Cycle debugging results in a runaway process that crashes the system

[glubsy]

• VS Code version: 1.21.1
• Extension version (available under the Extensions sidebar): XXX
• OS and version: Antergos (Arch Linux based) kernel 4.14.32-1-lts
• Python version (& distribution if applicable, e.g. Anaconda): python3.6
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): virtualenv

Actual behavior

itertools.cycle() returns an infinite sequence of items. When debugging, hovering the mouse over the cycle() object displays details about its items.
VSCode will allocate memory ad infinitum and crash the entire system when it runs out of memory (in less than 2 second on my machine with 16GB or RAM).

Expected behavior

No crash occurs, maybe only some items in the iterator should be displayed? With a hardcoded limit of only a few items perhaps? Or just not expanded at all just to be safe.

Steps to reproduce:

1. import itertools
2. Create an itertools.cycle() like so mycycle = itertools.cycle((1,2,3))
3. Start debugging
4. After the cycle class has been instantiated and populated (e.g. from a list), hover the mouse cursor above its symbol mycycle
5. Click "items" to expand the list in the popup details view.

Logs

No logs, system hard locks, no way to recover but to cold reboot the system.

I don't know TypeScript, but it looks like the problem is located around here maybe: https://github.com/DonJayamanne/pythonVSCode/blob/master/src/client/debugger/Main.ts#L637

Maybe cycles should be marked as not expandable?

[glubsy]

Initially reported there https://github.com/DonJayamanne/pythonVSCode/issues/1410

A bit confused as to which is the real upstream repo.

[brettcannon]

This is the main repo. The other one is Don's personal dev repo at this point.

[brettcannon]

I've reported this upstream as our experimental debugger raises an exception and I can confirm on the old debugger that a massive amount of memory suddenly gets consumed (at least on Windows 10 the inspection fails and then if you finish debugging the memory goes back down).

[brettcannon]

Part of the problem is there is no good way to know that an iterator is infinite, only finite if they define __len__ or __len_hint__. Probably just need to have a cap on the number of inspected items in the debugger.

[DonJayamanne]

Upstream issue microsoft/ptvsd#409