move revisions into a vector indexed by durability

src/runtime.rs

@@ -155,17 +155,13 @@ where
     /// Read current value of the revision counter.
     #[inline]
     pub(crate) fn current_revision(&self) -> Revision {
-        self.shared_state.revision.load()
+        self.shared_state.revisions[0].load()
     }
 
     /// The revision in which constants last changed.
     #[inline]
     pub(crate) fn durability_last_changed_revision(&self, d: Durability) -> Revision {
-        if d == Durability::CONSTANT {
-            self.shared_state.constant_revision.load()
-        } else {
-            self.current_revision()
-        }
+        self.shared_state.revisions[d.index()].load()
     }
 
     /// Read current value of the revision counter.
@@ -289,7 +285,7 @@ where
         // To modify the revision, we need the lock.
         let _lock = self.shared_state.query_lock.write();
 
-        let old_revision = self.shared_state.revision.fetch_then_increment();
+        let old_revision = self.shared_state.revisions[0].fetch_then_increment();
         assert_eq!(current_revision, old_revision);
 
         let new_revision = current_revision.next();
@@ -444,11 +440,8 @@ where
     /// dependent on constants (which otherwise might not get
     /// re-evaluated).
     pub(crate) fn mark_durability_as_changed(&self, d: Durability) {
-        if d == Durability::CONSTANT {
-            self.runtime
-                .shared_state
-                .constant_revision
-                .store(self.new_revision);
+        for rev in &self.runtime.shared_state.revisions[1..=d.index()] {
+            rev.store(self.new_revision);
         }
     }
 }
@@ -470,28 +463,40 @@ struct SharedState<DB: Database> {
     /// to ensure a higher-level consistency property.
     query_lock: RwLock<()>,
 
-    /// Stores the current revision. This is an `AtomicU64` because
-    /// it may be *read* at any point without holding the
-    /// `query_lock`. Updates, however, require the `query_lock` to be
-    /// acquired. (See `query_lock` for details.)
-    revision: AtomicRevision,
-
     /// This is typically equal to `revision` -- set to `revision+1`
     /// when a new revision is pending (which implies that the current
     /// revision is canceled).
     pending_revision: AtomicRevision,
 
-    /// The last time that a value marked as "constant" changed.  Like
-    /// `revision` and `pending_revision`, this is readable without
-    /// any lock but requires the query-lock to be write-locked for
-    /// updates
-    constant_revision: AtomicRevision,
+    /// Stores the "last change" revision for values of each duration.
+    /// This vector is always of length at least 1 (for Durability 0)
+    /// but its total length depends on the number of durations. The
+    /// element at index 0 is special as it represents the "current
+    /// revision".  In general, we have the invariant that revisions
+    /// in here are *declining* -- that is, `revisions[i] >=
+    /// revisions[i + 1]`, for all `i`. This is because when you
+    /// modify a value with durability D, that implies that values
+    /// with durability less than D may have changed too.
+    revisions: Vec<AtomicRevision>,
 
     /// The dependency graph tracks which runtimes are blocked on one
     /// another, waiting for queries to terminate.
     dependency_graph: Mutex<DependencyGraph<DB>>,
 }
 
+impl<DB: Database> SharedState<DB> {
+    fn with_durabilities(durabilities: usize) -> Self {
+        SharedState {
+            next_id: AtomicUsize::new(1),
+            storage: Default::default(),
+            query_lock: Default::default(),
+            revisions: (0..durabilities).map(|_| AtomicRevision::start()).collect(),
+            pending_revision: AtomicRevision::start(),
+            dependency_graph: Default::default(),
+        }
+    }
+}
+
 impl<DB> std::panic::RefUnwindSafe for SharedState<DB>
 where
     DB: Database,
@@ -501,15 +506,7 @@ where
 
 impl<DB: Database> Default for SharedState<DB> {
     fn default() -> Self {
-        SharedState {
-            next_id: AtomicUsize::new(1),
-            storage: Default::default(),
-            query_lock: Default::default(),
-            revision: AtomicRevision::start(),
-            pending_revision: AtomicRevision::start(),
-            constant_revision: AtomicRevision::start(),
-            dependency_graph: Default::default(),
-        }
+        Self::with_durabilities(2)
     }
 }
 
@@ -527,7 +524,7 @@ where
         };
         fmt.debug_struct("SharedState")
             .field("query_lock", &query_lock)
-            .field("revision", &self.revision)
+            .field("revisions", &self.revisions)
             .field("pending_revision", &self.pending_revision)
             .finish()
     }
@@ -617,6 +614,10 @@ impl Durability {
     pub(crate) fn and(self, c: Durability) -> Durability {
         Durability(self.0 & c.0)
     }
+
+    fn index(self) -> usize {
+        self.0 as usize
+    }
 }
 
 #[derive(Clone, Debug)]