StateMonitor: Remove on_change from Mutex

Improves syncing speed by about 10-15% on Android

state_monitor/src/lib.rs

@@ -112,13 +112,12 @@ struct StateMonitorShared {
     id: MonitorId,
     parent: Option<StateMonitor>,
     inner: BlockingMutex<StateMonitorInner>,
+    on_change: watch::Sender<()>,
 }
 
 struct StateMonitorInner {
     values: IndexMap<String, MonitoredValueHandle>,
     children: IndexMap<MonitorId, ChildEntry>,
-    // TODO: Why is this in mutex?
-    on_change: watch::Sender<()>,
 }
 
 struct ChildEntry {
@@ -175,8 +174,8 @@ impl StateMonitor {
                     inner: BlockingMutex::new(StateMonitorInner {
                         values: Default::default(),
                         children: Default::default(),
-                        on_change: watch::channel(()).0,
                     }),
+                    on_change: watch::channel(()).0,
                 });
 
                 e.insert(ChildEntry {
@@ -201,7 +200,7 @@ impl StateMonitor {
             // of this function because given that `self` exists it must be that `refcount` doesn't
             // drop to zero anywhere in this function (and thus won't be removed from parent).
             self.shared.increment_refcount();
-            self.shared.changed(self.shared.lock_inner());
+            self.shared.changed();
         }
 
         Self { shared: child }
@@ -249,7 +248,9 @@ impl StateMonitor {
             }
         };
 
-        self.shared.changed(lock);
+        drop(lock); // Drop ASAP
+
+        self.shared.changed();
 
         MonitoredValue {
             name,
@@ -316,7 +317,7 @@ impl Drop for StateMonitor {
         }
 
         entry.remove();
-        parent.shared.changed(parent_inner);
+        parent.shared.changed();
     }
 }
 
@@ -337,8 +338,8 @@ impl StateMonitorShared {
             inner: BlockingMutex::new(StateMonitorInner {
                 values: Default::default(),
                 children: Default::default(),
-                on_change: watch::channel(()).0,
             }),
+            on_change: watch::channel(()).0,
         })
     }
 
@@ -361,19 +362,11 @@ impl StateMonitorShared {
     }
 
     fn subscribe(self: &Arc<Self>) -> watch::Receiver<()> {
-        self.lock_inner().on_change.subscribe()
+        self.on_change.subscribe()
     }
 
-    // TODO: Does this need the lock?
-    fn changed(&self, lock: BlockingMutexGuard<'_, StateMonitorInner>) {
-        lock.on_change.send(()).unwrap_or(());
-
-        // Let's not lock self and parent at the same time to avoid potential deadlocks.
-        drop(lock);
-
-        if let Some(parent) = &self.parent {
-            parent.shared.changed(parent.shared.lock_inner());
-        }
+    fn changed(&self) {
+        self.on_change.send(()).unwrap_or(());
     }
 
     fn lock_inner(&self) -> BlockingMutexGuard<'_, StateMonitorInner> {
@@ -433,40 +426,33 @@ impl<T> MonitoredValue<T> {
     pub fn get(&self) -> MutexGuardWrap<'_, T> {
         MutexGuardWrap {
             monitor: self.monitor.clone(),
-            guard: Some(self.value.lock().unwrap()),
+            guard: self.value.lock().unwrap(),
         }
     }
 }
 
 pub struct MutexGuardWrap<'a, T> {
     monitor: StateMonitor,
-    // This is only None in the destructor.
-    guard: Option<BlockingMutexGuard<'a, T>>,
+    guard: BlockingMutexGuard<'a, T>,
 }
 
 impl<'a, T> core::ops::Deref for MutexGuardWrap<'a, T> {
     type Target = T;
 
     fn deref(&self) -> &T {
-        self.guard.as_ref().unwrap()
+        &*self.guard
     }
 }
 
 impl<'a, T> core::ops::DerefMut for MutexGuardWrap<'a, T> {
     fn deref_mut(&mut self) -> &mut T {
-        &mut *(self.guard.as_mut().unwrap())
+        &mut *self.guard
     }
 }
 
 impl<'a, T> Drop for MutexGuardWrap<'a, T> {
     fn drop(&mut self) {
-        {
-            // Unlock this before we try to lock the parent monitor.
-            self.guard.take();
-        }
-        self.monitor
-            .shared
-            .changed(self.monitor.shared.lock_inner());
+        self.monitor.shared.changed();
     }
 }
 
@@ -481,7 +467,7 @@ impl<T> Drop for MonitoredValue<T> {
                 v.refcount -= 1;
                 if v.refcount == 0 {
                     e.remove();
-                    self.monitor.shared.changed(lock);
+                    self.monitor.shared.changed();
                 }
             }
             Entry::Vacant(_) => unreachable!(),