fix(traces): avoid panic on empty /v0.6/stats payload

[lucaspimentel]

Overview

The /v0.6/stats handler unconditionally indexed stats.stats[0] to stamp the payload start time. When the request body is an empty msgpack map (0x80), stats_utils::get_stats_from_request_body returns a ClientStatsPayload with an empty stats vec, so stats.stats[0] panicked:

index out of bounds: the len is 0 but the index is 0

This is a pre-existing bug reproducible in the Lambda binary; it surfaced while smoke-testing the test-mode binary.

The fix guards the empty case before indexing: an empty payload has nothing to buffer, so we return 202 Accepted as a harmless no-op. When at least one bucket exists, we set .start via stats.first_mut().

Testing

Added a regression test (empty_stats_payload_does_not_panic) that feeds an empty msgpack map body and asserts the handler returns 202 Accepted without panicking and buffers nothing to the aggregator.

🤖

[datadog-datadog-prod-us1]

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 2 Pipeline jobs failed

DataDog/datadog-lambda-extension | bottlecap (amd64, fips)     

DataDog/datadog-lambda-extension | layer (arm64)     

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[Copilot]

Pull request overview

Fixes a crash in the trace stats ingestion path (/v0.6/stats) by handling the edge case where the decoded ClientStatsPayload contains zero stats buckets, preventing an out-of-bounds panic in the Lambda extension’s traces subsystem.

Changes:

• Guard against empty stats.stats and treat empty payloads as a harmless no-op returning 202 Accepted.
• Replace direct indexing (stats.stats[0]) with first_mut() to safely access the first bucket when present.
• Add a regression test ensuring an empty msgpack map body does not panic and does not enqueue anything to the aggregator channel.