truncate excess output per execution request

fixes JuliaImages/Images.jl#558, fixes JuliaLang#455

src/stdio.jl

@@ -32,20 +32,41 @@ end
 const bufs = Dict{String,IOBuffer}()
 const stream_interval = 0.1
 const max_bytes = 10*1024
+# max output per code cell is 512 kb
+const max_output_per_request = 1 << 19
 
-"""Continually read from (size limited) Libuv/OS buffer into an (effectively unlimited) `IObuffer`
-to avoid problems when the Libuv/OS buffer gets full (https://github.com/JuliaLang/julia/issues/8789).
-Send data immediately when buffer contains more than `max_bytes` bytes. Otherwise, if data is available
-it will be sent every `stream_interval` seconds (see the Timers set up in watch_stdio)"""
+"""
+Continually read from (size limited) Libuv/OS buffer into an `IObuffer` to avoid problems when
+the Libuv/OS buffer gets full (https://github.com/JuliaLang/julia/issues/8789). Send data immediately
+when buffer contains more than `max_bytes` bytes. Otherwise, if data is available it will be sent every
+`stream_interval` seconds (see the Timers set up in watch_stdio). Truncate the output to `max_output_per_request`
+bytes per execution request since excessive output can bring browsers to a grinding halt.
+"""
 function watch_stream(rd::IO, name::AbstractString)
     task_local_storage(:IJulia_task, "read $name task")
     try
         buf = IOBuffer()
         bufs[name] = buf
+        counter = 0
+        curr_msg = ""
         while !eof(rd) # blocks until something is available
             nb = nb_available(rd)
+            parent_id_exists = "msg_id" in keys(execute_msg.header)
+            continuation = curr_msg == execute_msg.header["msg_id"]
+            # if this a new stream then reset the byte counter
+            if parent_id_exists && !continuation
+                curr_msg = execute_msg.header["msg_id"]
+                counter = 0
+            end
             if nb > 0
-                write(buf, read(rd, nb))
+                counter += nb
+                # if this stream as surpassed the maximum output limit then ignore future bytes
+                if parent_id_exists && continuation && counter > max_output_per_request
+                    (counter - nb <= max_output_per_request) && println(orig_STDOUT[], "Excessive output truncated.")
+                    read(rd, nb) # read from libuv/os buffer and throw data away
+                else
+                    write(buf, read(rd, nb))
+                end
             end
             if buf.size > 0
                 if buf.size >= max_bytes