Add support for typed task Labels.

[PR98](#98) added support for associated a single untyped `Tag`
with each task and thread.  As we've gained experience with Tags, we've increasingly felt a need
to have a mechanism that is both better typed, and allows more than one tag to be associated with tasks.
This commit introduces `Labels`, which are inspired by `Extensions` in [the http crate](https://docs.rs/http/latest/http/struct.Extensions.html).
Users can attach any set of Labels to a task or thread, with the only caveat being that there can be
at most one Label for a given type T.  This is not too onerous a restriction, since one can use the
common [newtype idiom](https://doc.rust-lang.org/rust-by-example/generics/new_types.html) to easily
work around this.

For tracing, we also provide a newtype `TaskName` that can be converted to and from a `String`.
If the `TaskName` label is set for a task, tracing output will show the `TaskName` (in addition
to the `TaskId`) to make logs easier to read.

Since the current functionality provided by `Tag` is superseded by `Labels`, we also mark
`Tag` as deprecated.  They will be removed in a future release.

src/current.rs

@@ -5,9 +5,15 @@
 //! example, a tool that wants to check linearizability might want access to a global timestamp for
 //! events, which the [`context_switches`] function provides.
 
-use crate::runtime::execution::{ExecutionState, TASK_ID_TO_TAGS};
+#[allow(deprecated)]
+use crate::runtime::execution::TASK_ID_TO_TAGS;
+use crate::runtime::execution::{ExecutionState, LABELS};
 use crate::runtime::task::clock::VectorClock;
-pub use crate::runtime::task::{Tag, Taggable, TaskId};
+pub use crate::runtime::task::labels::Labels;
+pub use crate::runtime::task::{ChildLabelFn, TaskId, TaskName};
+#[allow(deprecated)]
+pub use crate::runtime::task::{Tag, Taggable};
+use std::fmt::Debug;
 use std::sync::Arc;
 
 /// The number of context switches that happened so far in the current Shuttle execution.
@@ -34,23 +40,74 @@ pub fn clock_for(task_id: TaskId) -> VectorClock {
     ExecutionState::with(|state| state.get_clock(task_id).clone())
 }
 
+/// Apply the given function to the Labels for the specified task
+pub fn with_labels_for_task<F, T>(task_id: TaskId, f: F) -> T
+where
+    F: FnOnce(&mut Labels) -> T,
+{
+    LABELS.with(|cell| {
+        let mut map = cell.borrow_mut();
+        let m = map.entry(task_id).or_default();
+        f(m)
+    })
+}
+
+/// Get a label of the given type for the specified task, if any
+#[inline]
+pub fn get_label_for_task<T: Clone + Debug + Send + Sync + 'static>(task_id: TaskId) -> Option<T> {
+    with_labels_for_task(task_id, |labels| labels.get().cloned())
+}
+
+/// Add the given label to the specified task, returning the old label for the type, if any
+#[inline]
+pub fn set_label_for_task<T: Clone + Debug + Send + Sync + 'static>(task_id: TaskId, value: T) -> Option<T> {
+    with_labels_for_task(task_id, |labels| labels.insert(value))
+}
+
+/// Remove a label of the given type for the specified task, returning the old label for the type, if any
+#[inline]
+pub fn remove_label_for_task<T: Clone + Debug + Send + Sync + 'static>(task_id: TaskId) -> Option<T> {
+    with_labels_for_task(task_id, |labels| labels.remove())
+}
+
+/// Get the debug name for a task
+#[inline]
+pub fn get_name_for_task(task_id: TaskId) -> Option<TaskName> {
+    get_label_for_task::<TaskName>(task_id)
+}
+
+/// Set the debug name for a task, returning the old name, if any
+#[inline]
+pub fn set_name_for_task(task_id: TaskId, task_name: impl Into<TaskName>) -> Option<TaskName> {
+    set_label_for_task::<TaskName>(task_id, task_name.into())
+}
+
+/// Gets the `TaskId` of the current task, or `None` if there is no current task.
+pub fn get_current_task() -> Option<TaskId> {
+    ExecutionState::with(|s| Some(s.try_current()?.id()))
+}
+
+/// Get the `TaskId` of the current task.  Panics if there is no current task.
+#[inline]
+pub fn me() -> TaskId {
+    get_current_task().unwrap()
+}
+
 /// Sets the `tag` field of the current task.
 /// Returns the `tag` which was there previously.
+#[allow(deprecated)]
 pub fn set_tag_for_current_task(tag: Arc<dyn Tag>) -> Option<Arc<dyn Tag>> {
     ExecutionState::set_tag_for_current_task(tag)
 }
 
 /// Gets the `tag` field of the current task.
+#[allow(deprecated)]
 pub fn get_tag_for_current_task() -> Option<Arc<dyn Tag>> {
     ExecutionState::get_tag_for_current_task()
 }
 
-/// Gets the `TaskId` of the current task, or `None` if there is no current task.
-pub fn get_current_task() -> Option<TaskId> {
-    ExecutionState::with(|s| Some(s.try_current()?.id()))
-}
-
 /// Gets the `tag` field of the specified task.
+#[allow(deprecated)]
 pub fn get_tag_for_task(task_id: TaskId) -> Option<Arc<dyn Tag>> {
     TASK_ID_TO_TAGS.with(|cell| {
         let map = cell.borrow();
@@ -59,6 +116,7 @@ pub fn get_tag_for_task(task_id: TaskId) -> Option<Arc<dyn Tag>> {
 }
 
 /// Sets the `tag` field of the specified task.
+#[allow(deprecated)]
 pub fn set_tag_for_task(task: TaskId, tag: Arc<dyn Tag>) -> Option<Arc<dyn Tag>> {
     ExecutionState::set_tag_for_task(task, tag)
 }

src/runtime/execution.rs

@@ -1,7 +1,8 @@
 use crate::runtime::failure::{init_panic_hook, persist_failure, persist_task_failure};
 use crate::runtime::storage::{StorageKey, StorageMap};
 use crate::runtime::task::clock::VectorClock;
-use crate::runtime::task::{Task, TaskId, DEFAULT_INLINE_TASKS};
+use crate::runtime::task::labels::Labels;
+use crate::runtime::task::{ChildLabelFn, Task, TaskId, TaskName, DEFAULT_INLINE_TASKS};
 use crate::runtime::thread::continuation::PooledContinuation;
 use crate::scheduler::{Schedule, Scheduler};
 use crate::thread::thread_fn;
@@ -18,6 +19,7 @@ use std::rc::Rc;
 use std::sync::Arc;
 use tracing::{trace, Span};
 
+#[allow(deprecated)]
 use super::task::Tag;
 
 // We use this scoped TLS to smuggle the ExecutionState, which is not 'static, across tasks that
@@ -28,9 +30,14 @@ scoped_thread_local! {
 
 thread_local! {
     #[allow(clippy::complexity)]
+    #[allow(deprecated)]
     pub(crate) static TASK_ID_TO_TAGS: RefCell<HashMap<TaskId, Arc<dyn Tag>>> = RefCell::new(HashMap::new());
 }
 
+thread_local! {
+    pub(crate) static LABELS: RefCell<HashMap<TaskId, Labels>> = RefCell::new(HashMap::new());
+}
+
 /// An `Execution` encapsulates a single run of a function under test against a chosen scheduler.
 /// Its only useful method is `Execution::run`, which executes the function to completion.
 ///
@@ -68,6 +75,11 @@ impl Execution {
             self.initial_schedule.clone(),
         ));
 
+        // Clear all Labels at the beginning of each execution
+        LABELS.with(|cell| {
+            cell.borrow_mut().clear();
+        });
+
         let _guard = init_panic_hook(config.clone());
 
         EXECUTION_STATE.set(&state, move || {
@@ -327,6 +339,33 @@ impl ExecutionState {
         Self::with(|s| s.current().id())
     }
 
+    fn set_labels_for_new_task(state: &ExecutionState, task_id: TaskId, name: Option<String>) {
+        LABELS.with(|cell| {
+            let mut map = cell.borrow_mut();
+
+            // If parent has labels, inherit them
+            if let Some(parent_task_id) = state.try_current().map(|t| t.id()) {
+                let parent_map = map.get(&parent_task_id);
+                if let Some(parent_map) = parent_map {
+                    let mut child_map = parent_map.clone();
+
+                    // If the parent has a `ChildLabelFn` set, use that to update the child's Labels
+                    if let Some(gen) = parent_map.get::<ChildLabelFn>() {
+                        (gen.0)(&mut child_map);
+                    }
+
+                    map.insert(task_id, child_map);
+                }
+            }
+
+            // Add any name assigned to the task to its set of Labels
+            if let Some(name) = name {
+                let m = map.entry(task_id).or_default();
+                m.insert(TaskName::from(name));
+            }
+        });
+    }
+
     /// Spawn a new task for a future. This doesn't create a yield point; the caller should do that
     /// if it wants to give the new task a chance to run immediately.
     pub(crate) fn spawn_future<F>(future: F, stack_size: usize, name: Option<String>) -> TaskId
@@ -339,6 +378,9 @@ impl ExecutionState {
 
             let task_id = TaskId(state.tasks.len());
             let tag = state.get_tag_or_default_for_current_task();
+
+            Self::set_labels_for_new_task(state, task_id, name.clone());
+
             let clock = state.increment_clock_mut(); // Increment the parent's clock
             clock.extend(task_id); // and extend it with an entry for the new task
 
@@ -372,6 +414,9 @@ impl ExecutionState {
             let parent_span_id = state.top_level_span.id();
             let task_id = TaskId(state.tasks.len());
             let tag = state.get_tag_or_default_for_current_task();
+
+            Self::set_labels_for_new_task(state, task_id, name.clone());
+
             let clock = if let Some(ref mut clock) = initial_clock {
                 clock
             } else {
@@ -424,6 +469,7 @@ impl ExecutionState {
         while Self::with(|state| state.storage.pop()).is_some() {}
 
         TASK_ID_TO_TAGS.with(|cell| cell.borrow_mut().clear());
+        LABELS.with(|cell| cell.borrow_mut().clear());
 
         #[cfg(debug_assertions)]
         Self::with(|state| state.has_cleaned_up = true);
@@ -651,6 +697,9 @@ impl ExecutionState {
         // 2) It creates a visual separation of scheduling decisions and `Task`-induced tracing.
         // Note that there is a case to be made for not `in_scope`-ing it, as that makes seeing the context
         // of the context switch clearer.
+        //
+        // Note also that changing this trace! statement requires changing the test `basic::labels::test_tracing_with_label_fn`
+        // which relies on this trace reporting the `runnable` tasks.
         self.top_level_span
             .in_scope(|| trace!(?runnable, next_task=?self.next_task));
 
@@ -669,18 +718,22 @@ impl ExecutionState {
 
     // Sets the `tag` field of the current task.
     // Returns the `tag` which was there previously.
+    #[allow(deprecated)]
     pub(crate) fn set_tag_for_current_task(tag: Arc<dyn Tag>) -> Option<Arc<dyn Tag>> {
         ExecutionState::with(|s| s.current_mut().set_tag(tag))
     }
 
+    #[allow(deprecated)]
     fn get_tag_or_default_for_current_task(&self) -> Option<Arc<dyn Tag>> {
         self.try_current().and_then(|current| current.get_tag())
     }
 
+    #[allow(deprecated)]
     pub(crate) fn get_tag_for_current_task() -> Option<Arc<dyn Tag>> {
         ExecutionState::with(|s| s.get_tag_or_default_for_current_task())
     }
 
+    #[allow(deprecated)]
     pub(crate) fn set_tag_for_task(task: TaskId, tag: Arc<dyn Tag>) -> Option<Arc<dyn Tag>> {
         ExecutionState::with(|s| s.get_mut(task).set_tag(tag))
     }