feat(memtrack): write dropped-event count to sidecar on shutdown

After the tracked command exits, read the eBPF dropped-event
counter, log a warning if non-zero, and persist the value as a
MemtrackMeta sidecar next to the events file. Lets the backend
mark a benchmark as incomplete instead of silently accepting a
truncated memory trace.

Refs COD-2577.

Author: not-matthias

crates/memtrack/src/main.rs

@@ -2,7 +2,9 @@ use clap::Parser;
 use ipc_channel::ipc;
 use memtrack::prelude::*;
 use memtrack::{MemtrackIpcMessage, Tracker, handle_ipc_message};
-use runner_shared::artifacts::{ArtifactExt, MemtrackArtifact, MemtrackEvent, MemtrackWriter};
+use runner_shared::artifacts::{
+    ArtifactExt, MemtrackArtifact, MemtrackEvent, MemtrackMeta, MemtrackWriter,
+};
 use std::os::unix::process::CommandExt;
 use std::path::{Path, PathBuf};
 use std::process::Command;
@@ -200,6 +202,30 @@ fn track_command(
         .join()
         .map_err(|_| anyhow::anyhow!("Failed to join writer thread"))??;
 
+    // Read the eBPF dropped-event counter after the run is complete.
+    // A non-zero value means the ring buffer overflowed and the trace is
+    // incomplete; we surface this both in the logs and in a sidecar file
+    // so the backend can invalidate the benchmark.
+    let dropped_events = match tracker_arc.lock() {
+        Ok(t) => t.dropped_events_count().unwrap_or_else(|e| {
+            warn!("Failed to read memtrack dropped-event counter: {e}");
+            0
+        }),
+        Err(_) => 0,
+    };
+    if dropped_events > 0 {
+        warn!(
+            "memtrack dropped {dropped_events} events because the ring buffer was full \
+             — this benchmark's memory trace is incomplete"
+        );
+    }
+
+    let meta = MemtrackMeta { dropped_events };
+    let meta_path = out_dir.join(MemtrackMeta::file_name(root_pid));
+    if let Err(e) = std::fs::write(&meta_path, serde_json::to_vec(&meta)?) {
+        warn!("Failed to write memtrack meta sidecar to {meta_path:?}: {e}");
+    }
+
     // IPC thread will exit when channel closes
     drop(ipc_handle);