Merge pull request rails#25624 from tinco/actioncable_write_race

Fix race condition in websocket stream write

Author: matthewd

actioncable/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Protect against concurrent writes to a websocket connection from
+    multiple threads; the underlying OS write is not always threadsafe.
+
+    *Tinco Andringa*
+
+
 ## Rails 5.0.0 (June 30, 2016) ##
 
 *   Fix development reloading support: new cable connections are now correctly

actioncable/lib/action_cable/connection/stream.rb

@@ -1,3 +1,5 @@
+require 'thread'
+
 module ActionCable
   module Connection
     #--
@@ -11,6 +13,7 @@ def initialize(event_loop, socket)
         @stream_send   = socket.env['stream.send']
 
         @rack_hijack_io = nil
+        @write_lock = Mutex.new
       end
 
       def each(&callback)
@@ -27,8 +30,10 @@ def shutdown
       end
 
       def write(data)
-        return @rack_hijack_io.write(data) if @rack_hijack_io
-        return @stream_send.call(data) if @stream_send
+        @write_lock.synchronize do
+          return @rack_hijack_io.write(data) if @rack_hijack_io
+          return @stream_send.call(data) if @stream_send
+        end
       rescue EOFError, Errno::ECONNRESET
         @socket_object.client_gone
       end