Fix (and test) for a deadlock that can happen while waiting for protocol info (#12633)

# Description

The local socket PR introduced a `Waitable` type, which could either
hold a value or be waited on until a value is available. Unlike a
channel, it would always return that value once set.

However, one issue with this design was that there was no way to detect
whether a value would ever be written. This splits the writer into a
different type `WaitableMut`, so that when it is dropped, waiting
threads can fail (because they'll never get a value).

# Tests + Formatting

A test has been added to `stress_internals` to make sure this fails in
the right way.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

crates/nu-plugin/src/plugin/interface/engine.rs

@@ -10,7 +10,7 @@ use crate::{
         PluginCall, PluginCallId, PluginCallResponse, PluginCustomValue, PluginInput, PluginOption,
         PluginOutput, ProtocolInfo,
     },
-    util::Waitable,
+    util::{Waitable, WaitableMut},
 };
 use nu_protocol::{
     engine::Closure, Config, IntoInterruptiblePipelineData, LabeledError, ListStream, PipelineData,
@@ -85,6 +85,8 @@ impl std::fmt::Debug for EngineInterfaceState {
 pub struct EngineInterfaceManager {
     /// Shared state
     state: Arc<EngineInterfaceState>,
+    /// The writer for protocol info
+    protocol_info_mut: WaitableMut<Arc<ProtocolInfo>>,
     /// Channel to send received PluginCalls to. This is removed after `Goodbye` is received.
     plugin_call_sender: Option<mpsc::Sender<ReceivedPluginCall>>,
     /// Receiver for PluginCalls. This is usually taken after initialization
@@ -103,15 +105,17 @@ impl EngineInterfaceManager {
     pub(crate) fn new(writer: impl PluginWrite<PluginOutput> + 'static) -> EngineInterfaceManager {
         let (plug_tx, plug_rx) = mpsc::channel();
         let (subscription_tx, subscription_rx) = mpsc::channel();
+        let protocol_info_mut = WaitableMut::new();
 
         EngineInterfaceManager {
             state: Arc::new(EngineInterfaceState {
-                protocol_info: Waitable::new(),
+                protocol_info: protocol_info_mut.reader(),
                 engine_call_id_sequence: Sequence::default(),
                 stream_id_sequence: Sequence::default(),
                 engine_call_subscription_sender: subscription_tx,
                 writer: Box::new(writer),
             }),
+            protocol_info_mut,
             plugin_call_sender: Some(plug_tx),
             plugin_call_receiver: Some(plug_rx),
             engine_call_subscriptions: BTreeMap::new(),
@@ -233,7 +237,7 @@ impl InterfaceManager for EngineInterfaceManager {
         match input {
             PluginInput::Hello(info) => {
                 let info = Arc::new(info);
-                self.state.protocol_info.set(info.clone())?;
+                self.protocol_info_mut.set(info.clone())?;
 
                 let local_info = ProtocolInfo::default();
                 if local_info.is_compatible_with(&info)? {

crates/nu-plugin/src/plugin/interface/engine/tests.rs

@@ -300,8 +300,7 @@ fn manager_consume_errors_on_sending_other_messages_before_hello() -> Result<(),
 
 fn set_default_protocol_info(manager: &mut EngineInterfaceManager) -> Result<(), ShellError> {
     manager
-        .state
+        .protocol_info_mut
-        .protocol_info
         .set(Arc::new(ProtocolInfo::default()))
 }
 

crates/nu-plugin/src/plugin/interface/plugin.rs

@@ -12,7 +12,7 @@ use crate::{
         PluginOutput, ProtocolInfo, StreamId, StreamMessage,
     },
     sequence::Sequence,
-    util::{with_custom_values_in, Waitable},
+    util::{with_custom_values_in, Waitable, WaitableMut},
 };
 use nu_protocol::{
     ast::Operator, CustomValue, IntoInterruptiblePipelineData, IntoSpanned, ListStream,
@@ -138,6 +138,8 @@ impl Drop for PluginCallState {
 pub struct PluginInterfaceManager {
     /// Shared state
     state: Arc<PluginInterfaceState>,
+    /// The writer for protocol info
+    protocol_info_mut: WaitableMut<Arc<ProtocolInfo>>,
     /// Manages stream messages and state
     stream_manager: StreamManager,
     /// State related to plugin calls
@@ -159,18 +161,20 @@ impl PluginInterfaceManager {
         writer: impl PluginWrite<PluginInput> + 'static,
     ) -> PluginInterfaceManager {
         let (subscription_tx, subscription_rx) = mpsc::channel();
+        let protocol_info_mut = WaitableMut::new();
 
         PluginInterfaceManager {
             state: Arc::new(PluginInterfaceState {
                 source,
                 process: pid.map(PluginProcess::new),
-                protocol_info: Waitable::new(),
+                protocol_info: protocol_info_mut.reader(),
                 plugin_call_id_sequence: Sequence::default(),
                 stream_id_sequence: Sequence::default(),
                 plugin_call_subscription_sender: subscription_tx,
                 error: OnceLock::new(),
                 writer: Box::new(writer),
             }),
+            protocol_info_mut,
             stream_manager: StreamManager::new(),
             plugin_call_states: BTreeMap::new(),
             plugin_call_subscription_receiver: subscription_rx,
@@ -464,7 +468,7 @@ impl InterfaceManager for PluginInterfaceManager {
         match input {
             PluginOutput::Hello(info) => {
                 let info = Arc::new(info);
-                self.state.protocol_info.set(info.clone())?;
+                self.protocol_info_mut.set(info.clone())?;
 
                 let local_info = ProtocolInfo::default();
                 if local_info.is_compatible_with(&info)? {
@@ -631,7 +635,14 @@ impl PluginInterface {
 
     /// Get the protocol info for the plugin. Will block to receive `Hello` if not received yet.
     pub fn protocol_info(&self) -> Result<Arc<ProtocolInfo>, ShellError> {
-        self.state.protocol_info.get()
+        self.state.protocol_info.get().and_then(|info| {
+            info.ok_or_else(|| ShellError::PluginFailedToLoad {
+                msg: format!(
+                    "Failed to get protocol info (`Hello` message) from the `{}` plugin",
+                    self.state.source.identity.name()
+                ),
+            })
+        })
     }
 
     /// Write the protocol info. This should be done after initialization

crates/nu-plugin/src/plugin/interface/plugin/tests.rs

@@ -321,8 +321,7 @@ fn manager_consume_errors_on_sending_other_messages_before_hello() -> Result<(),
 
 fn set_default_protocol_info(manager: &mut PluginInterfaceManager) -> Result<(), ShellError> {
     manager
-        .state
+        .protocol_info_mut
-        .protocol_info
         .set(Arc::new(ProtocolInfo::default()))
 }
 

crates/nu-plugin/src/util/mod.rs

@@ -3,5 +3,5 @@ mod waitable;
 mod with_custom_values_in;
 
 pub(crate) use mutable_cow::*;
-pub use waitable::Waitable;
+pub use waitable::*;
 pub use with_custom_values_in::*;

crates/nu-plugin/src/util/waitable.rs

@@ -1,18 +1,36 @@
 use std::sync::{
     atomic::{AtomicBool, Ordering},
-    Condvar, Mutex, MutexGuard, PoisonError,
+    Arc, Condvar, Mutex, MutexGuard, PoisonError,
 };
 
 use nu_protocol::ShellError;
 
-/// A container that may be empty, and allows threads to block until it has a value.
+/// A shared container that may be empty, and allows threads to block until it has a value.
-#[derive(Debug)]
+///
+/// This side is read-only - use [`WaitableMut`] on threads that might write a value.
+#[derive(Debug, Clone)]
 pub struct Waitable<T: Clone + Send> {
+    shared: Arc<WaitableShared<T>>,
+}
+
+#[derive(Debug)]
+pub struct WaitableMut<T: Clone + Send> {
+    shared: Arc<WaitableShared<T>>,
+}
+
+#[derive(Debug)]
+struct WaitableShared<T: Clone + Send> {
     is_set: AtomicBool,
-    mutex: Mutex<Option<T>>,
+    mutex: Mutex<SyncState<T>>,
     condvar: Condvar,
 }
 
+#[derive(Debug)]
+struct SyncState<T: Clone + Send> {
+    writers: usize,
+    value: Option<T>,
+}
+
 #[track_caller]
 fn fail_if_poisoned<'a, T>(
     result: Result<MutexGuard<'a, T>, PoisonError<MutexGuard<'a, T>>>,
@@ -26,75 +44,138 @@ fn fail_if_poisoned<'a, T>(
     }
 }
 
-impl<T: Clone + Send> Waitable<T> {
+impl<T: Clone + Send> WaitableMut<T> {
-    /// Create a new empty `Waitable`.
+    /// Create a new empty `WaitableMut`. Call [`.reader()`] to get [`Waitable`].
-    pub fn new() -> Waitable<T> {
+    pub fn new() -> WaitableMut<T> {
-        Waitable {
+        WaitableMut {
+            shared: Arc::new(WaitableShared {
                 is_set: AtomicBool::new(false),
-            mutex: Mutex::new(None),
+                mutex: Mutex::new(SyncState {
+                    writers: 1,
+                    value: None,
+                }),
                 condvar: Condvar::new(),
+            }),
         }
     }
 
-    /// Wait for a value to be available and then clone it.
+    pub fn reader(&self) -> Waitable<T> {
+        Waitable {
+            shared: self.shared.clone(),
+        }
+    }
+
+    /// Set the value and let waiting threads know.
     #[track_caller]
-    pub fn get(&self) -> Result<T, ShellError> {
+    pub fn set(&self, value: T) -> Result<(), ShellError> {
-        let guard = fail_if_poisoned(self.mutex.lock())?;
+        let mut sync_state = fail_if_poisoned(self.shared.mutex.lock())?;
-        if let Some(value) = (*guard).clone() {
+        self.shared.is_set.store(true, Ordering::SeqCst);
-            Ok(value)
+        sync_state.value = Some(value);
+        self.shared.condvar.notify_all();
+        Ok(())
+    }
+}
+
+impl<T: Clone + Send> Default for WaitableMut<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<T: Clone + Send> Clone for WaitableMut<T> {
+    fn clone(&self) -> Self {
+        let shared = self.shared.clone();
+        shared
+            .mutex
+            .lock()
+            .expect("failed to lock mutex to increment writers")
+            .writers += 1;
+        WaitableMut { shared }
+    }
+}
+
+impl<T: Clone + Send> Drop for WaitableMut<T> {
+    fn drop(&mut self) {
+        // Decrement writers...
+        if let Ok(mut sync_state) = self.shared.mutex.lock() {
+            sync_state.writers = sync_state
+                .writers
+                .checked_sub(1)
+                .expect("would decrement writers below zero");
+        }
+        // and notify waiting threads so they have a chance to see it.
+        self.shared.condvar.notify_all();
+    }
+}
+
+impl<T: Clone + Send> Waitable<T> {
+    /// Wait for a value to be available and then clone it.
+    ///
+    /// Returns `Ok(None)` if there are no writers left that could possibly place a value.
+    #[track_caller]
+    pub fn get(&self) -> Result<Option<T>, ShellError> {
+        let sync_state = fail_if_poisoned(self.shared.mutex.lock())?;
+        if let Some(value) = sync_state.value.clone() {
+            Ok(Some(value))
+        } else if sync_state.writers == 0 {
+            // There can't possibly be a value written, so no point in waiting.
+            Ok(None)
         } else {
-            let guard = fail_if_poisoned(self.condvar.wait_while(guard, |g| g.is_none()))?;
+            let sync_state = fail_if_poisoned(
-            Ok((*guard)
+                self.shared
-                .clone()
+                    .condvar
-                .expect("checked already for Some but it was None"))
+                    .wait_while(sync_state, |g| g.writers > 0 && g.value.is_none()),
+            )?;
+            Ok(sync_state.value.clone())
         }
     }
 
     /// Clone the value if one is available, but don't wait if not.
     #[track_caller]
     pub fn try_get(&self) -> Result<Option<T>, ShellError> {
-        let guard = fail_if_poisoned(self.mutex.lock())?;
+        let sync_state = fail_if_poisoned(self.shared.mutex.lock())?;
-        Ok((*guard).clone())
+        Ok(sync_state.value.clone())
     }
 
     /// Returns true if value is available.
     #[track_caller]
     pub fn is_set(&self) -> bool {
-        self.is_set.load(Ordering::SeqCst)
+        self.shared.is_set.load(Ordering::SeqCst)
-    }
-
-    /// Set the value and let waiting threads know.
-    #[track_caller]
-    pub fn set(&self, value: T) -> Result<(), ShellError> {
-        let mut guard = fail_if_poisoned(self.mutex.lock())?;
-        self.is_set.store(true, Ordering::SeqCst);
-        *guard = Some(value);
-        self.condvar.notify_all();
-        Ok(())
-    }
-}
-
-impl<T: Clone + Send> Default for Waitable<T> {
-    fn default() -> Self {
-        Self::new()
     }
 }
 
 #[test]
 fn set_from_other_thread() -> Result<(), ShellError> {
-    use std::sync::Arc;
+    let waitable_mut = WaitableMut::new();
-
+    let waitable = waitable_mut.reader();
-    let waitable = Arc::new(Waitable::new());
-    let waitable_clone = waitable.clone();
 
     assert!(!waitable.is_set());
 
     std::thread::spawn(move || {
-        waitable_clone.set(42).expect("error on set");
+        waitable_mut.set(42).expect("error on set");
     });
 
-    assert_eq!(42, waitable.get()?);
+    assert_eq!(Some(42), waitable.get()?);
     assert_eq!(Some(42), waitable.try_get()?);
     assert!(waitable.is_set());
     Ok(())
 }
+
+#[test]
+fn dont_deadlock_if_waiting_without_writer() {
+    use std::time::Duration;
+
+    let (tx, rx) = std::sync::mpsc::channel();
+    let writer = WaitableMut::<()>::new();
+    let waitable = writer.reader();
+    // Ensure there are no writers
+    drop(writer);
+    std::thread::spawn(move || {
+        let _ = tx.send(waitable.get());
+    });
+    let result = rx
+        .recv_timeout(Duration::from_secs(10))
+        .expect("timed out")
+        .expect("error");
+    assert!(result.is_none());
+}

crates/nu_plugin_stress_internals/src/main.rs

@@ -11,6 +11,7 @@ use serde_json::{json, Value};
 struct Options {
     refuse_local_socket: bool,
     advertise_local_socket: bool,
+    exit_before_hello: bool,
     exit_early: bool,
     wrong_version: bool,
     local_socket_path: Option<String>,
@@ -28,6 +29,7 @@ pub fn main() -> Result<(), Box<dyn Error>> {
     let mut opts = Options {
         refuse_local_socket: has_env("STRESS_REFUSE_LOCAL_SOCKET"),
         advertise_local_socket: has_env("STRESS_ADVERTISE_LOCAL_SOCKET"),
+        exit_before_hello: has_env("STRESS_EXIT_BEFORE_HELLO"),
         exit_early: has_env("STRESS_EXIT_EARLY"),
         wrong_version: has_env("STRESS_WRONG_VERSION"),
         local_socket_path: None,
@@ -75,6 +77,11 @@ pub fn main() -> Result<(), Box<dyn Error>> {
         output.flush()?;
     }
 
+    // Test exiting without `Hello`
+    if opts.exit_before_hello {
+        std::process::exit(1)
+    }
+
     // Send `Hello` message
     write(
         &mut output,

tests/plugins/stress_internals.rs

@@ -1,3 +1,5 @@
+use std::{sync::mpsc, time::Duration};
+
 use nu_test_support::nu_with_plugins;
 
 fn ensure_stress_env_vars_unset() {
@@ -75,6 +77,30 @@ fn test_failing_local_socket_fallback() {
     assert!(result.out.contains("local_socket_path: None"));
 }
 
+#[test]
+fn test_exit_before_hello_stdio() {
+    ensure_stress_env_vars_unset();
+    // This can deadlock if not handled properly, so we try several times and timeout
+    for _ in 0..5 {
+        let (tx, rx) = mpsc::channel();
+        std::thread::spawn(move || {
+            let result = nu_with_plugins!(
+                cwd: ".",
+                envs: vec![
+                    ("STRESS_EXIT_BEFORE_HELLO", "1"),
+                ],
+                plugin: ("nu_plugin_stress_internals"),
+                "stress_internals"
+            );
+            let _ = tx.send(result);
+        });
+        let result = rx
+            .recv_timeout(Duration::from_secs(15))
+            .expect("timed out. probably a deadlock");
+        assert!(!result.status.success());
+    }
+}
+
 #[test]
 fn test_exit_early_stdio() {
     ensure_stress_env_vars_unset();