Ignore forked but not exec'd child processes

CHANGELOG.md

@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 ## Changed
+## Fixed
+- Lading will now ignore child processes when polling /proc if the children are
+  forked but not exec'd.
 
 ## [0.25.7]
 ## Changed

lading/src/observer/linux/procfs.rs

@@ -3,10 +3,7 @@ mod memory;
 mod stat;
 mod uptime;
 
-use std::{
-    collections::{VecDeque, hash_map::Entry},
-    io,
-};
+use std::{collections::VecDeque, io};
 
 use metrics::gauge;
 use nix::errno::Errno;
@@ -87,13 +84,36 @@ impl Sampler {
         let mut processes_found: i32 = 0;
         let mut pids_skipped: FxHashSet<i32> = FxHashSet::default();
 
+        // Clear process_info at the start of each poll. A process is capable of
+        // changing its details in key ways between polls.
+        self.process_info.clear();
+
         // Every sample run we collect all the child processes rooted at the
         // parent. As noted by the procfs documentation is this done by
         // dereferencing the `/proc/<pid>/root` symlink.
-        let mut pids: FxHashSet<i32> = FxHashSet::default();
+        // We must be sure to initialize the parent process info.
+        let parent_pid = self.parent.pid();
+        let parent_info = match initialize_process_info(parent_pid).await {
+            Ok(Some(info)) => info,
+            Ok(None) => {
+                warn!("Could not initialize parent process info");
+                return Ok(());
+            }
+            Err(e) => {
+                warn!("Error initializing parent process info: {:?}", e);
+                return Ok(());
+            }
+        };
+        self.process_info.insert(parent_pid, parent_info);
+
         let mut processes: VecDeque<Process> = VecDeque::with_capacity(16); // an arbitrary smallish number
-        processes.push_back(Process::new(self.parent.pid())?);
-        pids.insert(self.parent.pid());
+        processes.push_back(Process::new(parent_pid)?);
+
+        // We need to track the pids of processes we've already seen to avoid
+        // duplicate reads of process info. A child pid may be listed multiple
+        // times, see below loop for details.
+        let mut pids: FxHashSet<i32> = FxHashSet::default();
+        pids.insert(parent_pid);
 
         while let Some(process) = processes.pop_back() {
             // Search for child processes. This is done by querying for every
@@ -102,6 +122,11 @@ impl Sampler {
             // id equal to their pid. It's also possible for a pid to list
             // itself as a child so we reference the pid hashset above to avoid
             // infinite loops.
+            //
+            // We take special care to avoid processes that are forked but not
+            // exec'd. We do this because unexeced processes register the heap
+            // memory of the parent process in their smaps, leading to a double
+            // counting.
             if let Ok(tasks) = process.tasks() {
                 for task in tasks.flatten() {
                     if let Ok(mut children) = task.children() {
@@ -111,9 +136,37 @@ impl Sampler {
                         {
                             let pid = child.pid();
                             if !pids.contains(&pid) {
-                                // We have not seen this process and do need to
-                                // record it for child scanning and sampling if
-                                // it proves to be a process.
+                                // This is a new process. We initialize its
+                                // process info and then determine by
+                                // examination of the exe/cmdline if the process
+                                // is not exec'd -- meaning we will not poll it
+                                // -- or if it's a process in good standing and
+                                // we'll attempt to poll it.
+                                let child_info = match initialize_process_info(pid).await {
+                                    Ok(Some(info)) => info,
+                                    Ok(None) => continue,
+                                    Err(e) => {
+                                        warn!(
+                                            "Couldn't initialize process info for pid {}: {:?}",
+                                            pid, e
+                                        );
+                                        continue;
+                                    }
+                                };
+
+                                // Check if this is a forked but not execed
+                                // process.
+                                let parent_info = self
+                                    .process_info
+                                    .get(&process.pid())
+                                    .expect("parent process info should exist");
+                                if child_info.exe == parent_info.exe
+                                    && child_info.cmdline == parent_info.cmdline
+                                {
+                                    continue;
+                                }
+
+                                self.process_info.insert(pid, child_info);
                                 processes.push_back(child);
                                 pids.insert(pid);
                             }
@@ -137,9 +190,6 @@ impl Sampler {
             }
         }
 
-        // Update the process_info map to only hold processes seen by the current poll call.
-        self.process_info.retain(|pid, _| pids.contains(pid));
-
         gauge!("total_rss_bytes").set(aggr.rss as f64);
         gauge!("total_pss_bytes").set(aggr.pss as f64);
         gauge!("processes_found").set(processes_found as f64);
@@ -188,50 +238,8 @@ impl Sampler {
             return Ok(false);
         }
 
-        // If we haven't seen this process before, initialize its ProcessInfo.
-        match self.process_info.entry(pid) {
-            Entry::Occupied(_) => { /* Already initialized */ }
-            Entry::Vacant(entry) => {
-                let exe = match proc_exe(pid).await {
-                    Ok(exe) => exe,
-                    Err(e) => {
-                        warn!("Couldn't read exe for pid {}: {:?}", pid, e);
-                        // The pid may have exited since we scanned it or we may not
-                        // have sufficient permission.
-                        return Ok(false);
-                    }
-                };
-                let comm = match proc_comm(pid).await {
-                    Ok(comm) => comm,
-                    Err(e) => {
-                        warn!("Couldn't read comm for pid {}: {:?}", pid, e);
-                        // The pid may have exited since we scanned it or we may not
-                        // have sufficient permission.
-                        return Ok(false);
-                    }
-                };
-                let cmdline = match proc_cmdline(pid).await {
-                    Ok(cmdline) => cmdline,
-                    Err(e) => {
-                        warn!("Couldn't read cmdline for pid {}: {:?}", pid, e);
-                        // The pid may have exited since we scanned it or we may not
-                        // have sufficient permission.
-                        return Ok(false);
-                    }
-                };
-                let pid_s = format!("{pid}");
-                let stat_sampler = stat::Sampler::new();
-                entry.insert(ProcessInfo {
-                    cmdline,
-                    exe,
-                    comm,
-                    pid_s,
-                    stat_sampler,
-                });
-            }
-        }
-
-        // SAFETY: We've just inserted this pid into the map.
+        // SAFETY: We've inserted process info into this map when polling for
+        // child processes.
         let pinfo = self
             .process_info
             .get_mut(&pid)
@@ -348,6 +356,43 @@ impl Sampler {
     }
 }
 
+/// Initialize [`ProcessInfo`] for a process. Returns `None` if the process should be skipped.
+async fn initialize_process_info(pid: i32) -> Result<Option<ProcessInfo>, Error> {
+    let exe = match proc_exe(pid).await {
+        Ok(exe) => exe,
+        Err(e) => {
+            warn!("Couldn't read exe for pid {}: {:?}", pid, e);
+            return Ok(None);
+        }
+    };
+    let comm = match proc_comm(pid).await {
+        Ok(comm) => comm,
+        Err(e) => {
+            warn!("Couldn't read comm for pid {}: {:?}", pid, e);
+            return Ok(None);
+        }
+    };
+    let cmdline = match proc_cmdline(pid).await {
+        Ok(cmdline) => cmdline,
+        Err(e) => {
+            warn!("Couldn't read cmdline for pid {}: {:?}", pid, e);
+            return Ok(None);
+        }
+    };
+
+    let pid_s = format!("{pid}");
+    let stat_sampler = stat::Sampler::new();
+    let info = ProcessInfo {
+        cmdline,
+        exe,
+        comm,
+        pid_s,
+        stat_sampler,
+    };
+
+    Ok(Some(info))
+}
+
 /// Read `/proc/{pid}/comm`
 async fn proc_comm(pid: i32) -> Result<String, Error> {
     let comm_path = format!("/proc/{pid}/comm");