From eadadb7f339fa45a619e572ffd044ce5c91f7fe6 Mon Sep 17 00:00:00 2001
From: Colin Rofls <colin@cmyr.net>
Date: Thu, 13 Jul 2023 11:45:58 -0400
Subject: [PATCH] Halt runloop when jobs panic

This involved quite a bit of fiddling. I couldn't get rayon's built-in
panic_handler to work, so I ended up rolling my own solution, where we
manually catch unwinds that occur in spawned jobs.

When a panic occurs, we will set an atomic flag and return an error. We
check this flag before beginning any scheduled job, and return
immediately if it is set.
---
 fontc/src/main.rs     |  5 ++--
 fontc/src/work.rs     |  2 ++
 fontc/src/workload.rs | 64 +++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 66 insertions(+), 5 deletions(-)

diff --git a/fontc/src/main.rs b/fontc/src/main.rs
index 36f429a2..9f88be8b 100644
--- a/fontc/src/main.rs
+++ b/fontc/src/main.rs
@@ -12,8 +12,9 @@ fn main() -> Result<(), Error> {
             let ts = buf.timestamp_micros();
             writeln!(
                 buf,
-                "[{ts} {:?} {} {}] {}",
-                std::thread::current().id(),
+                "[{ts} {} {} {}] {}",
+                // we manually assign all threads a name
+                std::thread::current().name().unwrap_or("unknown"),
                 record.target(),
                 buf.default_level_style(record.level())
                     .value(record.level()),
diff --git a/fontc/src/work.rs b/fontc/src/work.rs
index 0b55e902..387ca0c6 100644
--- a/fontc/src/work.rs
+++ b/fontc/src/work.rs
@@ -18,6 +18,7 @@ use fontir::{
 pub enum AnyWorkError {
     Fe(FeError),
     Be(BeError),
+    Panic(String),
 }
 
 impl From<BeError> for AnyWorkError {
@@ -37,6 +38,7 @@ impl Display for AnyWorkError {
         match self {
             AnyWorkError::Be(e) => e.fmt(f),
             AnyWorkError::Fe(e) => e.fmt(f),
+            AnyWorkError::Panic(e) => write!(f, "Job panicked: '{e}'"),
         }
     }
 }
diff --git a/fontc/src/workload.rs b/fontc/src/workload.rs
index e44b7cdf..f5055687 100644
--- a/fontc/src/workload.rs
+++ b/fontc/src/workload.rs
@@ -1,6 +1,14 @@
 //! Tracking jobs to run
 
-use std::collections::{HashMap, HashSet};
+use rayon::ThreadPoolBuilder;
+use std::{
+    collections::{HashMap, HashSet},
+    panic::AssertUnwindSafe,
+    sync::{
+        atomic::{AtomicBool, Ordering},
+        Arc,
+    },
+};
 
 use crossbeam_channel::{Receiver, TryRecvError};
 use fontbe::orchestration::{AnyWorkId, Context as BeContext};
@@ -191,7 +199,15 @@ impl<'a> Workload<'a> {
         // Async work will send us it's ID on completion
         let (send, recv) = crossbeam_channel::unbounded::<(AnyWorkId, Result<(), AnyWorkError>)>();
 
-        rayon::in_place_scope(|scope| {
+        // a flag we set if we panic
+        let abort_queued_jobs = Arc::new(AtomicBool::new(false));
+        // build a custom threadpool. we use this to ensure all threads are named
+        let threadpool = ThreadPoolBuilder::new()
+            .thread_name(|n| format!("tp{n}"))
+            .build()
+            .expect("failed to init thread pool");
+
+        threadpool.in_place_scope(|scope| {
             // Whenever a task completes see if it was the last incomplete dependency of other task(s)
             // and spawn them if it was
             // TODO timeout and die it if takes too long to make forward progress or we're spinning w/o progress
@@ -228,8 +244,38 @@ impl<'a> Workload<'a> {
                         job.write_access.clone(),
                     );
 
+                    let abort = abort_queued_jobs.clone();
+
                     scope.spawn(move |_| {
-                        let result = work.exec(work_context);
+                        if abort.load(Ordering::Relaxed) {
+                            log::trace!("Aborting {:?}", work.id());
+                            return;
+                        }
+                        // # Unwind Safety
+                        //
+                        // 'unwind safety' does not impact memory safety, but
+                        // it may impact program correctness; the thread may have
+                        // left shared memory in an inconsistent state.
+                        //
+                        // I believe this is not a concern for us, as we cancel any
+                        // pending jobs after seeing a panic and jobs that depend
+                        // on state produced by the panicking job must be scheduled
+                        // after it. Unless we have jobs that are mutating
+                        // shared resources then I think this is fine.
+                        //
+                        // references:
+                        // <https://doc.rust-lang.org/nomicon/exception-safety.html#exception-safety>
+                        // <https://doc.rust-lang.org/std/panic/trait.UnwindSafe.html>
+                        let result = match std::panic::catch_unwind(AssertUnwindSafe(|| {
+                            work.exec(work_context)
+                        })) {
+                            Ok(result) => result,
+                            Err(err) => {
+                                let msg = get_panic_message(err);
+                                abort.store(true, Ordering::Relaxed);
+                                Err(AnyWorkError::Panic(msg))
+                            }
+                        };
                         if let Err(e) = send.send((id.clone(), result)) {
                             log::error!("Unable to write {id:?} to completion channel: {e}");
                         }
@@ -387,3 +433,15 @@ impl<'a> Workload<'a> {
         self.success.difference(&pre_success).cloned().collect()
     }
 }
+
+// taken from std:
+// <https://github.com/rust-lang/rust/blob/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/std/src/panicking.rs#L247-L253>
+fn get_panic_message(msg: Box<dyn std::any::Any + Send + 'static>) -> String {
+    match msg.downcast_ref::<&'static str>() {
+        Some(s) => s.to_string(),
+        None => match msg.downcast_ref::<String>() {
+            Some(s) => s.to_owned(),
+            None => "Box<dyn Any>".to_owned(),
+        },
+    }
+}