Move preopen_dir handlers from wasmtime to wasi-common

src/lib.rs

@@ -29,3 +29,4 @@ pub mod memory;
 pub mod wasm32;
 
 pub use ctx::{WasiCtx, WasiCtxBuilder};
+pub use sys::preopen_dir;

src/sys/unix/mod.rs

@@ -2,6 +2,14 @@ pub mod fdentry;
 mod host_impl;
 pub mod hostcalls;
 
-pub fn dev_null() -> std::fs::File {
-    std::fs::File::open("/dev/null").expect("failed to open /dev/null")
+use std::fs::File;
+use std::io;
+use std::path::Path;
+
+pub fn dev_null() -> File {
+    File::open("/dev/null").expect("failed to open /dev/null")
+}
+
+pub fn preopen_dir<P: AsRef<Path>>(path: P) -> io::Result<File> {
+    File::open(path)
 }

src/sys/windows/mod.rs

@@ -3,7 +3,25 @@ mod host_impl;
 pub mod hostcalls;
 
 use std::fs::File;
+use std::io;
+use std::path::Path;
 
 pub fn dev_null() -> File {
     File::open("NUL").expect("failed to open NUL")
 }
+
+pub fn preopen_dir<P: AsRef<Path>>(path: P) -> io::Result<File> {
+    use std::fs::OpenOptions;
+    use std::os::windows::fs::OpenOptionsExt;
+    use winapi::um::winbase::FILE_FLAG_BACKUP_SEMANTICS;
+
+    // To open a directory using CreateFile, specify the
+    // FILE_FLAG_BACKUP_SEMANTICS flag as part of dwFileFlags...
+    // cf. https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-createfile2
+    OpenOptions::new()
+        .create(false)
+        .write(true)
+        .read(true)
+        .attributes(FILE_FLAG_BACKUP_SEMANTICS)
+        .open(path)
+}