Merge 'Fix clear_page_cache method and rollback' from Preston Thorpe

Previously we were iterating over every entry in the page cache,
clearing the dirty flag from each page.

Reviewed-by: Pere Diaz Bou <pere-altea@homail.com>
Reviewed-by: Nikita Sivukhin (@sivukhin)

Closes #2988

core/storage/page_cache.rs

@@ -528,20 +528,36 @@ impl PageCache {
     }
 
     pub fn clear(&mut self) -> Result<(), CacheError> {
-        for e in self.entries.iter() {
+        if self.map.len() == 0 {
+            // Fast path: nothing to do.
+            self.clock_hand = NULL;
+            return Ok(());
+        }
+
+        for node in self.map.iter() {
+            let e = &self.entries[node.slot_index];
             if let Some(ref p) = e.page {
                 if p.is_dirty() {
                     return Err(CacheError::Dirty { pgno: p.get().id });
                 }
+            }
+        }
+        let mut used_slots = Vec::with_capacity(self.map.len());
+        for node in self.map.iter() {
+            used_slots.push(node.slot_index);
+        }
+        // don't touch already-free slots at all.
+        for &i in &used_slots {
+            if let Some(p) = self.entries[i].page.take() {
                 p.clear_loaded();
                 let _ = p.get().contents.take();
             }
+            self.entries[i].clear_ref();
+            self.entries[i].reset_links();
         }
-        self.entries.fill(PageCacheEntry::empty());
-        self.map.clear();
         self.clock_hand = NULL;
-        self.freelist.clear();
-        for i in (0..self.capacity).rev() {
+        self.map = PageHashMap::new(self.capacity);
+        for &i in used_slots.iter().rev() {
             self.freelist.push(i);
         }
         Ok(())
@@ -631,16 +647,6 @@ impl PageCache {
         self.capacity
     }
 
-    pub fn unset_dirty_all_pages(&mut self) {
-        let entries = &self.entries;
-        for entry in entries.iter() {
-            if entry.page.is_none() {
-                continue;
-            }
-            entry.page.as_ref().unwrap().clear_dirty();
-        }
-    }
-
     #[cfg(test)]
     fn verify_cache_integrity(&self) {
         let map = &self.map;

core/storage/pager.rs

@@ -1563,12 +1563,15 @@ impl Pager {
     /// of a rollback or in case we want to invalidate page cache after starting a read transaction
     /// right after new writes happened which would invalidate current page cache.
     pub fn clear_page_cache(&self) {
-        self.dirty_pages.borrow_mut().clear();
-        self.page_cache.write().unset_dirty_all_pages();
-        self.page_cache
-            .write()
-            .clear()
-            .expect("Failed to clear page cache");
+        let dirty_pages = self.dirty_pages.borrow();
+        let mut cache = self.page_cache.write();
+        for page_id in dirty_pages.iter() {
+            let page_key = PageCacheKey::new(*page_id);
+            if let Some(page) = cache.get(&page_key).unwrap_or(None) {
+                page.clear_dirty();
+            }
+        }
+        cache.clear().expect("Failed to clear page cache");
     }
 
     /// Checkpoint in Truncate mode and delete the WAL file. This method is _only_ to be called
@@ -2118,6 +2121,7 @@ impl Pager {
         is_write: bool,
     ) -> Result<(), LimboError> {
         tracing::debug!(schema_did_change);
+        self.clear_page_cache();
         if is_write {
             self.dirty_pages.borrow_mut().clear();
         } else {
@@ -2126,12 +2130,7 @@ impl Pager {
                 "dirty pages should be empty for read txn"
             );
         }
-        let mut cache = self.page_cache.write();
-
         self.reset_internal_states();
-
-        cache.unset_dirty_all_pages();
-        cache.clear().expect("failed to clear page cache");
         if schema_did_change {
             connection.schema.replace(connection._db.clone_schema()?);
         }