Add NoSeek (#487)

* Add NoSeek

* Add NoSeek to implement Seek for types that won't
need to seek, but have the trait impl.

* fixup! Add NoSeek

add more docs

src/lib.rs

@@ -343,6 +343,9 @@ The feature `bits` enables the `bitvec` crate to use when reading and writing, w
 This however slows down the reading and writing process if your code doesn't use `bits` and the `bit_offset`
 in `from_bytes`.
 
+# NoSeek
+Unseekable streams such as [TcpStream](https://doc.rust-lang.org/std/net/struct.TcpStream.html) are supported through the [NoSeek](noseek::NoSeek) wrapper.
+
 */
 #![warn(missing_docs)]
 #![cfg_attr(not(feature = "std"), no_std)]
@@ -377,6 +380,7 @@ pub mod attributes;
 pub mod ctx;
 pub mod error;
 mod impls;
+pub mod noseek;
 pub mod prelude;
 pub mod reader;
 pub mod writer;

src/noseek.rs

@@ -0,0 +1,124 @@
+//! Wrapper type that provides a fake [`Seek`] implementation.
+
+#[cfg(not(feature = "std"))]
+use alloc::{string::String, vec::Vec};
+
+use crate::no_std_io::{Read, Result, Seek, SeekFrom, Write};
+use no_std_io::io::ErrorKind;
+
+/// A wrapper that provides a limited implementation of
+/// [`Seek`] for unseekable [`Read`] and [`Write`] streams.
+///
+/// This is useful when reading or writing from unseekable streams where deku
+/// does not *actually* need to seek to successfully parse or write the data.
+///
+/// This implementation was taken from [binrw](https://docs.rs/binrw/0.14.0/binrw/io/struct.NoSeek.html)
+pub struct NoSeek<T> {
+    /// The original stream.
+    inner: T,
+    /// The virtual position of the seekable stream.
+    pos: u64,
+}
+
+impl<T> NoSeek<T> {
+    /// Creates a new seekable wrapper for the given value.
+    pub fn new(inner: T) -> Self {
+        NoSeek { inner, pos: 0 }
+    }
+
+    /// Gets a mutable reference to the underlying value.
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.inner
+    }
+
+    /// Gets a reference to the underlying value.
+    pub fn get_ref(&self) -> &T {
+        &self.inner
+    }
+
+    /// Consumes this wrapper, returning the underlying value.
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+}
+
+impl<T> Seek for NoSeek<T> {
+    fn seek(&mut self, pos: SeekFrom) -> Result<u64> {
+        match pos {
+            SeekFrom::Start(n) if self.pos == n => Ok(n),
+            SeekFrom::Current(0) => Ok(self.pos),
+            // https://github.com/rust-lang/rust/issues/86442
+            #[cfg(feature = "std")]
+            _ => Err(std::io::Error::new(
+                ErrorKind::Other,
+                "seek on unseekable file",
+            )),
+            #[cfg(not(feature = "std"))]
+            _ => panic!("seek on unseekable file"),
+        }
+    }
+
+    #[cfg(feature = "std")]
+    fn stream_position(&mut self) -> Result<u64> {
+        Ok(self.pos)
+    }
+}
+
+impl<T: Read> Read for NoSeek<T> {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        let n = self.inner.read(buf)?;
+        self.pos += n as u64;
+        Ok(n)
+    }
+
+    #[cfg(feature = "std")]
+    fn read_vectored(&mut self, bufs: &mut [std::io::IoSliceMut<'_>]) -> Result<usize> {
+        let n = self.inner.read_vectored(bufs)?;
+        self.pos += n as u64;
+        Ok(n)
+    }
+
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
+        let n = self.inner.read_to_end(buf)?;
+        self.pos += n as u64;
+        Ok(n)
+    }
+
+    #[cfg(feature = "std")]
+    fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
+        let n = self.inner.read_to_string(buf)?;
+        self.pos += n as u64;
+        Ok(n)
+    }
+
+    fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
+        self.inner.read_exact(buf)?;
+        self.pos += buf.len() as u64;
+        Ok(())
+    }
+}
+
+impl<T: Write> Write for NoSeek<T> {
+    fn write(&mut self, buf: &[u8]) -> Result<usize> {
+        let n = self.inner.write(buf)?;
+        self.pos += n as u64;
+        Ok(n)
+    }
+
+    fn flush(&mut self) -> Result<()> {
+        self.inner.flush()
+    }
+
+    #[cfg(feature = "std")]
+    fn write_vectored(&mut self, bufs: &[std::io::IoSlice<'_>]) -> Result<usize> {
+        let n = self.inner.write_vectored(bufs)?;
+        self.pos += n as u64;
+        Ok(n)
+    }
+
+    fn write_all(&mut self, buf: &[u8]) -> Result<()> {
+        self.inner.write_all(buf)?;
+        self.pos += buf.len() as u64;
+        Ok(())
+    }
+}

tests/test_from_reader.rs

@@ -1,3 +1,4 @@
+use deku::noseek::NoSeek;
 use deku::prelude::*;
 use no_std_io::io::Seek;
 
@@ -32,6 +33,12 @@ fn test_from_reader_struct() {
     let (amt_read, ret_read) = TestDeku::from_reader((&mut c, total_read)).unwrap();
     assert_eq!(amt_read, 16);
     assert_eq!(TestDeku(0b1010), ret_read);
+
+    let inner = &c.into_inner();
+    let mut s = NoSeek::new(inner.as_slice());
+    let (amt_read, ret_read) = TestDeku::from_reader((&mut s, 0)).unwrap();
+    assert_eq!(amt_read, 4);
+    assert_eq!(TestDeku(0b0110), ret_read);
 }
 
 #[cfg(feature = "bits")]
@@ -57,4 +64,11 @@ fn test_from_reader_enum() {
     let (amt_read, ret_read) = TestDeku::from_reader((&mut c, first_amt_read)).unwrap();
     assert_eq!(amt_read, 6 + first_amt_read);
     assert_eq!(TestDeku::VariantB(0b10), ret_read);
+
+    c.rewind();
+    let inner = &c.into_inner();
+    let mut s = NoSeek::new(inner.as_slice());
+    let (amt_read, ret_read) = TestDeku::from_reader((&mut s, 0)).unwrap();
+    assert_eq!(first_amt_read, 8);
+    assert_eq!(TestDeku::VariantA(0b0110), ret_read);
 }

tests/test_seek.rs

@@ -1,4 +1,4 @@
-use deku::prelude::*;
+use deku::{noseek::NoSeek, prelude::*};
 use hexlit::hex;
 use rstest::*;
 
@@ -25,7 +25,6 @@ pub struct Test {
 )]
 fn test_seek(input: &[u8], expected: Test) {
     let input = input.to_vec();
-
     let mut cursor = std::io::Cursor::new(input.clone());
     let (_, ret_read) = Test::from_reader((&mut cursor, 0)).unwrap();
 
@@ -115,3 +114,36 @@ fn test_seek_ctx_end(input: &[u8], expected: SeekCtxBeforeEnd) {
     let _ = ret_read.to_writer(&mut writer, ()).unwrap();
     assert_eq!(buf, input);
 }
+
+#[derive(DekuRead, DekuWrite, Debug, PartialEq, Eq)]
+#[deku(seek_from_start = "0")]
+pub struct SeekCtxNoSeek {
+    byte: u8,
+}
+
+#[rstest(input, expected,
+    case(&hex!("03"), SeekCtxNoSeek { byte: 0x03 }),
+    case(&hex!("ff"), SeekCtxNoSeek { byte: 0xff }),
+)]
+fn test_seek_ctx_no_seek(input: &[u8], expected: SeekCtxNoSeek) {
+    use std::io::Cursor;
+    let input = input.to_vec();
+
+    let mut cursor = std::io::Cursor::new(input.clone());
+    let mut reader = Reader::new(&mut cursor);
+    let ret_read = SeekCtxNoSeek::from_reader_with_ctx(&mut reader, ()).unwrap();
+
+    assert_eq!(ret_read, expected);
+
+    let mut buf = vec![];
+    let mut cursor = Cursor::new(&mut buf);
+    let mut writer = Writer::new(&mut cursor);
+    let _ = ret_read.to_writer(&mut writer, ()).unwrap();
+    assert_eq!(buf, input);
+
+    let mut buf = vec![];
+    let mut cursor = NoSeek::new(&mut buf);
+    let mut writer = Writer::new(&mut cursor);
+    let _ = ret_read.to_writer(&mut writer, ()).unwrap();
+    assert_eq!(buf, input);
+}

tests/test_struct.rs

@@ -198,6 +198,11 @@ fn test_big_endian() {
     let new_bytes = a.to_bytes().unwrap();
     assert_eq!(bytes, &*new_bytes);
 
+    let bytes = &[0x11, 0x22, 0x33];
+    let a = A::from_bytes((bytes, 0)).unwrap().1;
+    let new_bytes = a.to_bytes().unwrap();
+    assert_eq!(bytes, &*new_bytes);
+
     #[derive(PartialEq, Debug, DekuRead, DekuWrite)]
     pub struct B {
         #[deku(bytes = "2", endian = "big")]