Fix INT64_MIN block_count DoS in C SDK

[kalt2212]

Summary

Fixes a signed integer overflow / denial of service in the Apache Avro C SDK's array and map value readers.

Details

In read_array_value() and read_map_value() (lang/c/src/value-read.c), a negative block_count from the wire is negated with block_count * -1. When block_count == INT64_MIN, the negation is undefined behavior; in practice the value stays negative (or wraps), and the subsequent while (block_count != 0) loop iterates ~2^63 times, hanging the process on just 11 bytes of input (a trivial remote DoS).

The patch rejects INT64_MIN explicitly with avro_set_error("Invalid array block count") / avro_set_error("Invalid map block count") and returns EINVAL, mirroring how other invalid encodings are handled.

Testing

• PoC verified before the fix: 11-byte input with block_count = INT64_MIN hangs the process; after the fix it returns EINVAL immediately.
• Existing C SDK test suite passes.

AI tool use disclosure: AI was used in part for code audit and patch drafting.