Unbounded nested components can trigger `RecursionError` in `repr()` / `str()`

[gistrec]

Summary

Deeply nested iCalendar components can be parsed successfully, but formatting the resulting object with repr(), str(), or an f-string can raise RecursionError.

The recursive path appears to be Component.__repr__():

# src/icalendar/cal/component.py
def __repr__(self):
    """String representation of class with all of it's subcomponents."""
    subs = ", ".join(str(it) for it in self.subcomponents)
    return f"{self.name or type(self).__name__}({dict(self)}{', ' + subs if subs else ''})"

This can cause a denial-of-service in applications that parse untrusted .ics files and later log or format the parsed calendar object.

Reproducer

import icalendar

ics = (
    "BEGIN:VCALENDAR\r\n"
    "VERSION:2.0\r\n"
    "PRODID:-//x//x//EN\r\n"
    + "BEGIN:VEVENT\r\n" * 500
    + "UID:t@e\r\n"
    "DTSTAMP:20260101T000000Z\r\n"
    + "END:VEVENT\r\n" * 500
    + "END:VCALENDAR\r\n"
)

cal = icalendar.Calendar.from_ical(ics)  # succeeds
str(cal)  # RecursionError

Observed behavior

Operation	Result
Calendar.from_ical(ics)	succeeds
repr(cal) / str(cal) / f"{cal}"	raises RecursionError
cal.to_ical()	succeeds
cal.walk()	succeeds

The payload is small, around 13 KB.

Impact

This is a denial-of-service / robustness issue. Example scenarios:

• a CalDAV server or calendar import service logs a parsed calendar object;
• an email invitation parser includes the object in diagnostics;
• error reporting formats local variables containing the parsed calendar.

The crash is not in parsing itself, but in later diagnostic/logging paths.

Possible fixes

Any of these should mitigate the issue:

1. Reject excessive nesting during parsing, for example with a component nesting-depth limit.
2. Make __repr__ iterative or depth-limited.
3. Truncate subcomponent output in __repr__ after a safe depth.

A parser-level limit would prevent this class of issue more generally, but a bounded __repr__ may be less invasive if compatibility is a concern.

[gistrec]

The impact appears limited to denial-of-service in applications that format parsed calendar objects from untrusted input

I opened a minimal fix in #1371. It keeps the current output format for normal calendars and only replaces the recursive __repr__ implementation with an iterative one

[SashankBhamidi]

This is included in v7.1.2.