Fix: Denial of Service (Stack Overflow & O(N^2) CPU) in OptionConverter

[OxBat]

Summary

I identified a Denial of Service vulnerability in OptionConverter::substVars.
The component handles variable substitution (e.g., ${key}) recursively.

The Issues:

1. Stack Overflow (Crash): substVarsSafely is recursive but had no depth limit. A configuration with deeply nested variables (e.g., 20,000 levels) caused a Segmentation Fault.
2. Algorithmic Complexity (Freeze): The cycle detection logic used a linked list (LogStringChain) that required traversing the parent chain at every step, resulting in O(N^2) complexity.

The Fix:
I refactored the internal substVarsSafely function:

1. Replaced the custom LogStringChain linked list with a std::vector<LogString> for history tracking (faster lookup).
2. Added a MAX_SUBST_DEPTH (20). If recursion exceeds this depth, it stops and logs a warning, preventing the crash.

[rm5248]

can you add a unit test to ensure that this is working correctly? This unit test can be added to the already existing optioncovertertestcase.cpp

[rm5248]

there's actually already a test case for recursive:

logging-log4cxx/src/test/cpp/helpers/optionconvertertestcase.cpp

Line 149 in 28a443e

void varSubstRecursiveReferenceTest()

Why is that test not failing, or is that test not properly catching the problem that this is fixing?

[OxBat]

The existing test only catches circular dependencies (loops). My fix targets stack overflows caused by deep recursion without loops.

I just added varSubstDepthLimitTest to cover this specific case (it builds a chain of 25 items to ensure we stop gracefully at the limit).