core/storage: Create FileMemoryStorage

This is basically a copy of DatabaseStorage but outside the fs compilation flag, this way, we can access MemoryIO regardless the storage medium.

core/storage/database.rs

@@ -70,3 +70,52 @@ impl DatabaseFile {
         Self { file }
     }
 }
+
+pub struct FileMemoryStorage {
+    file: Arc<dyn crate::io::File>,
+}
+
+unsafe impl Send for FileMemoryStorage {}
+unsafe impl Sync for FileMemoryStorage {}
+
+impl DatabaseStorage for FileMemoryStorage {
+    fn read_page(&self, page_idx: usize, c: Completion) -> Result<()> {
+        let r = match c {
+            Completion::Read(ref r) => r,
+            _ => unreachable!(),
+        };
+        let size = r.buf().len();
+        assert!(page_idx > 0);
+        if !(512..=65536).contains(&size) || size & (size - 1) != 0 {
+            return Err(LimboError::NotADB);
+        }
+        let pos = (page_idx - 1) * size;
+        self.file.pread(pos, c)?;
+        Ok(())
+    }
+
+    fn write_page(
+        &self,
+        page_idx: usize,
+        buffer: Arc<RefCell<Buffer>>,
+        c: Completion,
+    ) -> Result<()> {
+        let buffer_size = buffer.borrow().len();
+        assert!(buffer_size >= 512);
+        assert!(buffer_size <= 65536);
+        assert_eq!(buffer_size & (buffer_size - 1), 0);
+        let pos = (page_idx - 1) * buffer_size;
+        self.file.pwrite(pos, buffer, c)?;
+        Ok(())
+    }
+
+    fn sync(&self, c: Completion) -> Result<()> {
+        self.file.sync(c)
+    }
+}
+
+impl FileMemoryStorage {
+    pub fn new(file: Arc<dyn crate::io::File>) -> Self {
+        Self { file }
+    }
+}