Auto merge of rust-lang#3809 - RalfJung:fd-refcell, r=oli-obk

FD: remove big surrounding RefCell, simplify socketpair

A while ago, I added the big implicit RefCell for all file descriptions since it avoided interior mutability in `eventfd`. However, this requires us to hold the RefCell "lock" around the entire invocation of the `read`/`write` methods on an FD, which is not great. For instance, if an FD wants to update epoll notifications from inside its `read`/`write`, it is very crucial that the notification check does not end up accessing the FD itself. Such cycles, however, occur naturally:
- eventfd wants to update notifications for itself
- socketfd wants to update notifications on its "peer", which will in turn check *its* peer to see whether that buffer is empty -- and my peer's peer is myself.

This then also lets us simplify socketpair, which currently holds a weak reference to its peer *and* a weak reference to the peer's buffer -- that was previously needed precisely to avoid this issue.

src/tools/miri/src/shims/unix/fd.rs

@@ -2,9 +2,9 @@
 //! standard file descriptors (stdin/stdout/stderr).
 
 use std::any::Any;
-use std::cell::{Ref, RefCell, RefMut};
 use std::collections::BTreeMap;
 use std::io::{self, ErrorKind, IsTerminal, Read, SeekFrom, Write};
+use std::ops::Deref;
 use std::rc::Rc;
 use std::rc::Weak;
 
@@ -27,9 +27,9 @@ pub trait FileDescription: std::fmt::Debug + Any {
 
     /// Reads as much as possible into the given buffer, and returns the number of bytes read.
     fn read<'tcx>(
-        &mut self,
+        &self,
+        _self_ref: &FileDescriptionRef,
         _communicate_allowed: bool,
-        _fd_id: FdId,
         _bytes: &mut [u8],
         _ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<usize>> {
@@ -38,9 +38,9 @@ pub trait FileDescription: std::fmt::Debug + Any {
 
     /// Writes as much as possible from the given buffer, and returns the number of bytes written.
     fn write<'tcx>(
-        &mut self,
+        &self,
+        _self_ref: &FileDescriptionRef,
         _communicate_allowed: bool,
-        _fd_id: FdId,
         _bytes: &[u8],
         _ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<usize>> {
@@ -50,7 +50,7 @@ pub trait FileDescription: std::fmt::Debug + Any {
     /// Reads as much as possible into the given buffer from a given offset,
     /// and returns the number of bytes read.
     fn pread<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _bytes: &mut [u8],
         _offset: u64,
@@ -62,7 +62,7 @@ pub trait FileDescription: std::fmt::Debug + Any {
     /// Writes as much as possible from the given buffer starting at a given offset,
     /// and returns the number of bytes written.
     fn pwrite<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _bytes: &[u8],
         _offset: u64,
@@ -74,7 +74,7 @@ pub trait FileDescription: std::fmt::Debug + Any {
     /// Seeks to the given offset (which can be relative to the beginning, end, or current position).
     /// Returns the new position from the start of the stream.
     fn seek<'tcx>(
-        &mut self,
+        &self,
         _communicate_allowed: bool,
         _offset: SeekFrom,
     ) -> InterpResult<'tcx, io::Result<u64>> {
@@ -111,14 +111,9 @@ pub trait FileDescription: std::fmt::Debug + Any {
 
 impl dyn FileDescription {
     #[inline(always)]
-    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
+    pub fn downcast<T: Any>(&self) -> Option<&T> {
         (self as &dyn Any).downcast_ref()
     }
-
-    #[inline(always)]
-    pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
-        (self as &mut dyn Any).downcast_mut()
-    }
 }
 
 impl FileDescription for io::Stdin {
@@ -127,17 +122,17 @@ impl FileDescription for io::Stdin {
     }
 
     fn read<'tcx>(
-        &mut self,
+        &self,
+        _self_ref: &FileDescriptionRef,
         communicate_allowed: bool,
-        _fd_id: FdId,
         bytes: &mut [u8],
         _ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<usize>> {
         if !communicate_allowed {
             // We want isolation mode to be deterministic, so we have to disallow all reads, even stdin.
             helpers::isolation_abort_error("`read` from stdin")?;
         }
-        Ok(Read::read(self, bytes))
+        Ok(Read::read(&mut { self }, bytes))
     }
 
     fn is_tty(&self, communicate_allowed: bool) -> bool {
@@ -151,14 +146,14 @@ impl FileDescription for io::Stdout {
     }
 
     fn write<'tcx>(
-        &mut self,
+        &self,
+        _self_ref: &FileDescriptionRef,
         _communicate_allowed: bool,
-        _fd_id: FdId,
         bytes: &[u8],
         _ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<usize>> {
         // We allow writing to stderr even with isolation enabled.
-        let result = Write::write(self, bytes);
+        let result = Write::write(&mut { self }, bytes);
         // Stdout is buffered, flush to make sure it appears on the
         // screen.  This is the write() syscall of the interpreted
         // program, we want it to correspond to a write() syscall on
@@ -180,9 +175,9 @@ impl FileDescription for io::Stderr {
     }
 
     fn write<'tcx>(
-        &mut self,
+        &self,
+        _self_ref: &FileDescriptionRef,
         _communicate_allowed: bool,
-        _fd_id: FdId,
         bytes: &[u8],
         _ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<usize>> {
@@ -206,9 +201,9 @@ impl FileDescription for NullOutput {
     }
 
     fn write<'tcx>(
-        &mut self,
+        &self,
+        _self_ref: &FileDescriptionRef,
         _communicate_allowed: bool,
-        _fd_id: FdId,
         bytes: &[u8],
         _ecx: &mut MiriInterpCx<'tcx>,
     ) -> InterpResult<'tcx, io::Result<usize>> {
@@ -221,26 +216,23 @@ impl FileDescription for NullOutput {
 #[derive(Clone, Debug)]
 pub struct FileDescWithId<T: FileDescription + ?Sized> {
     id: FdId,
-    file_description: RefCell<Box<T>>,
+    file_description: Box<T>,
 }
 
 #[derive(Clone, Debug)]
 pub struct FileDescriptionRef(Rc<FileDescWithId<dyn FileDescription>>);
 
-impl FileDescriptionRef {
-    fn new(fd: impl FileDescription, id: FdId) -> Self {
-        FileDescriptionRef(Rc::new(FileDescWithId {
-            id,
-            file_description: RefCell::new(Box::new(fd)),
-        }))
-    }
+impl Deref for FileDescriptionRef {
+    type Target = dyn FileDescription;
 
-    pub fn borrow(&self) -> Ref<'_, dyn FileDescription> {
-        Ref::map(self.0.file_description.borrow(), |fd| fd.as_ref())
+    fn deref(&self) -> &Self::Target {
+        &*self.0.file_description
     }
+}
 
-    pub fn borrow_mut(&self) -> RefMut<'_, dyn FileDescription> {
-        RefMut::map(self.0.file_description.borrow_mut(), |fd| fd.as_mut())
+impl FileDescriptionRef {
+    fn new(fd: impl FileDescription, id: FdId) -> Self {
+        FileDescriptionRef(Rc::new(FileDescWithId { id, file_description: Box::new(fd) }))
     }
 
     pub fn close<'tcx>(
@@ -256,7 +248,7 @@ impl FileDescriptionRef {
                 // Remove entry from the global epoll_event_interest table.
                 ecx.machine.epoll_interests.remove(id);
 
-                RefCell::into_inner(fd.file_description).close(communicate_allowed, ecx)
+                fd.file_description.close(communicate_allowed, ecx)
             }
             None => Ok(Ok(())),
         }
@@ -269,16 +261,6 @@ impl FileDescriptionRef {
     pub fn get_id(&self) -> FdId {
         self.0.id
     }
-
-    /// Function used to retrieve the readiness events of a file description and insert
-    /// an `EpollEventInstance` into the ready list if the file description is ready.
-    pub(crate) fn check_and_update_readiness<'tcx>(
-        &self,
-        ecx: &mut InterpCx<'tcx, MiriMachine<'tcx>>,
-    ) -> InterpResult<'tcx, ()> {
-        use crate::shims::unix::linux::epoll::EvalContextExt;
-        ecx.check_and_update_readiness(self.get_id(), || self.borrow_mut().get_epoll_ready_events())
-    }
 }
 
 /// Holds a weak reference to the actual file description.
@@ -334,11 +316,20 @@ impl FdTable {
         fds
     }
 
-    /// Insert a new file description to the FdTable.
-    pub fn insert_new(&mut self, fd: impl FileDescription) -> i32 {
+    pub fn new_ref(&mut self, fd: impl FileDescription) -> FileDescriptionRef {
         let file_handle = FileDescriptionRef::new(fd, self.next_file_description_id);
         self.next_file_description_id = FdId(self.next_file_description_id.0.strict_add(1));
-        self.insert_ref_with_min_fd(file_handle, 0)
+        file_handle
+    }
+
+    /// Insert a new file description to the FdTable.
+    pub fn insert_new(&mut self, fd: impl FileDescription) -> i32 {
+        let fd_ref = self.new_ref(fd);
+        self.insert(fd_ref)
+    }
+
+    pub fn insert(&mut self, fd_ref: FileDescriptionRef) -> i32 {
+        self.insert_ref_with_min_fd(fd_ref, 0)
     }
 
     /// Insert a file description, giving it a file descriptor that is at least `min_fd`.
@@ -368,17 +359,7 @@ impl FdTable {
         new_fd
     }
 
-    pub fn get(&self, fd: i32) -> Option<Ref<'_, dyn FileDescription>> {
-        let fd = self.fds.get(&fd)?;
-        Some(fd.borrow())
-    }
-
-    pub fn get_mut(&self, fd: i32) -> Option<RefMut<'_, dyn FileDescription>> {
-        let fd = self.fds.get(&fd)?;
-        Some(fd.borrow_mut())
-    }
-
-    pub fn get_ref(&self, fd: i32) -> Option<FileDescriptionRef> {
+    pub fn get(&self, fd: i32) -> Option<FileDescriptionRef> {
         let fd = self.fds.get(&fd)?;
         Some(fd.clone())
     }
@@ -397,7 +378,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     fn dup(&mut self, old_fd: i32) -> InterpResult<'tcx, Scalar> {
         let this = self.eval_context_mut();
 
-        let Some(dup_fd) = this.machine.fds.get_ref(old_fd) else {
+        let Some(dup_fd) = this.machine.fds.get(old_fd) else {
             return Ok(Scalar::from_i32(this.fd_not_found()?));
         };
         Ok(Scalar::from_i32(this.machine.fds.insert_ref_with_min_fd(dup_fd, 0)))
@@ -406,7 +387,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     fn dup2(&mut self, old_fd: i32, new_fd: i32) -> InterpResult<'tcx, Scalar> {
         let this = self.eval_context_mut();
 
-        let Some(dup_fd) = this.machine.fds.get_ref(old_fd) else {
+        let Some(dup_fd) = this.machine.fds.get(old_fd) else {
             return Ok(Scalar::from_i32(this.fd_not_found()?));
         };
         if new_fd != old_fd {
@@ -492,7 +473,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             }
             let start = this.read_scalar(&args[2])?.to_i32()?;
 
-            match this.machine.fds.get_ref(fd) {
+            match this.machine.fds.get(fd) {
                 Some(dup_fd) =>
                     Ok(Scalar::from_i32(this.machine.fds.insert_ref_with_min_fd(dup_fd, start))),
                 None => Ok(Scalar::from_i32(this.fd_not_found()?)),
@@ -565,7 +546,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         let communicate = this.machine.communicate();
 
         // We temporarily dup the FD to be able to retain mutable access to `this`.
-        let Some(fd) = this.machine.fds.get_ref(fd) else {
+        let Some(fd) = this.machine.fds.get(fd) else {
             trace!("read: FD not found");
             return Ok(Scalar::from_target_isize(this.fd_not_found()?, this));
         };
@@ -576,14 +557,14 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         // `usize::MAX` because it is bounded by the host's `isize`.
         let mut bytes = vec![0; usize::try_from(count).unwrap()];
         let result = match offset {
-            None => fd.borrow_mut().read(communicate, fd.get_id(), &mut bytes, this),
+            None => fd.read(&fd, communicate, &mut bytes, this),
             Some(offset) => {
                 let Ok(offset) = u64::try_from(offset) else {
                     let einval = this.eval_libc("EINVAL");
                     this.set_last_error(einval)?;
                     return Ok(Scalar::from_target_isize(-1, this));
                 };
-                fd.borrow_mut().pread(communicate, &mut bytes, offset, this)
+                fd.pread(communicate, &mut bytes, offset, this)
             }
         };
 
@@ -629,19 +610,19 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         let bytes = this.read_bytes_ptr_strip_provenance(buf, Size::from_bytes(count))?.to_owned();
         // We temporarily dup the FD to be able to retain mutable access to `this`.
-        let Some(fd) = this.machine.fds.get_ref(fd) else {
+        let Some(fd) = this.machine.fds.get(fd) else {
             return Ok(Scalar::from_target_isize(this.fd_not_found()?, this));
         };
 
         let result = match offset {
-            None => fd.borrow_mut().write(communicate, fd.get_id(), &bytes, this),
+            None => fd.write(&fd, communicate, &bytes, this),
             Some(offset) => {
                 let Ok(offset) = u64::try_from(offset) else {
                     let einval = this.eval_libc("EINVAL");
                     this.set_last_error(einval)?;
                     return Ok(Scalar::from_target_isize(-1, this));
                 };
-                fd.borrow_mut().pwrite(communicate, &bytes, offset, this)
+                fd.pwrite(communicate, &bytes, offset, this)
             }
         };