feat(memtrack): record per-benchmark pid in execution timestamps

Memory mode tracks the whole runner process tree, so between
Start/StopProfiler the harness parent and sibling worker processes emit
allocation events into every measured region. The CurrentBenchmark FIFO
command already carries the pid that ran each benchmark; capture it
alongside the uri in ExecutionTimestamps.bench_pid_by_ts (serde-default
for artifacts written before this field) so the parser can scope events
to the benchmarking process.

Author: GuillaumeLagrange

crates/runner-shared/src/artifacts/execution_timestamps.rs

@@ -1,3 +1,4 @@
+use libc::pid_t;
 use serde::{Deserialize, Serialize};
 
 use crate::fifo::MarkerType;
@@ -6,14 +7,25 @@ use crate::fifo::MarkerType;
 pub struct ExecutionTimestamps {
     pub uri_by_ts: Vec<(u64, String)>,
     pub markers: Vec<MarkerType>,
+    /// PID of the process that ran each benchmark, keyed by the timestamp of its
+    /// `CurrentBenchmark` command (the same timestamp used in `uri_by_ts`).
+    /// Consumers pair it positionally with `uri_by_ts` once both are sorted by
+    /// timestamp. Empty for artifacts written before pid capture existed.
+    #[serde(default)]
+    pub bench_pid_by_ts: Vec<(u64, pid_t)>,
 }
 impl super::ArtifactExt for ExecutionTimestamps {}
 
 impl ExecutionTimestamps {
-    pub fn new(uri_by_ts: &[(u64, String)], markers: &[crate::fifo::MarkerType]) -> Self {
+    pub fn new(
+        uri_by_ts: &[(u64, String)],
+        markers: &[crate::fifo::MarkerType],
+        bench_pid_by_ts: &[(u64, pid_t)],
+    ) -> Self {
         Self {
             uri_by_ts: uri_by_ts.to_vec(),
             markers: markers.to_vec(),
+            bench_pid_by_ts: bench_pid_by_ts.to_vec(),
         }
     }
 }

src/executor/shared/fifo.rs

@@ -170,6 +170,7 @@ impl RunnerFifo {
         std::process::ExitStatus,
     )> {
         let mut bench_order_by_timestamp = Vec::<(u64, String)>::new();
+        let mut bench_pid_by_ts = Vec::<(u64, pid_t)>::new();
         let mut bench_pids = HashSet::<pid_t>::new();
         let mut markers = Vec::<MarkerType>::new();
 
@@ -206,7 +207,9 @@ impl RunnerFifo {
                 // Fall through to shared implementation for standard commands
                 match &cmd {
                     FifoCommand::CurrentBenchmark { pid, uri } => {
-                        bench_order_by_timestamp.push((get_current_time(), uri.to_string()));
+                        let ts = get_current_time();
+                        bench_order_by_timestamp.push((ts, uri.to_string()));
+                        bench_pid_by_ts.push((ts, *pid));
                         bench_pids.insert(*pid);
                         self.send_cmd(FifoCommand::Ack).await?;
                     }
@@ -273,8 +276,11 @@ impl RunnerFifo {
                     debug!(
                         "Process terminated with status: {exit_status}, stopping the command handler"
                     );
-                    let marker_result =
-                        ExecutionTimestamps::new(&bench_order_by_timestamp, &markers);
+                    let marker_result = ExecutionTimestamps::new(
+                        &bench_order_by_timestamp,
+                        &markers,
+                        &bench_pid_by_ts,
+                    );
                     let fifo_data = FifoBenchmarkData {
                         integration,
                         bench_pids,