make Store::apply cancel safe (#53)

patch-db/src/store.rs

@@ -6,6 +6,7 @@ use std::sync::atomic::AtomicU64;
 use std::sync::Arc;
 
 use fd_lock_rs::FdLock;
+use json_patch::PatchError;
 use json_ptr::{JsonPointer, SegList};
 use lazy_static::lazy_static;
 use serde::{Deserialize, Serialize};
@@ -221,43 +222,63 @@ impl Store {
     pub(crate) async fn apply(&mut self, patch: DiffPatch) -> Result<Option<Arc<Revision>>, Error> {
         use tokio::io::AsyncWriteExt;
 
+        // eject if noop
         if (patch.0).0.is_empty() {
             return Ok(None);
         }
 
+        struct TentativeUpdated<'a> {
+            store: &'a mut Store,
+            undo: Option<json_patch::Undo<'a>>,
+        }
+        impl<'a> TentativeUpdated<'a> {
+            fn new(store: &'a mut Store, patch: &'a DiffPatch) -> Result<Self, PatchError> {
+                let undo = json_patch::patch(&mut store.persistent, &*patch)?;
+                store.revision += 1;
+                Ok(Self {
+                    store,
+                    undo: Some(undo),
+                })
+            }
+        }
+        impl<'a> Drop for TentativeUpdated<'a> {
+            fn drop(&mut self) {
+                if let Some(undo) = self.undo.take() {
+                    undo.apply(&mut self.store.persistent);
+                    self.store.revision -= 1;
+                }
+            }
+        }
+
         #[cfg(feature = "tracing")]
         tracing::trace!("Attempting to apply patch: {:?}", patch);
 
+        // apply patch in memory
         let patch_bin = serde_cbor::to_vec(&*patch)?;
-        let persistent_undo = json_patch::patch(&mut self.persistent, &*patch)?;
-        self.revision += 1;
-        if let Err(_e) = if self.revision % 4096 == 0 {
-            self.compress().await
+        let mut updated = TentativeUpdated::new(self, &patch)?;
+
+        if updated.store.revision % 4096 == 0 {
+            updated.store.compress().await?
         } else {
-            async {
-                if self.file.stream_position().await? != self.file_cursor {
-                    self.file.set_len(self.file_cursor).await?;
-                    self.file.seek(SeekFrom::Start(self.file_cursor)).await?;
-                }
-                self.file.write_all(&patch_bin).await?;
-                self.file.flush().await?;
-                self.file.sync_all().await?;
-                self.file_cursor += patch_bin.len() as u64;
-                Ok::<_, Error>(())
+            if updated.store.file.stream_position().await? != updated.store.file_cursor {
+                updated
+                    .store
+                    .file
+                    .set_len(updated.store.file_cursor)
+                    .await?;
+                updated
+                    .store
+                    .file
+                    .seek(SeekFrom::Start(updated.store.file_cursor))
+                    .await?;
             }
-            .await
-        } {
-            #[cfg(feature = "tracing")]
-            tracing::error!("Error saving patch to disk: {}, attempting to compress", _e);
-            if let Err(e) = self.compress().await {
-                #[cfg(feature = "tracing")]
-                tracing::error!("Compression failed: {}", e);
-                persistent_undo.apply(&mut self.persistent);
-                self.revision -= 1;
-                return Err(e);
-            }
-        };
-        drop(persistent_undo);
+            updated.store.file.write_all(&patch_bin).await?;
+            updated.store.file.flush().await?;
+            updated.store.file.sync_all().await?;
+            updated.store.file_cursor += patch_bin.len() as u64;
+        }
+        drop(updated.undo.take());
+        drop(updated);
 
         let id = self.revision;
         let res = Arc::new(Revision { id, patch });