feat(tvix/nix-compat): add BytesReader r/7894

This adds AsyncRead counterpart for read_bytes.

Change-Id: I751da9944984c7a523abee305f8f8a050e705f04
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11385
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: Brian Olsen <me@griff.name>
Autosubmit: flokli <flokli@flokli.de>

3 files changed, 472 insertions, 1 deletions

diff --git a/tvix/nix-compat/src/wire/bytes_reader.rs b/tvix/nix-compat/src/wire/bytes_reader.rs
new file mode 100644
index 000000000000..b1dcebcc6092
--- /dev/null
+++ b/tvix/nix-compat/src/wire/bytes_reader.rs
@@ -0,0 +1,465 @@
+use pin_project_lite::pin_project;
+use std::{
+    ops::RangeBounds,
+    task::{ready, Poll},
+};
+use tokio::io::AsyncRead;
+
+use crate::wire::bytes::padding_len;
+
+use super::bytes_writer::{BytesPacketPosition, LEN_SIZE};
+
+pin_project! {
+    /// Reads a "bytes wire packet" from the underlying reader.
+    /// The format is the same as in [crate::wire::bytes::read_bytes],
+    /// however this structure provides a [AsyncRead] interface,
+    /// allowing to not having to pass around the entire payload in memory.
+    ///
+    /// After being constructed with the underlying reader and an allowed size,
+    /// subsequent requests to poll_read will return payload data until the end
+    /// of the packet is reached.
+    ///
+    /// Internally, it will first read over the size packet, filling payload_size,
+    /// ensuring it fits allowed_size, then return payload data.
+    /// It will only signal EOF (returning `Ok(())` without filling the buffer anymore)
+    /// when all padding has been successfully consumed too.
+    ///
+    /// This also means, it's important for a user to always read to the end,
+    /// and not just call read_exact - otherwise it might not skip over the
+    /// padding, and return garbage when reading the next packet.
+    ///
+    /// In case of an error due to size constraints, or in case of not reading
+    /// all the way to the end (and getting a EOF), the underlying reader is no
+    /// longer usable and might return garbage.
+    pub struct BytesReader<R, S>
+    where
+    R: AsyncRead,
+    S: RangeBounds<u64>,
+
+    {
+        #[pin]
+        inner: R,
+
+        allowed_size: S,
+        payload_size: [u8; 8],
+        state: BytesPacketPosition,
+    }
+}
+
+impl<R, S> BytesReader<R, S>
+where
+    R: AsyncRead + Unpin,
+    S: RangeBounds<u64>,
+{
+    /// Constructs a new BytesReader, using the underlying passed reader.
+    pub fn new(r: R, allowed_size: S) -> Self {
+        Self {
+            inner: r,
+            allowed_size,
+            payload_size: [0; 8],
+            state: BytesPacketPosition::Size(0),
+        }
+    }
+}
+/// Returns an error if the passed usize is 0.
+fn ensure_nonzero_bytes_read(bytes_read: usize) -> Result<usize, std::io::Error> {
+    if bytes_read == 0 {
+        Err(std::io::Error::new(
+            std::io::ErrorKind::UnexpectedEof,
+            "underlying reader returned EOF",
+        ))
+    } else {
+        Ok(bytes_read)
+    }
+}
+
+impl<R, S> AsyncRead for BytesReader<R, S>
+where
+    R: AsyncRead,
+    S: RangeBounds<u64>,
+{
+    fn poll_read(
+        self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+        buf: &mut tokio::io::ReadBuf<'_>,
+    ) -> Poll<std::io::Result<()>> {
+        let mut this = self.project();
+
+        // Use a loop, so we can deal with (multiple) state transitions.
+        loop {
+            match *this.state {
+                BytesPacketPosition::Size(LEN_SIZE) => {
+                    // used in case an invalid size was signalled.
+                    Err(std::io::Error::new(
+                        std::io::ErrorKind::InvalidData,
+                        "signalled package size not in allowed range",
+                    ))?
+                }
+                BytesPacketPosition::Size(pos) => {
+                    // try to read more of the size field.
+                    // We wrap a BufRead around this.payload_size here, and set_filled.
+                    let mut read_buf = tokio::io::ReadBuf::new(this.payload_size);
+                    read_buf.advance(pos);
+                    ready!(this.inner.as_mut().poll_read(cx, &mut read_buf))?;
+
+                    ensure_nonzero_bytes_read(read_buf.filled().len() - pos)?;
+
+                    let total_size_read = read_buf.filled().len();
+                    if total_size_read == LEN_SIZE {
+                        // If the entire payload size was read, parse it
+                        let payload_size = u64::from_le_bytes(*this.payload_size);
+
+                        if !this.allowed_size.contains(&payload_size) {
+                            // If it's not in the allowed
+                            // range, transition to failure mode
+                            // `BytesPacketPosition::Size(LEN_SIZE)`, where only
+                            // an error is returned.
+                            *this.state = BytesPacketPosition::Size(LEN_SIZE)
+                        } else if payload_size == 0 {
+                            // If the payload size is 0, move on to reading padding directly.
+                            *this.state = BytesPacketPosition::Padding(0)
+                        } else {
+                            // Else, transition to reading the payload.
+                            *this.state = BytesPacketPosition::Payload(0)
+                        }
+                    } else {
+                        // If we still need to read more of payload size, update
+                        // our position in the state.
+                        *this.state = BytesPacketPosition::Size(total_size_read)
+                    }
+                }
+                BytesPacketPosition::Payload(pos) => {
+                    let signalled_size = u64::from_le_bytes(*this.payload_size);
+                    // We don't enter this match arm at all if we're expecting empty payload
+                    debug_assert!(signalled_size > 0, "signalled size must be larger than 0");
+
+                    // Read from the underlying reader into buf
+                    // We cap the ReadBuf to the size of the payload, as we
+                    // don't want to leak padding to the caller.
+                    let bytes_read = ensure_nonzero_bytes_read({
+                        // Reducing these two u64 to usize on 32bits is fine - we
+                        // only care about not reading too much, not too less.
+                        let mut limited_buf = buf.take((signalled_size - pos) as usize);
+                        ready!(this.inner.as_mut().poll_read(cx, &mut limited_buf))?;
+                        limited_buf.filled().len()
+                    })?;
+
+                    // SAFETY: we just did populate this, but through limited_buf.
+                    unsafe { buf.assume_init(bytes_read) }
+                    buf.advance(bytes_read);
+
+                    if pos + bytes_read as u64 == signalled_size {
+                        // If we now read all payload, transition to padding
+                        // state.
+                        *this.state = BytesPacketPosition::Padding(0);
+                    } else {
+                        // if we didn't read everything yet, update our position
+                        // in the state.
+                        *this.state = BytesPacketPosition::Payload(pos + bytes_read as u64);
+                    }
+
+                    // We return from poll_read here.
+                    // This is important, as any error (or even Pending) from
+                    // the underlying reader on the next read (be it padding or
+                    // payload) would require us to roll back buf, as generally
+                    // a AsyncRead::poll_read may not advance the buffer in case
+                    // of a nonsuccessful read.
+                    // It can't be misinterpreted as EOF, as we definitely *did*
+                    // write something into buf if we come to here (we pass
+                    // `ensure_nonzero_bytes_read`).
+                    return Ok(()).into();
+                }
+                BytesPacketPosition::Padding(pos) => {
+                    // Consume whatever padding is left, ensuring it's all null
+                    // bytes. Only return `Ready(Ok(()))` once we're past the
+                    // padding (or in cases where polling the inner reader
+                    // returns `Poll::Pending`).
+                    let signalled_size = u64::from_le_bytes(*this.payload_size);
+                    let total_padding_len = padding_len(signalled_size) as usize;
+
+                    let padding_len_remaining = total_padding_len - pos;
+                    if padding_len_remaining != 0 {
+                        // create a buffer only accepting the number of remaining padding bytes.
+                        let mut buf = [0; 8];
+                        let mut padding_buf = tokio::io::ReadBuf::new(&mut buf);
+                        let mut padding_buf = padding_buf.take(padding_len_remaining);
+
+                        // read into padding_buf.
+                        ready!(this.inner.as_mut().poll_read(cx, &mut padding_buf))?;
+                        let bytes_read = ensure_nonzero_bytes_read(padding_buf.filled().len())?;
+
+                        *this.state = BytesPacketPosition::Padding(pos + bytes_read);
+
+                        // ensure the bytes are not null bytes
+                        if !padding_buf.filled().iter().all(|e| *e == b'\0') {
+                            return Err(std::io::Error::new(
+                                std::io::ErrorKind::InvalidData,
+                                "padding is not all zeroes",
+                            ))
+                            .into();
+                        }
+
+                        // if we still have padding to read, run the loop again.
+                        continue;
+                    }
+                    // return EOF
+                    return Ok(()).into();
+                }
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::time::Duration;
+
+    use crate::wire::bytes::write_bytes;
+    use hex_literal::hex;
+    use lazy_static::lazy_static;
+    use rstest::rstest;
+    use tokio::io::AsyncReadExt;
+    use tokio_test::{assert_err, io::Builder};
+
+    use super::*;
+
+    /// The maximum length of bytes packets we're willing to accept in the test
+    /// cases.
+    const MAX_LEN: u64 = 1024;
+
+    lazy_static! {
+        pub static ref LARGE_PAYLOAD: Vec<u8> = (0..255).collect::<Vec<u8>>().repeat(4 * 1024);
+    }
+
+    /// Helper function, calling the (simpler) write_bytes with the payload.
+    /// We use this to create data we want to read from the wire.
+    async fn produce_packet_bytes(payload: &[u8]) -> Vec<u8> {
+        let mut exp = vec![];
+        write_bytes(&mut exp, payload).await.unwrap();
+        exp
+    }
+
+    /// Read bytes packets of various length, and ensure read_to_end returns the
+    /// expected payload.
+    #[rstest]
+    #[case::empty(&[])] // empty bytes packet
+    #[case::size_1b(&[0xff])] // 1 bytes payload
+    #[case::size_8b(&hex!("0001020304050607"))] // 8 bytes payload (no padding)
+    #[case::size_9b( &hex!("000102030405060708"))] // 9 bytes payload (7 bytes padding)
+    #[case::size_1m(LARGE_PAYLOAD.as_slice())] // larger bytes packet
+    #[tokio::test]
+    async fn read_payload_correct(#[case] payload: &[u8]) {
+        let mut mock = Builder::new()
+            .read(&produce_packet_bytes(payload).await)
+            .build();
+
+        let mut r = BytesReader::new(&mut mock, ..=LARGE_PAYLOAD.len() as u64);
+        let mut buf = Vec::new();
+        r.read_to_end(&mut buf).await.expect("must succeed");
+
+        assert_eq!(payload, &buf[..]);
+    }
+
+    /// Fail if the bytes packet is larger than allowed
+    #[tokio::test]
+    async fn read_bigger_than_allowed_fail() {
+        let payload = LARGE_PAYLOAD.as_slice();
+        let mut mock = Builder::new()
+            .read(&produce_packet_bytes(payload).await[0..8]) // We stop reading after the size packet
+            .build();
+
+        let mut r = BytesReader::new(&mut mock, ..2048);
+        let mut buf = Vec::new();
+        assert_err!(r.read_to_end(&mut buf).await);
+    }
+
+    /// Fail if the bytes packet is smaller than allowed
+    #[tokio::test]
+    async fn read_smaller_than_allowed_fail() {
+        let payload = &[0x00, 0x01, 0x02];
+        let mut mock = Builder::new()
+            .read(&produce_packet_bytes(payload).await[0..8]) // We stop reading after the size packet
+            .build();
+
+        let mut r = BytesReader::new(&mut mock, 1024..2048);
+        let mut buf = Vec::new();
+        assert_err!(r.read_to_end(&mut buf).await);
+    }
+
+    /// Fail if the padding is not all zeroes
+    #[tokio::test]
+    async fn read_fail_if_nonzero_padding() {
+        let payload = &[0x00, 0x01, 0x02];
+        let mut packet_bytes = produce_packet_bytes(payload).await;
+        // Flip some bits in the padding
+        packet_bytes[12] = 0xff;
+        let mut mock = Builder::new().read(&packet_bytes).build(); // We stop reading after the faulty bit
+
+        let mut r = BytesReader::new(&mut mock, ..MAX_LEN);
+        let mut buf = Vec::new();
+
+        r.read_to_end(&mut buf).await.expect_err("must fail");
+    }
+
+    /// Start a 9 bytes payload packet, but have the underlying reader return
+    /// EOF in the middle of the size packet (after 4 bytes).
+    /// We should get an unexpected EOF error, already when trying to read the
+    /// first byte (of payload)
+    #[tokio::test]
+    async fn read_9b_eof_during_size() {
+        let payload = &hex!("FF0102030405060708");
+        let mut mock = Builder::new()
+            .read(&produce_packet_bytes(payload).await[..4])
+            .build();
+
+        let mut r = BytesReader::new(&mut mock, ..MAX_LEN);
+        let mut buf = [0u8; 1];
+
+        assert_eq!(
+            r.read_exact(&mut buf).await.expect_err("must fail").kind(),
+            std::io::ErrorKind::UnexpectedEof
+        );
+
+        assert_eq!(&[0], &buf, "buffer should stay empty");
+    }
+
+    /// Start a 9 bytes payload packet, but have the underlying reader return
+    /// EOF in the middle of the payload (4 bytes into the payload).
+    /// We should get an unexpected EOF error, after reading the first 4 bytes
+    /// (successfully).
+    #[tokio::test]
+    async fn read_9b_eof_during_payload() {
+        let payload = &hex!("FF0102030405060708");
+        let mut mock = Builder::new()
+            .read(&produce_packet_bytes(payload).await[..8 + 4])
+            .build();
+
+        let mut r = BytesReader::new(&mut mock, ..MAX_LEN);
+        let mut buf = [0; 9];
+
+        r.read_exact(&mut buf[..4]).await.expect("must succeed");
+
+        assert_eq!(
+            r.read_exact(&mut buf[4..=4])
+                .await
+                .expect_err("must fail")
+                .kind(),
+            std::io::ErrorKind::UnexpectedEof
+        );
+    }
+
+    /// Start a 9 bytes payload packet, but return an error at various stages *after* the actual payload.
+    /// read_exact with a 9 bytes buffer is expected to succeed, but any further
+    /// read, as well as read_to_end are expected to fail.
+    #[rstest]
+    #[case::before_padding(8 + 9)]
+    #[case::during_padding(8 + 9 + 2)]
+    #[case::after_padding(8 + 9 + padding_len(9) as usize)]
+    #[tokio::test]
+    async fn read_9b_eof_after_payload(#[case] offset: usize) {
+        let payload = &hex!("FF0102030405060708");
+        let mut mock = Builder::new()
+            .read(&produce_packet_bytes(payload).await[..offset])
+            .build();
+
+        let mut r = BytesReader::new(&mut mock, ..MAX_LEN);
+        let mut buf = [0; 9];
+
+        // read_exact of the payload will succeed, but a subsequent read will
+        // return UnexpectedEof error.
+        r.read_exact(&mut buf).await.expect("should succeed");
+        assert_eq!(
+            r.read_exact(&mut buf[4..=4])
+                .await
+                .expect_err("must fail")
+                .kind(),
+            std::io::ErrorKind::UnexpectedEof
+        );
+
+        // read_to_end will fail.
+        let mut mock = Builder::new()
+            .read(&produce_packet_bytes(payload).await[..8 + payload.len()])
+            .build();
+
+        let mut r = BytesReader::new(&mut mock, ..MAX_LEN);
+        let mut buf = Vec::new();
+        assert_eq!(
+            r.read_to_end(&mut buf).await.expect_err("must fail").kind(),
+            std::io::ErrorKind::UnexpectedEof
+        );
+    }
+
+    /// Start a 9 bytes payload packet, but return an error after a certain position.
+    /// Ensure that error is propagated.
+    #[rstest]
+    #[case::during_size(4)]
+    #[case::before_payload(8)]
+    #[case::during_payload(8 + 4)]
+    #[case::before_padding(8 + 4)]
+    #[case::during_padding(8 + 9 + 2)]
+    #[tokio::test]
+    async fn propagate_error_from_reader(#[case] offset: usize) {
+        let payload = &hex!("FF0102030405060708");
+        let mut mock = Builder::new()
+            .read(&produce_packet_bytes(payload).await[..offset])
+            .read_error(std::io::Error::new(std::io::ErrorKind::Other, "foo"))
+            .build();
+
+        let mut r = BytesReader::new(&mut mock, ..MAX_LEN);
+        let mut buf = Vec::new();
+
+        let err = r.read_to_end(&mut buf).await.expect_err("must fail");
+        assert_eq!(
+            err.kind(),
+            std::io::ErrorKind::Other,
+            "error kind must match"
+        );
+
+        assert_eq!(
+            err.into_inner().unwrap().to_string(),
+            "foo",
+            "error payload must contain foo"
+        );
+    }
+
+    /// If there's an error right after the padding, we don't propagate it, as
+    /// we're done reading. We just return EOF.
+    #[tokio::test]
+    async fn no_error_after_eof() {
+        let payload = &hex!("FF0102030405060708");
+        let mut mock = Builder::new()
+            .read(&produce_packet_bytes(payload).await)
+            .read_error(std::io::Error::new(std::io::ErrorKind::Other, "foo"))
+            .build();
+
+        let mut r = BytesReader::new(&mut mock, ..MAX_LEN);
+        let mut buf = Vec::new();
+
+        r.read_to_end(&mut buf).await.expect("must succeed");
+        assert_eq!(buf.as_slice(), payload);
+    }
+
+    /// Introduce various stalls in various places of the packet, to ensure we
+    /// handle these cases properly, too.
+    #[rstest]
+    #[case::beginning(0)]
+    #[case::before_payload(8)]
+    #[case::during_payload(8 + 4)]
+    #[case::before_padding(8 + 4)]
+    #[case::during_padding(8 + 9 + 2)]
+    #[tokio::test]
+    async fn read_payload_correct_pending(#[case] offset: usize) {
+        let payload = &hex!("FF0102030405060708");
+        let mut mock = Builder::new()
+            .read(&produce_packet_bytes(payload).await[..offset])
+            .wait(Duration::from_nanos(0))
+            .read(&produce_packet_bytes(payload).await[offset..])
+            .build();
+
+        let mut r = BytesReader::new(&mut mock, ..=LARGE_PAYLOAD.len() as u64);
+        let mut buf = Vec::new();
+        r.read_to_end(&mut buf).await.expect("must succeed");
+
+        assert_eq!(payload, &buf[..]);
+    }
+}
diff --git a/tvix/nix-compat/src/wire/bytes_writer.rs b/tvix/nix-compat/src/wire/bytes_writer.rs
index 933b0e6efa6c..8bd0a2d00ef6 100644
--- a/tvix/nix-compat/src/wire/bytes_writer.rs
+++ b/tvix/nix-compat/src/wire/bytes_writer.rs
@@ -6,7 +6,7 @@ use tokio::io::AsyncWrite;
 use super::bytes::EMPTY_BYTES;
 
 /// The length of the size field, in bytes is always 8.
-const LEN_SIZE: usize = 8;
+pub(crate) const LEN_SIZE: usize = 8;
 
 pin_project! {
     /// Writes a "bytes wire packet" to the underlying writer.
@@ -52,6 +52,8 @@ pin_project! {
 /// move to the beginning of the next one:
 /// - Size(LEN_SIZE) must be expressed as Payload(0)
 /// - Payload(self.payload_len) must be expressed as Padding(0)
+/// There's one exception - Size(LEN_SIZE) in the reader represents a failure
+/// state we enter in case the allowed size doesn't match the allowed range.
 ///
 /// Padding(padding_len) means we're at the end of the bytes wire packet.
 #[derive(Clone, Debug, PartialEq, Eq)]
diff --git a/tvix/nix-compat/src/wire/mod.rs b/tvix/nix-compat/src/wire/mod.rs
index cb14e4d420b7..56ea364dffb7 100644
--- a/tvix/nix-compat/src/wire/mod.rs
+++ b/tvix/nix-compat/src/wire/mod.rs
@@ -5,8 +5,12 @@
 pub mod bytes;
 
 #[cfg(feature = "async")]
+mod bytes_reader;
+#[cfg(feature = "async")]
 mod bytes_writer;
 #[cfg(feature = "async")]
+pub use bytes_reader::BytesReader;
+#[cfg(feature = "async")]
 pub use bytes_writer::BytesWriter;
 
 #[cfg(feature = "async")]