Apply snapshots automatically on empty taskdbs

taskchampion/src/server/local.rs

@@ -160,6 +160,10 @@ impl Server for LocalServer {
         // the local server never requests a snapshot, so it should never get one
         unreachable!()
     }
+
+    fn get_snapshot(&mut self) -> anyhow::Result<Option<(VersionId, Snapshot)>> {
+        Ok(None)
+    }
 }
 
 #[cfg(test)]

taskchampion/src/server/remote/mod.rs

@@ -152,4 +152,25 @@ impl Server for RemoteServer {
             .send_bytes(ciphertext.as_ref())
             .map(|_| ())?)
     }
+
+    fn get_snapshot(&mut self) -> anyhow::Result<Option<(VersionId, Snapshot)>> {
+        let url = format!("{}/v1/client/snapshot", self.origin);
+        match self
+            .agent
+            .get(&url)
+            .set("X-Client-Key", &self.client_key.to_string())
+            .call()
+        {
+            Ok(resp) => {
+                let version_id = get_uuid_header(&resp, "X-Version-Id")?;
+                let ciphertext = Ciphertext::from_resp(resp, SNAPSHOT_CONTENT_TYPE)?;
+                let snapshot = ciphertext
+                    .open(&self.encryption_secret, version_id)?
+                    .payload;
+                Ok(Some((version_id, snapshot)))
+            }
+            Err(ureq::Error::Status(status, _)) if status == 404 => Ok(None),
+            Err(err) => Err(err.into()),
+        }
+    }
 }

taskchampion/src/server/test.rs

@@ -12,9 +12,8 @@ struct Version {
     history_segment: HistorySegment,
 }
 
-#[derive(Clone)]
-
 /// TestServer implements the Server trait with a test implementation.
+#[derive(Clone)]
 pub(crate) struct TestServer(Arc<Mutex<Inner>>);
 
 pub(crate) struct Inner {
@@ -35,6 +34,7 @@ impl TestServer {
             snapshot: None,
         })))
     }
+    // feel free to add any test utility functions here
 
     /// Get a boxed Server implementation referring to this TestServer
     pub(crate) fn server(&self) -> Box<dyn Server> {
@@ -51,6 +51,12 @@ impl TestServer {
         let inner = self.0.lock().unwrap();
         inner.snapshot.as_ref().cloned()
     }
+
+    /// Delete a version from storage
+    pub(crate) fn delete_version(&mut self, parent_version_id: VersionId) {
+        let mut inner = self.0.lock().unwrap();
+        inner.versions.remove(&parent_version_id);
+    }
 }
 
 impl Server for TestServer {
@@ -119,4 +125,9 @@ impl Server for TestServer {
         inner.snapshot = Some((version_id, snapshot));
         Ok(())
     }
+
+    fn get_snapshot(&mut self) -> anyhow::Result<Option<(VersionId, Snapshot)>> {
+        let inner = self.0.lock().unwrap();
+        Ok(inner.snapshot.clone())
+    }
 }

taskchampion/src/server/types.rs

@@ -65,4 +65,6 @@ pub trait Server {
 
     /// Add a snapshot on the server
     fn add_snapshot(&mut self, version_id: VersionId, snapshot: Snapshot) -> anyhow::Result<()>;
+
+    fn get_snapshot(&mut self) -> anyhow::Result<Option<(VersionId, Snapshot)>>;
 }

taskchampion/src/storage/mod.rs

@@ -105,6 +105,16 @@ pub trait StorageTxn {
     /// Note that this is the only way items are removed from the set.
     fn clear_working_set(&mut self) -> Result<()>;
 
+    /// Check whether this storage is entirely empty
+    fn is_empty(&mut self) -> Result<bool> {
+        let mut empty = true;
+        empty = empty && self.all_tasks()?.is_empty();
+        empty = empty && self.get_working_set()? == vec![None];
+        empty = empty && self.base_version()? == Uuid::nil();
+        empty = empty && self.operations()?.is_empty();
+        Ok(empty)
+    }
+
     /// Commit any changes made in the transaction.  It is an error to call this more than
     /// once.
     fn commit(&mut self) -> Result<()>;

taskchampion/src/taskdb/snapshot.rs

@@ -70,7 +70,6 @@ impl SnapshotTasks {
     }
 }
 
-#[allow(dead_code)]
 /// Generate a snapshot (compressed, unencrypted) for the current state of the taskdb in the given
 /// storage.
 pub(super) fn make_snapshot(txn: &mut dyn StorageTxn) -> anyhow::Result<Vec<u8>> {
@@ -78,7 +77,6 @@ pub(super) fn make_snapshot(txn: &mut dyn StorageTxn) -> anyhow::Result<Vec<u8>>
     all_tasks.encode()
 }
 
-#[allow(dead_code)]
 /// Apply the given snapshot (compressed, unencrypted) to the taskdb's storage.
 pub(super) fn apply_snapshot(
     txn: &mut dyn StorageTxn,
@@ -87,14 +85,8 @@ pub(super) fn apply_snapshot(
 ) -> anyhow::Result<()> {
     let all_tasks = SnapshotTasks::decode(snapshot)?;
 
-    // first, verify that the taskdb truly is empty
+    // double-check emptiness
-    let mut empty = true;
+    if !txn.is_empty()? {
-    empty = empty && txn.all_tasks()?.is_empty();
-    empty = empty && txn.get_working_set()? == vec![None];
-    empty = empty && txn.base_version()? == Uuid::nil();
-    empty = empty && txn.operations()?.is_empty();
-
-    if !empty {
         anyhow::bail!("Cannot apply snapshot to a non-empty task database");
     }
 

taskchampion/src/taskdb/sync.rs

@@ -16,6 +16,15 @@ pub(super) fn sync(
     txn: &mut dyn StorageTxn,
     avoid_snapshots: bool,
 ) -> anyhow::Result<()> {
+    // if this taskdb is entirely empty, then start by getting and applying a snapshot
+    if txn.is_empty()? {
+        trace!("storage is empty; attempting to apply a snapshot");
+        if let Some((version, snap)) = server.get_snapshot()? {
+            snapshot::apply_snapshot(txn, version, snap.as_ref())?;
+            trace!("applied snapshot for version {}", version);
+        }
+    }
+
     // retry synchronizing until the server accepts our version (this allows for races between
     // replicas trying to sync to the same server).  If the server insists on the same base
     // version twice, then we have diverged.
@@ -293,24 +302,23 @@ mod test {
     }
 
     #[test]
-    fn test_sync_adds_snapshot() -> anyhow::Result<()> {
+    fn test_sync_add_snapshot_start_with_snapshot() -> anyhow::Result<()> {
-        let test_server = TestServer::new();
+        let mut test_server = TestServer::new();
 
         let mut server: Box<dyn Server> = test_server.server();
         let mut db1 = newdb();
 
         let uuid = Uuid::new_v4();
-        db1.apply(Operation::Create { uuid }).unwrap();
+        db1.apply(Operation::Create { uuid })?;
         db1.apply(Operation::Update {
             uuid,
             property: "title".into(),
             value: Some("my first task".into()),
             timestamp: Utc::now(),
-        })
+        })?;
-        .unwrap();
 
         test_server.set_snapshot_urgency(SnapshotUrgency::High);
-        sync(&mut server, db1.storage.txn()?.as_mut(), false).unwrap();
+        sync(&mut server, db1.storage.txn()?.as_mut(), false)?;
 
         // assert that a snapshot was added
         let base_version = db1.storage.txn()?.base_version()?;
@@ -322,6 +330,26 @@ mod test {
         let tasks = SnapshotTasks::decode(&s)?.into_inner();
         assert_eq!(tasks[0].0, uuid);
 
+        // update the taskdb and sync again
+        db1.apply(Operation::Update {
+            uuid,
+            property: "title".into(),
+            value: Some("my first task, updated".into()),
+            timestamp: Utc::now(),
+        })?;
+        sync(&mut server, db1.storage.txn()?.as_mut(), false)?;
+
+        // delete the first version, so that db2 *must* initialize from
+        // the snapshot
+        test_server.delete_version(Uuid::nil());
+
+        // sync to a new DB and check that we got the expected results
+        let mut db2 = newdb();
+        sync(&mut server, db2.storage.txn()?.as_mut(), false)?;
+
+        let task = db2.get_task(uuid)?.unwrap();
+        assert_eq!(task.get("title").unwrap(), "my first task, updated");
+
         Ok(())
     }