fix(tvix): Avoid buffering file into memory in builtins.hashFile

Right now `builtins.hashFile` always reads the entire file into memory
before hashing, which is not ideal for large files. This replaces
`read_to_string` with `open_file` which allows calculating the hash of
the file without buffering it entirely into memory. Other callers can
continue to buffer into memory if they choose, but they still use the
`open_file` VM request and then call `read_to_string` or `read_to_end`
on the `std::io::Reader`.

Fixes b/380

Change-Id: Ifa1c8324bcee8f751604b0b449feab875c632fda
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11236
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/hash.rs

@@ -6,18 +6,22 @@ use sha2::{digest::Output, Digest, Sha256, Sha512};
 
 use crate::ErrorKind;
 
-fn hash<D: Digest>(b: &[u8]) -> Output<D> {
+/// Reads through all data from the passed reader, and returns the resulting [Digest].
+/// The exact hash function used is left generic over all [Digest].
+fn hash<D: Digest + std::io::Write>(mut r: impl std::io::Read) -> Result<Output<D>, ErrorKind> {
     let mut hasher = D::new();
-    hasher.update(b);
-    hasher.finalize()
+    std::io::copy(&mut r, &mut hasher)?;
+    Ok(hasher.finalize())
 }
 
-pub fn hash_nix_string(algo: impl AsRef<[u8]>, s: impl AsRef<[u8]>) -> Result<String, ErrorKind> {
+/// For a given algo "string" and reader for data, calculate the digest
+/// and return it as a hexlower encoded [String].
+pub fn hash_nix_string(algo: impl AsRef<[u8]>, s: impl std::io::Read) -> Result<String, ErrorKind> {
     match algo.as_ref() {
-        b"md5" => Ok(HEXLOWER.encode(hash::<Md5>(s.as_ref()).as_bstr())),
-        b"sha1" => Ok(HEXLOWER.encode(hash::<Sha1>(s.as_ref()).as_bstr())),
-        b"sha256" => Ok(HEXLOWER.encode(hash::<Sha256>(s.as_ref()).as_bstr())),
-        b"sha512" => Ok(HEXLOWER.encode(hash::<Sha512>(s.as_ref()).as_bstr())),
+        b"md5" => Ok(HEXLOWER.encode(hash::<Md5>(s)?.as_bstr())),
+        b"sha1" => Ok(HEXLOWER.encode(hash::<Sha1>(s)?.as_bstr())),
+        b"sha256" => Ok(HEXLOWER.encode(hash::<Sha256>(s)?.as_bstr())),
+        b"sha512" => Ok(HEXLOWER.encode(hash::<Sha512>(s)?.as_bstr())),
         _ => Err(ErrorKind::UnknownHashType(
             algo.as_ref().as_bstr().to_string(),
         )),

tvix/eval/src/builtins/impure.rs

@@ -31,14 +31,13 @@ mod impure_builtins {
     }
 
     #[builtin("hashFile")]
-    #[allow(non_snake_case)]
-    async fn builtin_hashFile(co: GenCo, algo: Value, path: Value) -> Result<Value, ErrorKind> {
+    async fn builtin_hash_file(co: GenCo, algo: Value, path: Value) -> Result<Value, ErrorKind> {
         let path = match coerce_value_to_path(&co, path).await? {
             Err(cek) => return Ok(Value::from(cek)),
             Ok(p) => p,
         };
-        let s = generators::request_read_to_string(&co, path).await;
-        hash_nix_string(algo.to_str()?, s.to_str()?).map(Value::from)
+        let r = generators::request_open_file(&co, path).await;
+        Ok(hash_nix_string(algo.to_str()?, r).map(Value::from)?)
     }
 
     #[builtin("pathExists")]
@@ -79,7 +78,13 @@ mod impure_builtins {
     async fn builtin_read_file(co: GenCo, path: Value) -> Result<Value, ErrorKind> {
         match coerce_value_to_path(&co, path).await? {
             Err(cek) => Ok(Value::from(cek)),
-            Ok(path) => Ok(generators::request_read_to_string(&co, path).await),
+            Ok(path) => {
+                let mut buf = Vec::new();
+                generators::request_open_file(&co, path)
+                    .await
+                    .read_to_end(&mut buf)?;
+                Ok(Value::from(buf))
+            }
         }
     }
 }

tvix/eval/src/builtins/mod.rs

@@ -773,9 +773,8 @@ mod pure_builtins {
     }
 
     #[builtin("hashString")]
-    #[allow(non_snake_case)]
-    async fn builtin_hashString(co: GenCo, algo: Value, s: Value) -> Result<Value, ErrorKind> {
-        hash_nix_string(algo.to_str()?, s.to_str()?).map(Value::from)
+    async fn builtin_hash_string(co: GenCo, algo: Value, s: Value) -> Result<Value, ErrorKind> {
+        hash_nix_string(algo.to_str()?, std::io::Cursor::new(s.to_str()?)).map(Value::from)
     }
 
     #[builtin("head")]

tvix/eval/src/compiler/import.rs

@@ -6,7 +6,6 @@
 //! instance, or observers).
 
 use super::GlobalsMap;
-use bstr::ByteSlice;
 use genawaiter::rc::Gen;
 use std::rc::Weak;
 
@@ -39,9 +38,11 @@ async fn import_impl(
         return Ok(cached);
     }
 
-    // TODO(tazjin): make this return a string directly instead
-    let contents: Value = generators::request_read_to_string(&co, path.clone()).await;
-    let contents = contents.to_str()?.to_str()?.to_owned();
+    let mut reader = generators::request_open_file(&co, path.clone()).await;
+    // We read to a String instead of a Vec<u8> because rnix only supports
+    // string source files.
+    let mut contents = String::new();
+    reader.read_to_string(&mut contents)?;
 
     let parsed = rnix::ast::Root::parse(&contents);
     let errors = parsed.errors();

tvix/eval/src/io.rs

@@ -16,6 +16,7 @@
 //! how store paths are opened and so on.
 
 use std::{
+    fs::File,
     io,
     path::{Path, PathBuf},
 };
@@ -48,13 +49,8 @@ pub trait EvalIO {
     /// * `builtins.pathExists :: path -> bool`
     fn path_exists(&self, path: &Path) -> io::Result<bool>;
 
-    /// Read the file at the specified path to a `Vec<u8>`.
-    ///
-    /// This is used for the following language evaluation cases:
-    ///
-    /// * `builtins.readFile :: path -> string`
-    /// * `builtins.import :: path -> any`
-    fn read_to_end(&self, path: &Path) -> io::Result<Vec<u8>>;
+    /// Open the file at the specified path to a `io::Read`.
+    fn open(&self, path: &Path) -> io::Result<Box<dyn io::Read>>;
 
     /// Read the directory at the specified path and return the names
     /// of its entries associated with their [`FileType`].
@@ -99,8 +95,8 @@ impl EvalIO for StdIO {
         path.try_exists()
     }
 
-    fn read_to_end(&self, path: &Path) -> io::Result<Vec<u8>> {
-        std::fs::read(path)
+    fn open(&self, path: &Path) -> io::Result<Box<dyn io::Read>> {
+        Ok(Box::new(File::open(path)?))
     }
 
     fn read_dir(&self, path: &Path) -> io::Result<Vec<(bytes::Bytes, FileType)>> {
@@ -145,7 +141,7 @@ impl EvalIO for DummyIO {
         ))
     }
 
-    fn read_to_end(&self, _: &Path) -> io::Result<Vec<u8>> {
+    fn open(&self, _: &Path) -> io::Result<Box<dyn io::Read>> {
         Err(io::Error::new(
             io::ErrorKind::Unsupported,
             "I/O methods are not implemented in DummyIO",

tvix/eval/src/vm/generators.rs

@@ -102,8 +102,8 @@ pub enum VMRequest {
     /// Request that the VM imports the given path through its I/O interface.
     PathImport(PathBuf),
 
-    /// Request that the VM reads the given path to a string.
-    ReadToString(PathBuf),
+    /// Request that the VM opens the specified file and provides a reader.
+    OpenFile(PathBuf),
 
     /// Request that the VM checks whether the given path exists.
     PathExists(PathBuf),
@@ -170,8 +170,8 @@ impl Display for VMRequest {
                 write!(f, "import_cache_put({})", p.to_string_lossy())
             }
             VMRequest::PathImport(p) => write!(f, "path_import({})", p.to_string_lossy()),
-            VMRequest::ReadToString(p) => {
-                write!(f, "read_to_string({})", p.to_string_lossy())
+            VMRequest::OpenFile(p) => {
+                write!(f, "open_file({})", p.to_string_lossy())
             }
             VMRequest::PathExists(p) => write!(f, "path_exists({})", p.to_string_lossy()),
             VMRequest::ReadDir(p) => write!(f, "read_dir({})", p.to_string_lossy()),
@@ -199,6 +199,9 @@ pub enum VMResponse {
 
     /// VM response with a span to use at the current point.
     Span(LightSpan),
+
+    /// [std::io::Reader] produced by the VM in response to some IO operation.
+    Reader(Box<dyn std::io::Read>),
 }
 
 impl Display for VMResponse {
@@ -209,6 +212,7 @@ impl Display for VMResponse {
             VMResponse::Path(p) => write!(f, "path({})", p.to_string_lossy()),
             VMResponse::Directory(d) => write!(f, "dir(len = {})", d.len()),
             VMResponse::Span(_) => write!(f, "span"),
+            VMResponse::Reader(_) => write!(f, "reader"),
         }
     }
 }
@@ -425,18 +429,18 @@ where
                             message = VMResponse::Path(imported);
                         }
 
-                        VMRequest::ReadToString(path) => {
-                            let content = self
+                        VMRequest::OpenFile(path) => {
+                            let reader = self
                                 .io_handle
                                 .as_ref()
-                                .read_to_end(&path)
+                                .open(&path)
                                 .map_err(|e| ErrorKind::IO {
                                     path: Some(path),
                                     error: e.into(),
                                 })
                                 .with_span(&span, self)?;
 
-                            message = VMResponse::Value(content.into())
+                            message = VMResponse::Reader(reader)
                         }
 
                         VMRequest::PathExists(path) => {
@@ -730,9 +734,10 @@ pub(crate) async fn request_path_import(co: &GenCo, path: PathBuf) -> PathBuf {
     }
 }
 
-pub(crate) async fn request_read_to_string(co: &GenCo, path: PathBuf) -> Value {
-    match co.yield_(VMRequest::ReadToString(path)).await {
-        VMResponse::Value(value) => value,
+/// Request that the VM open a [std::io::Read] for the specified file.
+pub async fn request_open_file(co: &GenCo, path: PathBuf) -> Box<dyn std::io::Read> {
+    match co.yield_(VMRequest::OpenFile(path)).await {
+        VMResponse::Reader(value) => value,
         msg => panic!(
             "Tvix bug: VM responded with incorrect generator message: {}",
             msg