Fix data loss when compacting a document in fs_store

Problem: the `FsStore::compact` logic failed to load the data the caller
passed to it when compacting. This meant that if the compacted document
contained data which was not on disk then that data was lost.

Solution: load the document and merge it with existing on disk state

src/fs_store.rs

@@ -186,11 +186,14 @@ impl FsStore {
         tracing::debug!("compacting document");
         let paths = DocIdPaths::from(id);
 
+        let mut doc = automerge::Automerge::load(full_doc)
+            .map_err(|e| Error(ErrorKind::LoadDocToCompact(e)))?;
+
         // Load all the data we have into a doc
         match Chunks::load(&self.root, id) {
             Ok(Some(chunks)) => {
-                let doc = chunks
-                    .to_doc()
+                chunks
+                    .add_to_doc(&mut doc)
                     .map_err(|e| Error(ErrorKind::LoadDocToCompact(e)))?;
 
                 // Write the snapshot
@@ -225,11 +228,6 @@ impl FsStore {
                 }
             }
             Ok(None) => {
-                // The chunks are missing in storage for whatever reason
-                // Try to recreate the document from full_doc if possible
-                let doc = automerge::Automerge::load(full_doc)
-                    .map_err(|e| Error(ErrorKind::LoadDocToCompact(e)))?;
-
                 std::fs::create_dir_all(paths.level2_path(&self.root)).map_err(|e| {
                     Error(ErrorKind::CreateLevel2Path(
                         paths.level2_path(&self.root),
@@ -492,15 +490,16 @@ impl Chunks {
         }))
     }
 
-    fn to_doc(&self) -> Result<automerge::Automerge, automerge::AutomergeError> {
+    fn add_to_doc(&self, doc: &mut automerge::Automerge) -> Result<(), automerge::AutomergeError> {
         let mut bytes = Vec::new();
         for chunk in self.snapshots.values() {
             bytes.extend(chunk);
         }
         for chunk in self.incrementals.values() {
             bytes.extend(chunk);
         }
-        automerge::Automerge::load(&bytes)
+        doc.load_incremental(&bytes)?;
+        Ok(())
     }
 }
 
@@ -557,3 +556,43 @@ mod error {
         DeleteChunk(PathBuf, std::io::Error),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use automerge::{transaction::Transactable, AutoCommit, ReadDoc};
+    use tempfile::tempdir;
+
+    use crate::DocumentId;
+
+    use super::FsStore;
+
+    #[test]
+    fn compac_adds_new_changes_to_fs() {
+        let mut doc = AutoCommit::new();
+        doc.put(automerge::ROOT, "foo", "bar").unwrap();
+
+        let data_dir = tempdir().unwrap();
+
+        let doc_id = DocumentId::random();
+        let fs = FsStore::open(&data_dir).unwrap();
+        let change = doc
+            .get_changes(&[])
+            .into_iter()
+            .flat_map(|c| c.raw_bytes().to_vec())
+            .collect::<Vec<_>>();
+
+        fs.append(&doc_id, &change).unwrap();
+
+        doc.put(automerge::ROOT, "foo", "baz").unwrap();
+        let compacted = doc.save();
+
+        fs.compact(&doc_id, &compacted).unwrap();
+
+        let reloaded_raw = fs.get(&doc_id).unwrap().unwrap();
+        let reloaded = AutoCommit::load(&reloaded_raw).unwrap();
+        assert_eq!(
+            reloaded.get(&automerge::ROOT, "foo").unwrap().unwrap().0,
+            "baz".into()
+        );
+    }
+}