Limit recursion depth of mcpack2pb parser

[wwbmmm]

What problem does this PR solve?

Problem Summary:

The mcpack2pb parser (used by nshead_mcpack / ubrpc / public_pbrpc
requests) has no limit on the nesting depth of the input object/array.
For a protobuf message that references itself (e.g. a repeated
message field), the generated parser recurses once per nesting level,
so an input nested deep enough makes the recursion grow unbounded and
overflow the stack, crashing the process. The serializer has enforced
MAX_DEPTH (src/mcpack2pb/field_type.h) since the beginning, the
parser just never did.

What is changed and the side effects?

Changed:

• Thread the current nesting depth through mcpack2pb::UnparsedValue
  and the ObjectIterator / ArrayIterator constructors.
• Fail the parse (mark the input stream bad) as soon as the depth
  exceeds MAX_DEPTH (128), exactly like the serializer already does,
  so that the recursion stops at a bounded level instead of exhausting
  the stack.
• Add unit tests covering both a deeply-nested payload (rejected
  without crashing) and a moderately-nested one (parsed normally).

The depth limit is enforced on the iterator level, so existing code
generated by protoc-gen-mcpack is protected without regeneration:
no generated-code API is changed.

Side effects:

• Parsing now fails for mcpack input nested deeper than 128 levels
  (the same limit the serializer already applies). Such input is
  abnormal for any real workload, roughly matching
  json2pb_max_recursion_depth (100) used by the JSON parser.

• Performance effects: none (one integer compare per iterator).

• Breaking backward compatibility: no.

Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

Pull request overview

This PR adds a recursion/nesting depth limit to the mcpack2pb parser to prevent unbounded recursion on deeply nested mcpack objects/arrays (stack overflow), aligning parser behavior with the existing serializer depth cap.

Changes:

• Thread a nesting-depth counter through mcpack2pb::UnparsedValue and iterator constructors.
• Reject parsing when iterator nesting exceeds MAX_DEPTH to bound recursion.
• Add unit tests for deep (rejected) vs moderate (accepted) nesting on a small-stack thread.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/mcpack2pb/parser.h	Adds depth plumbing to UnparsedValue, ObjectIterator, and ArrayIterator declarations and constructors.
src/mcpack2pb/parser-inl.h	Enforces MAX_DEPTH in iterator init() and threads depth through as_object() / as_array().
src/mcpack2pb/parser.cpp	Propagates iterator depth into yielded UnparsedValue items during iteration.
test/brpc_mcpack2pb_unittest.cpp	Adds regression tests ensuring deep nesting is rejected without stack overflow (small-stack thread).

Suppressed comments (1)

src/mcpack2pb/parser.h:170

• The depth comment here says the top-level object starts at 0, but ObjectIterator(UnparsedValue&) initializes with value.depth() + 1 (and generated code builds the top-level UnparsedValue with depth=0). Clarifying that iterator depth is effectively 1-based in the common/auto-generated path will make the MAX_DEPTH enforcement easier to maintain.

    // Parse `n' bytes from `stream' as fields of an object.
    // `depth' is the nesting level of the object; the top-level object
    // starts at 0 and each nested iterator adds 1. Input nested deeper
    // than MAX_DEPTH is rejected to avoid stack overflow on unbounded
    // recursion (CWE-674), mirroring the serializer's limit.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

Suppressed comments (1)

src/mcpack2pb/parser.h:221

• ArrayIterator(InputStream*, size_t, size_t depth = 0) has the same issue as ObjectIterator: with the default depth of 0, nested UnparsedValue items will keep depth 0, so recursion depth never increases and MAX_DEPTH is never enforced for callers using this overload.

class ArrayIterator {
public:
    typedef UnparsedValue Field;

    ArrayIterator(InputStream* stream, size_t size, size_t depth = 0)
    { init(stream, size, depth); }
    explicit ArrayIterator(UnparsedValue& value)
    { init(value.stream(), value.size(), value.depth() + 1); }

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.