feat(tvix/nix-compat): read client setting from wire

Add the primitives necessary to read the client settings from the Nix
daemon wire protocol.

Introducing the read_string primitive. This trivial primitive parses a
read_bytes call, check the bytes are valid utf-8 bytes and wraps the
result in a String.

Change-Id: Ie1253523a6bd4e31e7924e9898a0898109da2fa0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11358
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nix-compat/src/wire/bytes.rs

@@ -1,4 +1,7 @@
-use std::ops::RangeBounds;
+use std::{
+    io::{Error, ErrorKind},
+    ops::RangeBounds,
+};
 
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
 
@@ -58,6 +61,21 @@ where
     Ok(buf)
 }
 
+/// Read a Nix daemon string from the AsyncWrite, encoded as utf8.
+/// Rejects reading more than `allowed_size` bytes
+///
+/// A Nix daemon string is made up of two distincts parts:
+/// 1. Its lenght, LE-encoded on 64 bits.
+/// 2. Its content. 0-padded on 64 bits.
+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 sequence of sized bits to a (hopefully buffered)
 /// [AsyncWriteExt] handle.
 ///