Add Buffer::cursor() method

This can be useful for providing access to buffers through core IO traits.

Author: khvzak

src/buffer.rs

@@ -1,3 +1,5 @@
+use std::io;
+
 #[cfg(feature = "serde")]
 use serde::ser::{Serialize, Serializer};
 
@@ -50,20 +52,33 @@ impl Buffer {
     #[track_caller]
     pub fn write_bytes(&self, offset: usize, bytes: &[u8]) {
         let lua = self.0.lua.lock();
-        let data = unsafe {
-            let (buf, size) = self.as_raw_parts(&lua);
-            std::slice::from_raw_parts_mut(buf, size)
-        };
+        let data = self.as_slice_mut(&lua);
         data[offset..offset + bytes.len()].copy_from_slice(bytes);
     }
 
+    /// Returns an adaptor implementing [`io::Read`], [`io::Write`] and [`io::Seek`] over the
+    /// buffer.
+    ///
+    /// Buffer operations are infallible, none of the read/write functions will return a Err.
+    pub fn cursor(self) -> impl io::Read + io::Write + io::Seek {
+        BufferCursor(self, 0)
+    }
+
     pub(crate) fn as_slice(&self, lua: &RawLua) -> &[u8] {
         unsafe {
             let (buf, size) = self.as_raw_parts(lua);
             std::slice::from_raw_parts(buf, size)
         }
     }
 
+    #[allow(clippy::mut_from_ref)]
+    fn as_slice_mut(&self, lua: &RawLua) -> &mut [u8] {
+        unsafe {
+            let (buf, size) = self.as_raw_parts(lua);
+            std::slice::from_raw_parts_mut(buf, size)
+        }
+    }
+
     #[cfg(feature = "luau")]
     unsafe fn as_raw_parts(&self, lua: &RawLua) -> (*mut u8, usize) {
         let mut size = 0usize;
@@ -78,6 +93,66 @@ impl Buffer {
     }
 }
 
+struct BufferCursor(Buffer, usize);
+
+impl io::Read for BufferCursor {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let lua = self.0 .0.lua.lock();
+        let data = self.0.as_slice(&lua);
+        if self.1 == data.len() {
+            return Ok(0);
+        }
+        let len = buf.len().min(data.len() - self.1);
+        buf[..len].copy_from_slice(&data[self.1..self.1 + len]);
+        self.1 += len;
+        Ok(len)
+    }
+}
+
+impl io::Write for BufferCursor {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        let lua = self.0 .0.lua.lock();
+        let data = self.0.as_slice_mut(&lua);
+        if self.1 == data.len() {
+            return Ok(0);
+        }
+        let len = buf.len().min(data.len() - self.1);
+        data[self.1..self.1 + len].copy_from_slice(&buf[..len]);
+        self.1 += len;
+        Ok(len)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+impl io::Seek for BufferCursor {
+    fn seek(&mut self, pos: io::SeekFrom) -> io::Result<u64> {
+        let lua = self.0 .0.lua.lock();
+        let data = self.0.as_slice(&lua);
+        let new_offset = match pos {
+            io::SeekFrom::Start(offset) => offset as i64,
+            io::SeekFrom::End(offset) => data.len() as i64 + offset,
+            io::SeekFrom::Current(offset) => self.1 as i64 + offset,
+        };
+        if new_offset < 0 {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "invalid seek to a negative position",
+            ));
+        }
+        if new_offset as usize > data.len() {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "invalid seek to a position beyond the end of the buffer",
+            ));
+        }
+        self.1 = new_offset as usize;
+        Ok(self.1 as u64)
+    }
+}
+
 #[cfg(feature = "serde")]
 impl Serialize for Buffer {
     fn serialize<S: Serializer>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error> {

tests/buffer.rs

@@ -1,5 +1,7 @@
 #![cfg(feature = "luau")]
 
+use std::io::{Read, Seek, SeekFrom, Write};
+
 use mlua::{Lua, Result, Value};
 
 #[test]
@@ -66,3 +68,56 @@ fn create_large_buffer() {
     let buf = lua.create_buffer_with_capacity(1024 * 1024).unwrap();
     assert_eq!(buf.len(), 1024 * 1024);
 }
+
+#[test]
+fn test_buffer_cursor() -> Result<()> {
+    let lua = Lua::new();
+    let mut cursor = lua.create_buffer(b"hello, world")?.cursor();
+
+    let mut data = Vec::new();
+    cursor.read_to_end(&mut data)?;
+    assert_eq!(data, b"hello, world");
+
+    // No more data to read
+    let mut one = [0u8; 1];
+    assert_eq!(cursor.read(&mut one)?, 0);
+
+    // Seek to start
+    cursor.seek(SeekFrom::Start(0))?;
+    cursor.read_exact(&mut one)?;
+    assert_eq!(one, [b'h']);
+
+    // Seek to end -5
+    cursor.seek(SeekFrom::End(-5))?;
+    let mut five = [0u8; 5];
+    cursor.read_exact(&mut five)?;
+    assert_eq!(&five, b"world");
+
+    // Seek to current -1
+    cursor.seek(SeekFrom::Current(-1))?;
+    cursor.read_exact(&mut one)?;
+    assert_eq!(one, [b'd']);
+
+    // Invalid seek
+    assert!(cursor.seek(SeekFrom::Current(-100)).is_err());
+    assert!(cursor.seek(SeekFrom::End(1)).is_err());
+
+    // Write data
+    let buf = lua.create_buffer_with_capacity(100)?;
+    cursor = buf.clone().cursor();
+
+    cursor.write_all(b"hello, ...")?;
+    cursor.seek(SeekFrom::Current(-3))?;
+    cursor.write_all(b"Rust!")?;
+
+    assert_eq!(&buf.read_bytes::<12>(0), b"hello, Rust!");
+
+    // Writing beyond the end of the buffer does nothing
+    cursor.seek(SeekFrom::End(0))?;
+    assert_eq!(cursor.write(b".")?, 0);
+
+    // Flush is no-op
+    cursor.flush()?;
+
+    Ok(())
+}