From 6290ef5647572456f3bfa22710c515f313d634e1 Mon Sep 17 00:00:00 2001
From: Lucas Pimentel <lucas.pimentel@datadoghq.com>
Date: Thu, 2 Jul 2026 18:04:45 -0400
Subject: [PATCH 1/2] fix(traces): avoid panic on empty /v0.6/stats payload
MIME-Version: 1.0
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An empty msgpack map request body deserializes to a stats payload with
no buckets, so indexing stats[0] to stamp the start time panicked with
"index out of bounds". Guard the empty case and return 202 Accepted as a
no-op instead. Adds a regression test.

🤖
---
 bottlecap/src/traces/stats_processor.rs | 39 ++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/bottlecap/src/traces/stats_processor.rs b/bottlecap/src/traces/stats_processor.rs
index 32bd76792..60f7eb26d 100644
--- a/bottlecap/src/traces/stats_processor.rs
+++ b/bottlecap/src/traces/stats_processor.rs
@@ -78,12 +78,20 @@ impl StatsProcessor for ServerlessStatsProcessor {
                 }
             };
 
+        // An empty stats payload (e.g. an empty msgpack map) has nothing to buffer, so
+        // treat it as a harmless no-op instead of indexing into an empty vec (which panics).
+        let Some(first_bucket) = stats.stats.first_mut() else {
+            debug!("Received empty trace stats payload; nothing to aggregate.");
+            return Ok((StatusCode::ACCEPTED, "Empty stats payload; nothing to aggregate.")
+                .into_response());
+        };
+
         let start = SystemTime::now();
         let timestamp = start
             .duration_since(UNIX_EPOCH)
             .unwrap_or_default()
             .as_nanos();
-        stats.stats[0].start = if let Ok(result) = u64::try_from(timestamp) {
+        first_bucket.start = if let Ok(result) = u64::try_from(timestamp) {
             result
         } else {
             let error_msg = "Error converting timestamp to u64";
@@ -109,3 +117,32 @@ impl StatsProcessor for ServerlessStatsProcessor {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use axum::body::Body;
+    use tokio::sync::mpsc;
+
+    #[tokio::test]
+    async fn empty_stats_payload_does_not_panic() {
+        // An empty msgpack map (0x80) deserializes to a ClientStatsPayload with an empty
+        // `stats` vec. Indexing `stats.stats[0]` used to panic here; assert we return 202
+        // Accepted instead and buffer nothing.
+        let req = Request::builder()
+            .body(Body::from(vec![0x80u8]))
+            .expect("failed to build request");
+
+        let (tx, mut rx) = mpsc::channel(1);
+        let response = ServerlessStatsProcessor {}
+            .process_stats(req, tx)
+            .await
+            .expect("process_stats returned an error");
+
+        assert_eq!(response.status(), StatusCode::ACCEPTED);
+        assert!(
+            rx.try_recv().is_err(),
+            "no stats payload should be buffered for an empty request"
+        );
+    }
+}

From 4ff0d099d7637e7784e700c7aa751c18cfecdf9b Mon Sep 17 00:00:00 2001
From: Lucas Pimentel <lucas.pimentel@datadoghq.com>
Date: Thu, 2 Jul 2026 18:04:47 -0400
Subject: [PATCH 2/2] style: apply cargo fmt to stats_processor
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

🤖
---
 bottlecap/src/traces/stats_processor.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/bottlecap/src/traces/stats_processor.rs b/bottlecap/src/traces/stats_processor.rs
index 60f7eb26d..91bb9e37a 100644
--- a/bottlecap/src/traces/stats_processor.rs
+++ b/bottlecap/src/traces/stats_processor.rs
@@ -82,7 +82,10 @@ impl StatsProcessor for ServerlessStatsProcessor {
         // treat it as a harmless no-op instead of indexing into an empty vec (which panics).
         let Some(first_bucket) = stats.stats.first_mut() else {
             debug!("Received empty trace stats payload; nothing to aggregate.");
-            return Ok((StatusCode::ACCEPTED, "Empty stats payload; nothing to aggregate.")
+            return Ok((
+                StatusCode::ACCEPTED,
+                "Empty stats payload; nothing to aggregate.",
+            )
                 .into_response());
         };