x/website: playground execution doesn't finish normally, but no error message is shown

[NanoUser3000]

What is the URL of the page with the issue?

https://go.dev/play/

What is your user agent?

I don't think this will be relevant
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:145.0) Gecko/20100101 Firefox/145.0
(The problem occurred also in Google Chrome on another computer having Windows 11)

Screenshot

What did you do?

I wrote some code for Advent of Code 2015, day 10, part 1

Here is the code:

package main

import (
	"fmt"
	"strings"
)

func part1(input string) {
	input = strings.TrimSpace(input)

	for range 40 {
		input = lookAndSay(input)
	}

	fmt.Println("part 1:", len(input))
}

func lookAndSay(s string) string {
	var result string

	for i := 0; i < len(s); {
		b := s[i]
		var j = 0
		for ; i < len(s) && s[i] == b; i, j = i+1, j+1 {
		}
		result += fmt.Sprintf("%d%c", j, b)
	}
	return result
}

func main() {
	fmt.Println("Start")
	part1(input)
	fmt.Println("End")
}

const input = `3113322113` 

What did you expect to see?

I expected to see "Start", then answer, then "End" in the output.

Or, potentially, an error. (e.g. timeout). Now that I think about it, this is what I would actually expect, given how intensive the 40 iterations turn out to be (you can find the details below).

What did you see instead?

There is 40 in one place in the code. For 40 and more (100, 1_000_000) iterations the program seems to be stopped before it finishes, yet the output and message are as if everything went ok (with the beginning of the output present, and a gray message Program exited., which is the same for normal, successful execution.

For 20 iterations, the program finishes no problem, there isn't a bug in it.

Some more info

Program consistently finishes for 25 iterations, and consistently doesn't for 30.
Also, for 30 I occasionally get timeout running program (picture below), followed by what happens usually (and I believe is not acceptable): beginning of the output and a Program exited. message.

Here is some info about how computationally intense is the execution.
The following are times measured on my computer (some info fwiw: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz, 16GiB memory),
and lengths of string processed during execution.

iterations	time	string lengths
20	~0.05s	up to ~1600
30	~-0.15s	~23k
35	~0.7s	~90k
40	~8s	~330k

[gabyhelp]

Related Issues

• x/playground: program continues running after timeout #56543
• x/playground: timeout running program but locally it works fine #57294
• x/playground: playground times out when executing simple snippet #66741 (closed)
• x/tools/playground: program does not stop #41007 (closed)
• x/playground: no error reported when program gets stuck #53307

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[dmitshur]

CC @findleyr, @golang/tools-team.

[linsite]

The timeout of execution in playground is 5s.
The execution indeed turned out to be a timeout error. Playground server did return a response, but the message was not shown on the web page.

[NanoUser3000]

Wasn't able to replicate.
Now it shown "timeout running program" for the first time, but the next 5 times it didn't show it, and there is no message (as seen below). Note that the "timeout running program" is often above the output.