sausage-vfs: layers (host, memory, generated, mounts), process guards, wasi:filesystem adapter

Checkpoint. More tests and the review findings follow.

vfs/tests/guests/files/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "vfs-files-guest"
+version = "0.1.0"
+edition = "2024"
+[workspace]
+[dependencies]
+tempfile = "3"
+rusqlite = { version = "0.38", features = ["bundled"] }

vfs/tests/guests/files/src/main.rs

@@ -0,0 +1,66 @@
+//! Exercises Rust filesystem clients through the virtual WASI filesystem.
+use std::{
+    fs::{self, OpenOptions},
+    io::{Read, Seek, SeekFrom, Write},
+};
+fn main() {
+    fs::create_dir("/work").unwrap();
+    fs::write("/work/a", b"hello").unwrap();
+    let mut f = OpenOptions::new()
+        .read(true)
+        .write(true)
+        .open("/work/a")
+        .unwrap();
+    f.seek(SeekFrom::Start(1)).unwrap();
+    f.write_all(b"A").unwrap();
+    f.set_len(4).unwrap();
+    f.sync_all().unwrap();
+    OpenOptions::new()
+        .append(true)
+        .open("/work/a")
+        .unwrap()
+        .write_all(b"!")
+        .unwrap();
+    assert_eq!(fs::read("/work/a").unwrap(), b"hAll!");
+    assert_eq!(fs::metadata("/work/a").unwrap().len(), 5);
+    fs::rename("/work/a", "/work/b").unwrap();
+    assert_eq!(fs::read("/work/../work/b").unwrap(), b"hAll!");
+    assert_eq!(fs::read("work/b").unwrap(), b"hAll!");
+    assert_eq!(fs::read_dir("/work").unwrap().count(), 1);
+    assert_eq!(
+        fs::write("/proc/no", b"no").unwrap_err().kind(),
+        std::io::ErrorKind::ReadOnlyFilesystem
+    );
+    assert_eq!(
+        fs::rename("/work/b", "/run/b").unwrap_err().kind(),
+        std::io::ErrorKind::CrossesDevices
+    );
+    let root = fs::read_dir("/")
+        .unwrap()
+        .map(|e| e.unwrap().file_name())
+        .collect::<Vec<_>>();
+    for name in ["proc", "run", "self"] {
+        assert!(root.iter().any(|n| n == name));
+    }
+    assert_eq!(fs::read("/proc/value").unwrap(), b"snapshot");
+    fs::write("/run/test", b"memory").unwrap();
+    assert_eq!(fs::read("/run/test").unwrap(), b"memory");
+    fs::remove_file("/work/b").unwrap();
+    fs::remove_dir("/work").unwrap();
+    // WASI std does not implement env::temp_dir, so select the folder explicitly.
+    let mut temp = tempfile::tempfile_in("/tmp").unwrap();
+    temp.write_all(b"temporary").unwrap();
+    temp.seek(SeekFrom::Start(0)).unwrap();
+    let mut text = String::new();
+    temp.read_to_string(&mut text).unwrap();
+    assert_eq!(text, "temporary");
+    let db = rusqlite::Connection::open("/database.sqlite").unwrap();
+    db.execute_batch("CREATE TABLE data (value TEXT); INSERT INTO data VALUES ('row');")
+        .unwrap();
+    drop(db);
+    let db = rusqlite::Connection::open("/database.sqlite").unwrap();
+    let row: String = db
+        .query_row("SELECT value FROM data", [], |r| r.get(0))
+        .unwrap();
+    assert_eq!(row, "row");
+}