YuLi 6b16281ab0 fix: limit AMF decoder nesting depth (#4787) ...

### Motivation
- Prevent unbounded recursive-descent parsing in the AMF decoder that
allowed deeply nested AMF objects/arrays to exhaust the process stack
and cause a pre-auth remote DoS.

### Description
- Add an RAII depth guard `AMFDecoder::DepthGuard` and a per-decoder
`depth` counter with `kMaxDepth = 64` to `AMFDecoder` in
`src/Rtmp/amf.h`/`src/Rtmp/amf.cpp`.
- Instrument `load_object()`, `load_ecma()`, and `load_arr()` to create
a `DepthGuard` at entry so nesting is incremented and reliably
decremented on return or exception, and throw
`std::runtime_error("Maximum AMF nesting depth exceeded")` when the
limit is exceeded.
- Add `tests/test_amf.cpp` to assert that a 64-level nested AMF
container is accepted and a 65-level nested container is rejected with
the expected error.
- Keep non-nested parsing behavior unchanged; the guard only enforces a
maximum container nesting depth.

### Testing
- Configured the project with tests enabled using `cmake -S . -B build
-DENABLE_TESTS=ON`, which generated the `test_amf` target successfully.
- Built the test target with `cmake --build build --target test_amf` and
performed smoke compiles of modified sources, with `src/Rtmp/amf.cpp`
compiling to object (`/tmp/amf.o`) successfully.
- Compiled the new test source with `c++ -std=gnu++11 -c
tests/test_amf.cpp` successfully and verified `git diff --cached
--check` passed.

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[Codex
Task](https://chatgpt.com/codex/cloud/tasks/task_e_6a63c9326b9883209c857c0f4fa5d0cc)

2026-07-27 22:56:18 +08:00

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