Reinstate is_changed

This reverts commit 02e513b.

src/engine/sim_engine/pool.rs

@@ -431,10 +431,10 @@ mod tests {
             .changed()
             .unwrap();
         let pool = engine.get_mut_pool(uuid).unwrap().1;
-        assert_eq!(
-            pool.destroy_filesystems(pool_name, &[]).unwrap().changed(),
-            None
-        );
+        assert!(match pool.destroy_filesystems(pool_name, &[]) {
+            Ok(uuids) => !uuids.is_changed(),
+            _ => false,
+        });
     }
 
     #[test]

src/engine/strat_engine/pool.rs

@@ -351,11 +351,8 @@ impl Pool for StratPool {
     ) -> StratisResult<SetDeleteAction<FilesystemUuid>> {
         let mut removed = Vec::new();
         for &uuid in fs_uuids {
-            if let Some(uuid) = self
-                .thin_pool
-                .destroy_filesystem(pool_name, uuid)?
-                .changed()
-            {
+            let changed = self.thin_pool.destroy_filesystem(pool_name, uuid)?;
+            if changed.is_changed() {
                 removed.push(uuid);
             }
         }

src/engine/types/actions.rs

@@ -13,6 +13,9 @@
 pub trait EngineAction {
     type Return;
 
+    /// Returns whether or not the action changed state.
+    fn is_changed(&self) -> bool;
+
     /// Returns the thing or things changed.
     fn changed(self) -> Option<Self::Return>;
 }
@@ -29,6 +32,13 @@ pub enum CreateAction<T> {
 impl<T> EngineAction for CreateAction<T> {
     type Return = T;
 
+    fn is_changed(&self) -> bool {
+        match *self {
+            CreateAction::Identity => false,
+            _ => true,
+        }
+    }
+
     fn changed(self) -> Option<T> {
         match self {
             CreateAction::Created(t) => Some(t),
@@ -52,6 +62,10 @@ impl<T> SetCreateAction<T> {
 impl<T> EngineAction for SetCreateAction<T> {
     type Return = Vec<T>;
 
+    fn is_changed(&self) -> bool {
+        !self.changed.is_empty()
+    }
+
     fn changed(self) -> Option<Vec<T>> {
         if self.changed.is_empty() {
             None
@@ -75,6 +89,13 @@ pub enum RenameAction<T> {
 impl<T> EngineAction for RenameAction<T> {
     type Return = T;
 
+    fn is_changed(&self) -> bool {
+        match *self {
+            RenameAction::Renamed(_) => true,
+            _ => false,
+        }
+    }
+
     fn changed(self) -> Option<T> {
         match self {
             RenameAction::Renamed(t) => Some(t),
@@ -95,6 +116,13 @@ pub enum DeleteAction<T> {
 impl<T> EngineAction for DeleteAction<T> {
     type Return = T;
 
+    fn is_changed(&self) -> bool {
+        match *self {
+            DeleteAction::Deleted(_) => true,
+            _ => false,
+        }
+    }
+
     fn changed(self) -> Option<T> {
         match self {
             DeleteAction::Deleted(t) => Some(t),