Pretty: ensure recursionsLeft is not zero before decrementing it

[niooss-ledger]

When value_to_pretty is called with recursionsLeft=0 and an Array or Map type, container_to_pretty is called with recursionsLeft - 1 = -1. This breaks the recursion limit check.

In practice, this can be triggered with a CBOR data containing 1023 Arrays, a Tag and many more Arrays:

$ python3 -c 'import sys;sys.stdout.buffer.write(b"\x9f" * 1023 + b"\xc0\x9f" + b"\x9f" * 100000 + b"\xff" * 101024)' | ./bin/cbordump
Segmentation fault (core dumped)

This segmentation fault is due to the stack growing too much, due to the quantity of recursive calls.

Fix this by reporting a proper error instead. The same input now produces:

[_ [_ [_ ... [_ 0([
-: internal error: too many nested containers found in recursive function
_ <nesting too deep, recursion stopped>

Moreover, using fewer nested arrays works fine:

$ python3 -c 'import sys;sys.stdout.buffer.write(b"\x9f" * 1023 + b"\xc0\x9f" + b"\x9f" * 1024 + b"\xff" * 2048)' |./bin/cbordump
[_ [_ [_ ... [_ 0([_ <nesting too deep, recursion stopped>])] ... ]]]

[thiagomacieira]

Looks ok, but it would be easier to just change container_to_pretty:

    if (!recursionsLeft <= 0) {

[niooss-ledger]

Actually, I was not sure whether negative values of recursionsLeft were acceptable. In some projects, values such as -1 are used to mean "disable the limit".

Here it seems recursionsLeft is always expected to start at CBOR_PARSER_MAX_RECURSIONS and stay positive. So I agree with your comment and will update the Pull Request.