Support fd_fdstat_get and fd_renumber on stdin/stdout/stderr (#631)

* Support fd_fdstat_get on stdin/stdout/stderr.

Add a routine for obtaining an `OsFile` containing a file descriptor for
stdin/stdout/stderr so that we can do fd_fdstat_get on them.

* Add a testcase for fd_fdstat_get etc. on stdin etc.

* Don't dup file descriptors in fd_renumber.

* Fix compilation on macOS

* Rename OsFile to OsHandle

This commits renames `OsFile` to `OsHandle` which seems to make
more sense semantically as it is permitted to hold a valid OS handle
to OS entities other than simply file/dir (e.g., socket, stream, etc.).
As such, this commit also renames methods on `Descriptor` struct
from `as_actual_file` to `as_file` as this in reality does pertain
ops on FS entities such as files/dirs, and `as_file` to `as_os_handle`
as in this case it can be anything, from file, through a socket, to
a stream.

* Fix compilation on Linux

* Introduce `OsHandleRef` for borrowing OS resources.

To prevent a `ManuallyDrop<OsHandleRef>` from outliving the resource it
holds on to, create an `OsHandleRef` class parameterized on the lifetime
of the `Descriptor`.

* Fix scoping to pub-priv and backport to snapshot_0

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

@@ -1,26 +1,27 @@
-use crate::fdentry::Descriptor;
+use crate::fdentry::{Descriptor, OsHandleRef};
 use crate::{wasi, Error, Result};
 use std::fs::File;
 use std::io;
+use std::mem::ManuallyDrop;
 use std::ops::{Deref, DerefMut};
 use std::os::windows::prelude::{AsRawHandle, FromRawHandle, RawHandle};
 
 #[derive(Debug)]
-pub(crate) struct OsFile(File);
+pub(crate) struct OsHandle(File);
 
-impl From<File> for OsFile {
+impl From<File> for OsHandle {
     fn from(file: File) -> Self {
         Self(file)
     }
 }
 
-impl AsRawHandle for OsFile {
+impl AsRawHandle for OsHandle {
     fn as_raw_handle(&self) -> RawHandle {
         self.0.as_raw_handle()
     }
 }
 
-impl Deref for OsFile {
+impl Deref for OsHandle {
     type Target = File;
 
     fn deref(&self) -> &Self::Target {
@@ -28,7 +29,7 @@ impl Deref for OsFile {
     }
 }
 
-impl DerefMut for OsFile {
+impl DerefMut for OsHandle {
     fn deref_mut(&mut self) -> &mut Self::Target {
         &mut self.0
     }
@@ -37,7 +38,7 @@ impl DerefMut for OsFile {
 impl AsRawHandle for Descriptor {
     fn as_raw_handle(&self) -> RawHandle {
         match self {
-            Self::OsFile(file) => file.as_raw_handle(),
+            Self::OsHandle(file) => file.as_raw_handle(),
             Self::Stdin => io::stdin().as_raw_handle(),
             Self::Stdout => io::stdout().as_raw_handle(),
             Self::Stderr => io::stderr().as_raw_handle(),
@@ -45,6 +46,14 @@ impl AsRawHandle for Descriptor {
     }
 }
 
+pub(crate) fn descriptor_as_oshandle<'lifetime>(
+    desc: &'lifetime Descriptor,
+) -> OsHandleRef<'lifetime> {
+    OsHandleRef::new(ManuallyDrop::new(OsHandle::from(unsafe {
+        File::from_raw_handle(desc.as_raw_handle())
+    })))
+}
+
 /// This function is unsafe because it operates on a raw file handle.
 pub(crate) unsafe fn determine_type_and_access_rights<Handle: AsRawHandle>(
     handle: &Handle,

crates/wasi-common/src/sys/windows/hostcalls_impl/fs.rs

@@ -5,7 +5,7 @@ use crate::ctx::WasiCtx;
 use crate::fdentry::FdEntry;
 use crate::helpers::systemtime_to_timestamp;
 use crate::hostcalls_impl::{fd_filestat_set_times_impl, Dirent, FileType, PathGet};
-use crate::sys::fdentry_impl::{determine_type_rights, OsFile};
+use crate::sys::fdentry_impl::determine_type_rights;
 use crate::sys::host_impl::{self, path_from_host};
 use crate::sys::hostcalls_impl::fs_helpers::PathGetExt;
 use crate::{wasi, Error, Result};