Initial reorg.

This is largely the same as #305, but updated for the current tree.

crates/wasi-common/src/sys/windows/mod.rs

@@ -0,0 +1,32 @@
+pub(crate) mod fdentry_impl;
+pub(crate) mod host_impl;
+pub(crate) mod hostcalls_impl;
+
+use crate::Result;
+use std::fs::{File, OpenOptions};
+use std::path::Path;
+
+pub(crate) fn dev_null() -> Result<File> {
+    OpenOptions::new()
+        .read(true)
+        .write(true)
+        .open("NUL")
+        .map_err(Into::into)
+}
+
+pub fn preopen_dir<P: AsRef<Path>>(path: P) -> 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)
+        .map_err(Into::into)
+}