Inside the Go playground

Francesc Campoy Flores
Developer Advocate, Gopher
@francesc
campoy@golang.org
http://campoy.cat

* Agenda

- What is the Go playground

- What could go wrong

- What did we do to avoid it

- An animated ASCII train

* The Go playground

.image playground/img/play.png 500 _
.caption [[http://play.golang.org][play.golang.org]]

* De facto pastebin of the Go community

.image playground/img/share.png 500 _
.caption [[http://play.golang.org/p/bJYnajZ6Kp]]

* The Go tour

.image playground/img/tour.png 500 _
.caption [[/tour/][go.dev/tour]]

* Executable examples on documentation

.image playground/img/examples.png 500 _
.caption [[/pkg/strings/#example_Fields][go.dev/pkg/strings]]

* Executable code on blog posts

.image playground/img/blog.png 500 _
.caption [[/blog/slices][go.dev/blog/slices]]

* Executable slides

.play playground/hello.go

These slides are driven by the `present` Go tool

	go get code.google.com/go.tools/cmd/present

* Naive implementation

* Architecture

.image playground/img/arch.png 500 _

* Backend

Let's start with something simple

- receive code
- compile it
- run it

* What could go wrong?

.image playground/img/areyousure.png 500 _

###########
## Issues #
###########

* Resource exhaustion

* Exhausting memory on the stack

`stack`overflow`

.play playground/stack.go

The runtime catches the error and panics.

* Too much memory on the heap

`out`of`memory`

.play playground/heap.go

Again the runtime catches the error and panics.

* Too much CPU time

.play playground/loop.go

* Stealing resources by sleeping

.play playground/sleep.go

A sleeping program still consumes resources.

Easy way of having a Denial of Service attack.

* Accessing things you shouldn't

* File system access

User code shouldn't be able to modify the backend's file system.

- Reading sensitive information

- Installing backdoors

- General mayhem

.play playground/removeall.go /func main/,/^}/

* Network access

.play playground/http.go /func main/,/^}/

* Use your imagination

.image playground/img/cat.jpg 500 _

###################
# Countermeasures #
###################

* Countermeasures

* Restricting resource usage with ulimit

Default limits are not safe enough.

`ulimit` could solve this.

	-d    maximum size of data segment or heap (in kbytes)

	-s    maximum size of stack segment (in kbytes)

	-t    maximum CPU time (in seconds)

	-v    maximum size of virtual memory (in kbytes)

* Native Client

Originally designed to execute native code in Chrome safely.

NaCl defines restrictions on the binaries being executed.

The code runs in a sandbox isolated from the underlying OS.

- No file access
- No network access

.image playground/img/nacl.png 300 _

* Isolating process execution with NaCl

We use NaCl to:

- limit CPU time

- limit memory

- isolate from the filesystem

- isolate from the network

Process can only write to stdout/stderr.

* Limiting user time

"No sleeping in the playground."

Custom runtime with a fake `time` package.

	func Sleep(d time.Duration) {
		panic("No sleeping in the playground")
	}

* Restoring functionality

* Faking the file system

The `syscall` package is the only link between user code and the OS kernel.

The playground runtime has a custom `syscall` package.

File system operations operate on a fake in-memory file system.

.play playground/file.go /func main/,

* Faking the network

All network operations also use the `syscall` package.

The network stack is also faked in-memory.

.play playground/net.go /func main/,/^}/

* Faking the network (continued)

.play playground/net.go /func dial/,/^}/

##########
## TIME ##
##########

* Sleeping in the playground

Go is about concurrency.

We need to demonstrate concurrency in blog posts and talks.

And demonstrating concurrency without `time` is hard.

* What to do if an open source project lacks a feature?

.image playground/img/gopherbw.png 500 _

* File a bug!

.image playground/img/bug.png 500 _
.caption [[https://code.google.com/p/go/issues/detail?id=4280][bug 4280]]

* Normal behavior

There's a special goroutine managing timers `T`.

A goroutine `G` calls `time.Sleep`:

1. `G` adds a timer to the timer heap.

2. `G` puts itself to sleep.

3. `T` tells the OS to wake it when the next timer expires and puts itself to sleep.

4. When `T` is woken up it looks at the timer on the top of the heap, and wakes the corresponding goroutine.

* Intermission: deadlocks

.play playground/deadlock.go

Many flavors of deadlocks.

One common property: all goroutines are asleep.

* New behavior

A goroutine `G` calls `time.Sleep`:

1. `G` adds a timer to the timer heap.

2. `G` puts itself to sleep.

3. The scheduler detects a deadlock, checks the timer heap for pending timers.

4. The internal clock is advanced to the next timer expiration.

5. The corresponding goroutines are woken up.

* Sleeping fast

Faking time allows precise sleep durations.

.play playground/sleepfast.go

* So there's no actual sleep?

The playground's `write` syscall inserts a timestamp before each write.

The front end translates that into a series of "events" that the browser can play back.

.play playground/sleep.go /func main/,

Returns directly

	{
		"Errors":"",
		"Events":[
			{"Message":"Good night\n","Delay":0},
			{"Message":"Good morning\n","Delay":28800000000000}
		]
	}

* So the bug was fixed

.image playground/img/andrew.png _ 1000
.caption [[http://play.golang.org/p/3fv0L3-z0s]]

* And people were happy

.image playground/img/brad.png _ 1000
.caption [[http://play.golang.org/p/rX_3WcpUOZ]]

* Very happy

.image playground/img/jan.png _ 1000
.caption [[http://play.golang.org/p/P-Dk0NH_vf]]

.image playground/img/mattn.png _ 1000
.caption [[http://play.golang.org/p/NOycgN2i6b]]

* References

These slides: [[/talks/2014/playground.slide]]

More about the Go tour:

- Inside the Go playground: [[/blog/playground][go.dev/blog/playground]]

- The Go tour: [[/tour/][go.dev/tour]]

More about Go on NaCl:

- Running Go under Native Client: [[https://code.google.com/p/go-wiki/wiki/NativeClient]]

- Go 1.3 Native Client Support: [[/s/go13nacl]]