Infinite loop in MergingSceneIndex

[cpichard]

While running the arnold usd testsuite with USD 25.11, we discovered a case where the merging scene index enters an infinite loop.

It happens in the function HdMergingSceneIndex::_GetInputEntriesByPath. Here is a copy of the code involved:

    if (_inputs.size() < 5) {
        // It is common for merging scene indexes to have few inputs,
        // ex: 2 or 3.
        // In that case, skip looking through the path table and use the full
        // list.
        return _inputs;
    }

    // Find the closest enclosing path table entry.
    for (SdfPath p = primPath; !p.IsEmpty(); p = p.GetParentPath()) {
        const _InputEntriesByPathTable::const_iterator it =
            _inputsPathTable.find(p);
        if (it != _inputsPathTable.end()) {
            return it->second;
        }
    }

_inputs.size() >= 5, so the runtime enters the loop that should find the path in the _inputsPathTable.
For some good or bad reasons primPath = '.' and each iteration the loop adds the parent path to the original relative path, ending up looking for paths like "./../../../../..", etc, and it never ends as the path is never empty.
The process ends up crashing.

This happens with USD 25.11 and was found on macOS.

[jesschimein]

Filed as internal issue #USD-11667

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