--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/src/vfs.rs	Mon Sep 06 11:39:59 2021 +0200
@@ -0,0 +1,73 @@
+use crate::errors::{HgError, IoErrorContext, IoResultExt};
+use memmap::{Mmap, MmapOptions};
+use std::io::ErrorKind;
+use std::path::{Path, PathBuf};
+
+/// Filesystem access abstraction for the contents of a given "base" diretory
+#[derive(Clone, Copy)]
+pub struct Vfs<'a> {
+    pub(crate) base: &'a Path,
+}
+
+impl Vfs<'_> {
+    pub fn join(&self, relative_path: impl AsRef<Path>) -> PathBuf {
+        self.base.join(relative_path)
+    }
+
+    pub fn read(
+        &self,
+        relative_path: impl AsRef<Path>,
+    ) -> Result<Vec<u8>, HgError> {
+        let path = self.join(relative_path);
+        std::fs::read(&path).when_reading_file(&path)
+    }
+
+    pub fn mmap_open(
+        &self,
+        relative_path: impl AsRef<Path>,
+    ) -> Result<Mmap, HgError> {
+        let path = self.base.join(relative_path);
+        let file = std::fs::File::open(&path).when_reading_file(&path)?;
+        // TODO: what are the safety requirements here?
+        let mmap = unsafe { MmapOptions::new().map(&file) }
+            .when_reading_file(&path)?;
+        Ok(mmap)
+    }
+
+    pub fn rename(
+        &self,
+        relative_from: impl AsRef<Path>,
+        relative_to: impl AsRef<Path>,
+    ) -> Result<(), HgError> {
+        let from = self.join(relative_from);
+        let to = self.join(relative_to);
+        std::fs::rename(&from, &to)
+            .with_context(|| IoErrorContext::RenamingFile { from, to })
+    }
+}
+
+fn fs_metadata(
+    path: impl AsRef<Path>,
+) -> Result<Option<std::fs::Metadata>, HgError> {
+    let path = path.as_ref();
+    match std::fs::metadata(path) {
+        Ok(meta) => Ok(Some(meta)),
+        Err(error) => match error.kind() {
+            // TODO: when we require a Rust version where `NotADirectory` is
+            // stable, invert this logic and return None for it and `NotFound`
+            // and propagate any other error.
+            ErrorKind::PermissionDenied => Err(error).with_context(|| {
+                IoErrorContext::ReadingMetadata(path.to_owned())
+            }),
+            _ => Ok(None),
+        },
+    }
+}
+
+pub(crate) fn is_dir(path: impl AsRef<Path>) -> Result<bool, HgError> {
+    Ok(fs_metadata(path)?.map_or(false, |meta| meta.is_dir()))
+}
+
+pub(crate) fn is_file(path: impl AsRef<Path>) -> Result<bool, HgError> {
+    Ok(fs_metadata(path)?.map_or(false, |meta| meta.is_file()))
+}