refactor(tvix/nix-compat): reorganize wire and bytes r/7896

Move everything bytes-related into its own module, and re-export
both bytes and primitive in a flat space from wire/mod.rs.

Expose this if a `wire` feature flag is set. We only have `async` stuff
in here.

Change-Id: Ia4ce4791f13a5759901cc9d6ce6bd6bbcca587c7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11389
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: Brian Olsen <me@griff.name>

1 files changed, 254 insertions, 0 deletions

diff --git a/tvix/nix-compat/src/wire/bytes/mod.rs b/tvix/nix-compat/src/wire/bytes/mod.rs
new file mode 100644
index 0000000000..9487536eb7
--- /dev/null
+++ b/tvix/nix-compat/src/wire/bytes/mod.rs
@@ -0,0 +1,254 @@
+use std::{
+    io::{Error, ErrorKind},
+    ops::RangeBounds,
+};
+use tokio::io::{AsyncReadExt, AsyncWriteExt};
+
+mod reader;
+pub use reader::BytesReader;
+mod writer;
+pub use writer::BytesWriter;
+
+use super::primitive;
+
+/// 8 null bytes, used to write out padding.
+const EMPTY_BYTES: &[u8; 8] = &[0u8; 8];
+
+/// The length of the size field, in bytes is always 8.
+const LEN_SIZE: usize = 8;
+
+#[allow(dead_code)]
+/// Read a "bytes wire packet" from the AsyncRead.
+/// Rejects reading more than `allowed_size` bytes of payload.
+///
+/// The packet is made up of three parts:
+/// - a length header, u64, LE-encoded
+/// - the payload itself
+/// - null bytes to the next 8 byte boundary
+///
+/// Ensures the payload size fits into the `allowed_size` passed,
+/// and that the padding is actual null bytes.
+///
+/// On success, the returned `Vec<u8>` only contains the payload itself.
+/// On failure (for example if a too large byte packet was sent), the reader
+/// becomes unusable.
+///
+/// This buffers the entire payload into memory, a streaming version will be
+/// added later.
+pub async fn read_bytes<R, S>(r: &mut R, allowed_size: S) -> std::io::Result<Vec<u8>>
+where
+    R: AsyncReadExt + Unpin,
+    S: RangeBounds<u64>,
+{
+    // read the length field
+    let len = primitive::read_u64(r).await?;
+
+    if !allowed_size.contains(&len) {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidData,
+            "signalled package size not in allowed range",
+        ));
+    }
+
+    // calculate the total length, including padding.
+    // byte packets are padded to 8 byte blocks each.
+    let padded_len = padding_len(len) as u64 + (len as u64);
+    let mut limited_reader = r.take(padded_len);
+
+    let mut buf = Vec::new();
+
+    let s = limited_reader.read_to_end(&mut buf).await?;
+
+    // make sure we got exactly the number of bytes, and not less.
+    if s as u64 != padded_len {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidData,
+            "got less bytes than expected",
+        ));
+    }
+
+    let (_content, padding) = buf.split_at(len as usize);
+
+    // ensure the padding is all zeroes.
+    if !padding.iter().all(|e| *e == b'\0') {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidData,
+            "padding is not all zeroes",
+        ));
+    }
+
+    // return the data without the padding
+    buf.truncate(len as usize);
+    Ok(buf)
+}
+
+/// Read a "bytes wire packet" of from the AsyncRead and tries to parse as string.
+/// Internally uses [read_bytes].
+/// Rejects reading more than `allowed_size` bytes of payload.
+pub async fn read_string<R, S>(r: &mut R, allowed_size: S) -> std::io::Result<String>
+where
+    R: AsyncReadExt + Unpin,
+    S: RangeBounds<u64>,
+{
+    let bytes = read_bytes(r, allowed_size).await?;
+    String::from_utf8(bytes).map_err(|e| Error::new(ErrorKind::InvalidData, e))
+}
+
+/// Writes a "bytes wire packet" to a (hopefully buffered) [AsyncWriteExt].
+///
+/// Accepts anything implementing AsRef<[u8]> as payload.
+///
+/// See [read_bytes] for a description of the format.
+///
+/// Note: if performance matters to you, make sure your
+/// [AsyncWriteExt] handle is buffered. This function is quite
+/// write-intesive.
+pub async fn write_bytes<W: AsyncWriteExt + Unpin, B: AsRef<[u8]>>(
+    w: &mut W,
+    b: B,
+) -> std::io::Result<()> {
+    // write the size packet.
+    primitive::write_u64(w, b.as_ref().len() as u64).await?;
+
+    // write the payload
+    w.write_all(b.as_ref()).await?;
+
+    // write padding if needed
+    let padding_len = padding_len(b.as_ref().len() as u64) as usize;
+    if padding_len != 0 {
+        w.write_all(&EMPTY_BYTES[..padding_len]).await?;
+    }
+    Ok(())
+}
+
+/// Computes the number of bytes we should add to len (a length in
+/// bytes) to be alined on 64 bits (8 bytes).
+fn padding_len(len: u64) -> u8 {
+    let modulo = len % 8;
+    if modulo == 0 {
+        0
+    } else {
+        8 - modulo as u8
+    }
+}
+
+/// Models the position inside a "bytes wire packet" that the reader or writer
+/// is in.
+/// It can be in three different stages, inside size, payload or padding fields.
+/// The number tracks the number of bytes written inside the specific field.
+/// There shall be no ambiguous states, at the end of a stage we immediately
+/// 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)]
+enum BytesPacketPosition {
+    Size(usize),
+    Payload(u64),
+    Padding(usize),
+}
+
+#[cfg(test)]
+mod tests {
+    use tokio_test::{assert_ok, io::Builder};
+
+    use super::*;
+    use hex_literal::hex;
+
+    /// The maximum length of bytes packets we're willing to accept in the test
+    /// cases.
+    const MAX_LEN: u64 = 1024;
+
+    #[tokio::test]
+    async fn test_read_8_bytes() {
+        let mut mock = Builder::new()
+            .read(&8u64.to_le_bytes())
+            .read(&12345678u64.to_le_bytes())
+            .build();
+
+        assert_eq!(
+            &12345678u64.to_le_bytes(),
+            read_bytes(&mut mock, 0u64..MAX_LEN)
+                .await
+                .unwrap()
+                .as_slice()
+        );
+    }
+
+    #[tokio::test]
+    async fn test_read_9_bytes() {
+        let mut mock = Builder::new()
+            .read(&9u64.to_le_bytes())
+            .read(&hex!("01020304050607080900000000000000"))
+            .build();
+
+        assert_eq!(
+            hex!("010203040506070809"),
+            read_bytes(&mut mock, 0u64..MAX_LEN)
+                .await
+                .unwrap()
+                .as_slice()
+        );
+    }
+
+    #[tokio::test]
+    async fn test_read_0_bytes() {
+        // A empty byte packet is essentially just the 0 length field.
+        // No data is read, and there's zero padding.
+        let mut mock = Builder::new().read(&0u64.to_le_bytes()).build();
+
+        assert_eq!(
+            hex!(""),
+            read_bytes(&mut mock, 0u64..MAX_LEN)
+                .await
+                .unwrap()
+                .as_slice()
+        );
+    }
+
+    #[tokio::test]
+    /// Ensure we don't read any further than the size field if the length
+    /// doesn't match the range we want to accept.
+    async fn test_read_reject_too_large() {
+        let mut mock = Builder::new().read(&100u64.to_le_bytes()).build();
+
+        read_bytes(&mut mock, 10..10)
+            .await
+            .expect_err("expect this to fail");
+    }
+
+    #[tokio::test]
+    async fn test_write_bytes_no_padding() {
+        let input = hex!("6478696f34657661");
+        let len = input.len() as u64;
+        let mut mock = Builder::new()
+            .write(&len.to_le_bytes())
+            .write(&input)
+            .build();
+        assert_ok!(write_bytes(&mut mock, &input).await)
+    }
+    #[tokio::test]
+    async fn test_write_bytes_with_padding() {
+        let input = hex!("322e332e3137");
+        let len = input.len() as u64;
+        let mut mock = Builder::new()
+            .write(&len.to_le_bytes())
+            .write(&hex!("322e332e31370000"))
+            .build();
+        assert_ok!(write_bytes(&mut mock, &input).await)
+    }
+
+    #[tokio::test]
+    async fn test_write_string() {
+        let input = "Hello, World!";
+        let len = input.len() as u64;
+        let mut mock = Builder::new()
+            .write(&len.to_le_bytes())
+            .write(&hex!("48656c6c6f2c20576f726c6421000000"))
+            .build();
+        assert_ok!(write_bytes(&mut mock, &input).await)
+    }
+}