Add fallback backend for all unsupported platforms

src/tree_store/page_store/file_backend/fallback.rs

@@ -0,0 +1,48 @@
+use crate::{DatabaseError, Result, StorageBackend};
+use std::fs::File;
+use std::io;
+use std::io::{Read, Seek, SeekFrom, Write};
+use std::sync::Mutex;
+
+/// Stores a database as a file on-disk.
+#[derive(Debug)]
+pub struct FileBackend {
+    file: Mutex<File>,
+}
+
+impl FileBackend {
+    /// Creates a new backend which stores data to the given file.
+    pub fn new(file: File) -> Result<Self, DatabaseError> {
+        Self {
+            file: Mutex::new(file),
+        }
+    }
+}
+
+impl StorageBackend for FileBackend {
+    fn len(&self) -> Result<u64, io::Error> {
+        Ok(self.file.lock().unwrap().metadata()?.len())
+    }
+
+    fn read(&self, offset: u64, len: usize) -> Result<Vec<u8>, io::Error> {
+        let mut result = vec![0; len];
+        let file = self.file.lock().unwrap();
+        file.seek(SeekFrom::Start(offset))?;
+        file.read_exact(&mut result)?;
+        Ok(result)
+    }
+
+    fn set_len(&self, len: u64) -> Result<(), io::Error> {
+        self.file.lock().unwrap().set_len(len)
+    }
+
+    fn sync_data(&self, _eventual: bool) -> Result<(), io::Error> {
+        self.file.lock().unwrap().sync_data()
+    }
+
+    fn write(&self, offset: u64, data: &[u8]) -> Result<(), io::Error> {
+        let file = self.file.lock().unwrap();
+        file.seek(SeekFrom::Start(offset))?;
+        file.write_all(data)
+    }
+}

src/tree_store/page_store/file_backend/mod.rs

@@ -9,6 +9,6 @@ mod windows;
 pub use windows::FileBackend;
 
 #[cfg(not(any(windows, unix, target_os = "wasi")))]
-mod unsupported;
+mod fallback;
 #[cfg(not(any(windows, unix, target_os = "wasi")))]
-pub use unsupported::FileBackend;
+pub use fallback::FileBackend;