Fix a crash when an observation is quickly cancelled

Author: layoutSubviews

GRDB/ValueObservation/ValueWriteOnlyObserver.swift

@@ -198,7 +198,10 @@ extension ValueWriteOnlyObserver {
         // from a database access.
         try writer.unsafeReentrantWrite { db in
             // Fetch & Start observing the database
-            let fetchedValue = try fetchAndStartObservation(db)
+            guard let fetchedValue = try fetchAndStartObservation(db) else {
+                // Likely a GRDB bug
+                fatalError("can't start a cancelled or failed observation")
+            }
 
             // Reduce
             return reduceQueue.sync {
@@ -218,12 +221,11 @@ extension ValueWriteOnlyObserver {
         // Start from a write access, so that self can register as a
         // transaction observer.
         writer.asyncWriteWithoutTransaction { db in
-            let isNotifying = self.lock.synchronized { self.notificationCallbacks != nil }
-            guard isNotifying else { return /* Cancelled */ }
-            
             do {
                 // Fetch & Start observing the database
-                let fetchedValue = try self.fetchAndStartObservation(db)
+                guard let fetchedValue = try self.fetchAndStartObservation(db) else {
+                    return /* Cancelled */
+                }
 
                 // Reduce
                 //
@@ -260,14 +262,13 @@ extension ValueWriteOnlyObserver {
     /// By grouping the initial fetch and the beginning of observation in a
     /// single database access, we are sure that no concurrent write can happen
     /// during the initial fetch, and that we won't miss any future change.
-    private func fetchAndStartObservation(_ db: Database) throws -> Reducer.Fetched {
+    private func fetchAndStartObservation(_ db: Database) throws -> Reducer.Fetched? {
         // TODO: [SR-214] remove -Opt suffix when we only support Xcode 12.5.1+
         let (eventsOpt, fetchOpt) = lock.synchronized {
             (notificationCallbacks?.events, databaseAccess?.fetch)
         }
         guard let events = eventsOpt, let fetch = fetchOpt else {
-            // Likely a GRDB bug
-            fatalError("can't start a cancelled or failed observation")
+            return nil /* Cancelled */
         }
 
         switch trackingMode {