCancel:
+      if (message.error().type() !=
+          protocol::FileTransfer_Error_Type_CANCELED) {
+        LOG(ERROR) << "File transfer error from client: " << message.error();
+      }
       Cancel();
       break;
     default:

remoting/proto/file_transfer.proto

@@ -33,27 +33,24 @@ message FileTransfer {
   // Next Id: 1
   message Success {}
 
-  // Bidirectional message canceling the transfer as the result of a user action
-  // or otherwise not due to an error. When Cancel is received, no more messages
-  // relating to the transfer should be sent, but the canceling end should be
-  // prepared to handle any messages that may already be on the wire.
-  // Next Id: 1
-  message Cancel {}
-
-  // Bidirectional message aborting the transfer due to an error. This may be
-  // sent by the sender to signal a read error, by the receiver to signal a
-  // write error, or by either side to signal any of the myriad of other things
-  // that can go wrong while attempting to transfer a file.
+  // Bidirectional message aborting the transfer due to an error or user
+  // cancellation. This may be sent by the sender to signal a read error, by the
+  // receiver to signal a write error, or by either side to signal any of the
+  // myriad of other things that can go wrong while attempting to transfer a
+  // file. When Error is received, no more messages relating to the transfer
+  // should be sent, but the end sending the error should be prepared to handle
+  // any messages that may already be on the wire.
   // Next Id: 6
   message Error {
     enum Type {
       UNSPECIFIED = 0;
-      UNEXPECTED_ERROR = 1;
-      PROTOCOL_ERROR = 2;
-      PERMISSION_DENIED = 3;
-      OUT_OF_DISK_SPACE = 4;
-      IO_ERROR = 5;
-      NOT_LOGGED_IN = 6;
+      CANCELED = 1;
+      UNEXPECTED_ERROR = 2;
+      PROTOCOL_ERROR = 3;
+      PERMISSION_DENIED = 4;
+      OUT_OF_DISK_SPACE = 5;
+      IO_ERROR = 6;
+      NOT_LOGGED_IN = 7;
     }
 
     // An error category to be used to select a user-displayed error message.
@@ -73,7 +70,6 @@ message FileTransfer {
     Data data = 2;
     End end = 3;
     Success success = 4;
-    Cancel cancel = 5;
-    Error error = 6;
+    Error error = 5;
   }
 }