Use an Arc for DependencyMap (#514)

Using Rc saves some atomic operations, at the cost of the result being
usable only from one thread. But lots of things are multi-threaded,
including Python, which causes issues trying to [wrap this
type](GothenburgBitFactory/taskchampion-py#11).
This isn't performance-critical code, so let's just us Arc.

This is a breaking change to the return type of
`Replica::dependency_map`.

This also adds a `Clone` derive to the type, just in case someone wants
the data without sharing.

src/depmap.rs

@@ -4,9 +4,9 @@ use uuid::Uuid;
 ///
 /// This information requires a scan of the working set to generate, so it is
 /// typically calculated once and re-used.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, Clone)]
 pub struct DependencyMap {
-    /// Edges of the dependency graph.  If (a, b) is in this array, then task a depends on tsak b.
+    /// Edges of the dependency graph.  If (a, b) is in this array, then task a depends on task b.
     edges: Vec<(Uuid, Uuid)>,
 }
 

src/replica.rs

@@ -11,7 +11,7 @@ use anyhow::Context;
 use chrono::{DateTime, Duration, Utc};
 use log::trace;
 use std::collections::HashMap;
-use std::rc::Rc;
+use std::sync::Arc;
 use uuid::Uuid;
 
 /// A replica represents an instance of a user's task data, providing an easy interface
@@ -37,7 +37,7 @@ pub struct Replica {
     added_undo_point: bool,
 
     /// The dependency map for this replica, if it has been calculated.
-    depmap: Option<Rc<DependencyMap>>,
+    depmap: Option<Arc<DependencyMap>>,
 }
 
 impl Replica {
@@ -151,7 +151,7 @@ impl Replica {
     ///
     /// Calculating this value requires a scan of the full working set and may not be performant.
     /// The [`TaskData`] API avoids generating this value.
-    pub fn dependency_map(&mut self, force: bool) -> Result<Rc<DependencyMap>> {
+    pub fn dependency_map(&mut self, force: bool) -> Result<Arc<DependencyMap>> {
         if force || self.depmap.is_none() {
             // note: we can't use self.get_task here, as that depends on a
             // DependencyMap
@@ -203,7 +203,7 @@ impl Replica {
                     }
                 }
             }
-            self.depmap = Some(Rc::new(dm));
+            self.depmap = Some(Arc::new(dm));
         }
 
         // at this point self.depmap is guaranteed to be Some(_)

src/task/task.rs

@@ -8,8 +8,8 @@ use chrono::prelude::*;
 use log::trace;
 use std::convert::AsRef;
 use std::convert::TryInto;
-use std::rc::Rc;
 use std::str::FromStr;
+use std::sync::Arc;
 use uuid::Uuid;
 
 /// A task, with a high-level interface.
@@ -28,7 +28,7 @@ pub struct Task {
     data: TaskData,
 
     // The dependency map for this replica, for rapidly computing synthetic tags.
-    depmap: Rc<DependencyMap>,
+    depmap: Arc<DependencyMap>,
 
     // True if an operation has alredy been emitted to update the `modified` property.
     updated_modified: bool,
@@ -80,7 +80,7 @@ fn uda_tuple_to_string(namespace: impl AsRef<str>, key: impl AsRef<str>) -> Stri
 }
 
 impl Task {
-    pub(crate) fn new(data: TaskData, depmap: Rc<DependencyMap>) -> Task {
+    pub(crate) fn new(data: TaskData, depmap: Arc<DependencyMap>) -> Task {
         Task {
             data,
             depmap,
@@ -538,8 +538,8 @@ mod test {
     use pretty_assertions::assert_eq;
     use std::collections::HashSet;
 
-    fn dm() -> Rc<DependencyMap> {
-        Rc::new(DependencyMap::new())
+    fn dm() -> Arc<DependencyMap> {
+        Arc::new(DependencyMap::new())
     }
 
     // Test task mutation by modifying a task and checking the assertions both on the