feat(nix-compat/nar/reader): provide passthrough buffered I/O r/7035

Allow taking advantage of the buffer of the underlying reader to avoid
unnecessary copies of file data.

We can't easily implement the methods of BufRead directly, since we
have some extra I/O to perform in the final consume() invocation.

That could be resolved at the cost of additional bookkeeping, but this
will suffice for now.

Change-Id: I8100cf0abd79e7469670b8596bd989be5db44a91
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10089
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

1 files changed, 75 insertions, 13 deletions

diff --git a/tvix/nix-compat/src/nar/reader/mod.rs b/tvix/nix-compat/src/nar/reader/mod.rs
index 4958a67de304..fa7ddc77f96c 100644
--- a/tvix/nix-compat/src/nar/reader/mod.rs
+++ b/tvix/nix-compat/src/nar/reader/mod.rs
@@ -7,7 +7,7 @@
 use std::io::{
     self, BufRead,
     ErrorKind::{InvalidData, UnexpectedEof},
-    Read,
+    Read, Write,
 };
 
 // Required reading for understanding this module.
@@ -111,6 +111,62 @@ impl<'a, 'r> FileReader<'a, 'r> {
     }
 }
 
+impl FileReader<'_, '_> {
+    /// Equivalent to [BufRead::fill_buf]
+    ///
+    /// We can't directly implement [BufRead], because [FileReader::consume] needs
+    /// to perform fallible I/O.
+    pub fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        if self.is_empty() {
+            return Ok(&[]);
+        }
+
+        let mut buf = self.reader.fill_buf()?;
+
+        if buf.is_empty() {
+            return Err(UnexpectedEof.into());
+        }
+
+        if buf.len() as u64 > self.len {
+            buf = &buf[..self.len as usize];
+        }
+
+        Ok(buf)
+    }
+
+    /// Analogous to [BufRead::consume], differing only in that it needs
+    /// to perform I/O in order to read padding and terminators.
+    pub fn consume(&mut self, n: usize) -> io::Result<()> {
+        if n == 0 {
+            return Ok(());
+        }
+
+        self.len = self
+            .len
+            .checked_sub(n as u64)
+            .expect("consumed bytes past EOF");
+
+        self.reader.consume(n);
+
+        if self.is_empty() {
+            self.finish()?;
+        }
+
+        Ok(())
+    }
+
+    /// Copy the (remaining) contents of the file into `dst`.
+    pub fn copy(&mut self, mut dst: impl Write) -> io::Result<()> {
+        while !self.is_empty() {
+            let buf = self.fill_buf()?;
+            let n = dst.write(buf)?;
+            self.consume(n)?;
+        }
+
+        Ok(())
+    }
+}
+
 impl Read for FileReader<'_, '_> {
     fn read(&mut self, mut buf: &mut [u8]) -> io::Result<usize> {
         if buf.is_empty() || self.is_empty() {
@@ -128,24 +184,30 @@ impl Read for FileReader<'_, '_> {
             return Err(UnexpectedEof.into());
         }
 
-        // If we've reached semantic EOF, consume and verify the padding and terminating TOK_PAR.
-        // Files are padded to 64 bits (8 bytes), just like any other byte string in the wire format.
         if self.is_empty() {
-            let pad = (self.pad & 7) as usize;
+            self.finish()?;
+        }
 
-            if pad != 0 {
-                let mut buf = [0; 8];
-                self.reader.read_exact(&mut buf[pad..])?;
+        Ok(n)
+    }
+}
 
-                if buf != [0; 8] {
-                    return Err(InvalidData.into());
-                }
-            }
+impl FileReader<'_, '_> {
+    /// We've reached semantic EOF, consume and verify the padding and terminating TOK_PAR.
+    /// Files are padded to 64 bits (8 bytes), just like any other byte string in the wire format.
+    fn finish(&mut self) -> io::Result<()> {
+        let pad = (self.pad & 7) as usize;
 
-            read::token(self.reader, &wire::TOK_PAR)?;
+        if pad != 0 {
+            let mut buf = [0; 8];
+            self.reader.read_exact(&mut buf[pad..])?;
+
+            if buf != [0; 8] {
+                return Err(InvalidData.into());
+            }
         }
 
-        Ok(n)
+        read::token(self.reader, &wire::TOK_PAR)
     }
 }