Introduce strongly-typed system primitives (#1561)

* Introduce strongly-typed system primitives

This commit does a lot of reshuffling and even some more. It introduces
strongly-typed system primitives which are: `OsFile`, `OsDir`, `Stdio`,
and `OsOther`. Those primitives are separate structs now, each implementing
a subset of `Handle` methods, rather than all being an enumeration of some
supertype such as `OsHandle`. To summarise the structs:

* `OsFile` represents a regular file, and implements fd-ops
  of `Handle` trait
* `OsDir` represents a directory, and primarily implements path-ops, plus
  `readdir` and some common fd-ops such as `fdstat`, etc.
* `Stdio` represents a stdio handle, and implements a subset of fd-ops
  such as `fdstat` _and_ `read_` and `write_vectored` calls
* `OsOther` currently represents anything else and implements a set similar
  to that implemented by `Stdio`

This commit is effectively an experiment and an excercise into better
understanding what's going on for each OS resource/type under-the-hood.
It's meant to give us some intuition in order to move on with the idea
of having strongly-typed handles in WASI both in the syscall impl as well
as at the libc level.

Some more minor changes include making `OsHandle` represent an OS-specific
wrapper for a raw OS handle (Unix fd or Windows handle). Also, since `OsDir`
is tricky across OSes, we also have a supertype of `OsHandle` called
`OsDirHandle` which may store a `DIR*` stream pointer (mainly BSD). Last but not
least, the `Filetype` and `Rights` are now computed when the resource is created,
rather than every time we call `Handle::get_file_type` and `Handle::get_rights`.
Finally, in order to facilitate the latter, I've converted `EntryRights` into
`HandleRights` and pushed them into each `Handle` implementor.

* Do not adjust rights on Stdio

* Clean up testing for TTY and escaping writes

* Implement AsFile for dyn Handle

This cleans up a lot of repeating boilerplate code todo with
dynamic dispatch.

* Delegate definition of OsDir to OS-specific modules

Delegates defining `OsDir` struct to OS-specific modules (BSD, Linux,
Emscripten, Windows). This way, `OsDir` can safely re-use `OsHandle`
for raw OS handle storage, and can store some aux data such as an
initialized stream ptr in case of BSD. As a result, we can safely
get rid of `OsDirHandle` which IMHO was causing unnecessary noise and
overcomplicating the design. On the other hand, delegating definition
of `OsDir` to OS-specific modules isn't super clean in and of itself
either. Perhaps there's a better way of handling this?

* Check if filetype of OS handle matches WASI filetype when creating

It seems prudent to check if the passed in `File` instance is of
type matching that of the requested WASI filetype. In other words,
we'd like to avoid situations where `OsFile` is created from a
pipe.

* Make AsFile fallible

Return `EBADF` in `AsFile` in case a `Handle` cannot be made into
a `std::fs::File`.

* Remove unnecessary as_file conversion

* Remove unnecessary check for TTY for Stdio handle type

* Fix incorrect stdio ctors on Unix

* Split Stdio into three separate types: Stdin, Stdout, Stderr

* Rename PendingEntry::File to PendingEntry::OsHandle to avoid confusion

* Rename OsHandle to RawOsHandle

Also, since `RawOsHandle` on *nix doesn't need interior mutability
wrt the inner raw file descriptor, we can safely swap the `RawFd`
for `File` instance.

* Add docs explaining what OsOther is

* Allow for stdio to be non-character-device (e.g., piped)

* Return error on bad preopen rather than panic

crates/wasi-common/src/sys/osdir.rs

@@ -0,0 +1,129 @@
+use super::sys_impl::oshandle::RawOsHandle;
+use super::{fd, path, AsFile};
+use crate::handle::{Handle, HandleRights};
+use crate::wasi::{types, Errno, Result};
+use log::{debug, error};
+use std::any::Any;
+use std::io;
+use std::ops::Deref;
+
+// TODO could this be cleaned up?
+// The actual `OsDir` struct is OS-dependent, therefore we delegate
+// its definition to OS-specific modules.
+pub(crate) use super::sys_impl::osdir::OsDir;
+
+impl Deref for OsDir {
+    type Target = RawOsHandle;
+
+    fn deref(&self) -> &Self::Target {
+        &self.handle
+    }
+}
+
+impl Handle for OsDir {
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+    fn try_clone(&self) -> io::Result<Box<dyn Handle>> {
+        let handle = self.handle.try_clone()?;
+        let new = Self::new(self.rights.get(), handle)?;
+        Ok(Box::new(new))
+    }
+    fn get_file_type(&self) -> types::Filetype {
+        types::Filetype::Directory
+    }
+    fn get_rights(&self) -> HandleRights {
+        self.rights.get()
+    }
+    fn set_rights(&self, rights: HandleRights) {
+        self.rights.set(rights)
+    }
+    // FdOps
+    fn fdstat_get(&self) -> Result<types::Fdflags> {
+        fd::fdstat_get(&*self.as_file()?)
+    }
+    fn fdstat_set_flags(&self, fdflags: types::Fdflags) -> Result<()> {
+        if let Some(new_file) = fd::fdstat_set_flags(&*self.as_file()?, fdflags)? {
+            self.handle.update_from(new_file);
+        }
+        Ok(())
+    }
+    fn filestat_get(&self) -> Result<types::Filestat> {
+        fd::filestat_get(&*self.as_file()?)
+    }
+    fn filestat_set_times(
+        &self,
+        atim: types::Timestamp,
+        mtim: types::Timestamp,
+        fst_flags: types::Fstflags,
+    ) -> Result<()> {
+        fd::filestat_set_times(&*self.as_file()?, atim, mtim, fst_flags)
+    }
+    fn readdir<'a>(
+        &'a self,
+        cookie: types::Dircookie,
+    ) -> Result<Box<dyn Iterator<Item = Result<(types::Dirent, String)>> + 'a>> {
+        fd::readdir(self, cookie)
+    }
+    // PathOps
+    fn create_directory(&self, path: &str) -> Result<()> {
+        path::create_directory(self, path)
+    }
+    fn openat(
+        &self,
+        path: &str,
+        read: bool,
+        write: bool,
+        oflags: types::Oflags,
+        fd_flags: types::Fdflags,
+    ) -> Result<Box<dyn Handle>> {
+        path::open(self, path, read, write, oflags, fd_flags)
+    }
+    fn link(
+        &self,
+        old_path: &str,
+        new_handle: Box<dyn Handle>,
+        new_path: &str,
+        follow: bool,
+    ) -> Result<()> {
+        let new_handle = match new_handle.as_any().downcast_ref::<Self>() {
+            None => {
+                error!("Tried to link with handle that's not an OsDir");
+                return Err(Errno::Badf);
+            }
+            Some(handle) => handle,
+        };
+        path::link(self, old_path, new_handle, new_path, follow)
+    }
+    fn symlink(&self, old_path: &str, new_path: &str) -> Result<()> {
+        path::symlink(old_path, self, new_path)
+    }
+    fn readlink(&self, path: &str, buf: &mut [u8]) -> Result<usize> {
+        path::readlink(self, path, buf)
+    }
+    fn readlinkat(&self, path: &str) -> Result<String> {
+        path::readlinkat(self, path)
+    }
+    fn rename(&self, old_path: &str, new_handle: Box<dyn Handle>, new_path: &str) -> Result<()> {
+        let new_handle = match new_handle.as_any().downcast_ref::<Self>() {
+            None => {
+                error!("Tried to rename with handle that's not an OsDir");
+                return Err(Errno::Badf);
+            }
+            Some(handle) => handle,
+        };
+        debug!("rename (old_dirfd, old_path)=({:?}, {:?})", self, old_path);
+        debug!(
+            "rename (new_dirfd, new_path)=({:?}, {:?})",
+            new_handle, new_path
+        );
+        path::rename(self, old_path, new_handle, new_path)
+    }
+    fn remove_directory(&self, path: &str) -> Result<()> {
+        debug!("remove_directory (dirfd, path)=({:?}, {:?})", self, path);
+        path::remove_directory(self, path)
+    }
+    fn unlink_file(&self, path: &str) -> Result<()> {
+        path::unlink_file(self, path)
+    }
+}