I am unable to reproduce this. How do you run those tests? When I run make test using macOS native Docker I have different step numbers:

...golang.org/x/playground (master $=) % git a
* 76d1738 - (HEAD -> master, origin/master, origin/HEAD) playground: support examples as tests

... SKIPPED ...
Step 45/48 : WORKDIR /app
Removing intermediate container 7782ffd5187f
 ---> 87a0a8813a76
Step 46/48 : RUN /go/bin/playground test
 ---> Running in 8188688d1019
App (project ID: "") is NOT caching results
OK
Removing intermediate container 8188688d1019
 ---> 226e01540b07
Step 47/48 : EXPOSE 8080
 ---> Running in d7deedd5a872
Removing intermediate container d7deedd5a872
 ---> cebe18a41b3c
Step 48/48 : ENTRYPOINT ["/go/bin/playground"]
 ---> Running in df745592a4fd
Removing intermediate container df745592a4fd
 ---> 91f5b1decaa6
Successfully built 91f5b1decaa6
Successfully tagged playground:latest
go test
PASS
ok  	golang.org/x/playground	0.018s

Does it happen for the commit 76d1738?

That particular test is the only one expecting output of two different kinds in Events slice: [{"testing: warning: no tests to run\n" "stderr" "0s"} {"PASS\n" "stdout" "0s"}]

Trying to write to stderr and stdout gives inconsistent output when writing both to stderr and stdout at the same "playground time":

package main

import (
	"fmt"
	"os"
)

func main() {
	fmt.Fprintln(os.Stderr, "Error")
	fmt.Fprintln(os.Stdout, "Hello")
}

https://play.golang.org/p/8du1N7eAxmq gives

Hello
Error

https://play.golang.org/p/yky_PWrcZvu gives

Error1
Hello1

This program produces correct order in my local playground.

And then I got to this function: https://github.com/golang/go/blob/master/src/runtime/sys_nacl_amd64p32.s#L63

TEXT runtime·write(SB),NOSPLIT,$16-20
	// If using fake time and writing to stdout or stderr,
	// emit playback header before actual data.

242050d

ping @minux

The problem is not about patched runtime.write function. It's about how data got copied in os/exec.writeDescriptor:

	c.goroutine = append(c.goroutine, func() error {
		_, err := io.Copy(w, pr)
		pr.Close() // in case io.Copy stopped due to write error
		return err
	})

Since we have two parallel writers, for stderr and stdout, io.Copy happens in goroutines competing with each other. Then there is no guarantee that copy operations happen in correct order. Fixing this would mean patching os/exec, but I doubt that it's worth it.

Another crude temporary fix would be to patch sandbox code to capture both kind of output into the same writer Stdout():

	rec := new(Recorder)
	cmd.Stdout = rec.Stdout()
	cmd.Stderr = rec.Stdout()

Since stdout and stderr output looks exactly the same on playground UI, then we could apply this patch until we find a proper fix.

Of course we can resort to just fixing the test and closing the issue. Then spawn another issue for fixing stdout/stderr messed up order.

@andybons, what do you think?

Change https://golang.org/cl/104857 mentions this issue: playground: fix the order of interleaved stdout and stderr

Change https://golang.org/cl/105235 mentions this issue: runtime: make fake timestamps in Playground writes strict

Change https://golang.org/cl/105496 mentions this issue: playground: use strict fake timestamps for sorting output

Change https://golang.org/cl/106216 mentions this issue: playground: patch runtime with https://golang.org/cl/105235

Change https://golang.org/cl/106475 mentions this issue: playground: add integration test for https://golang.org/cl/105235