Define a file descriptor reservation/commit/rollback abstraction.

Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>

Author: wedsonaf

rust/kernel/file.rs

@@ -82,3 +82,49 @@ impl Deref for FileRef {
         self.0.deref()
     }
 }
+
+/// A file descriptor reservation.
+///
+/// This allows the creation of a file descriptor in two steps: first, we reserve a slot for it,
+/// then we commit or drop the reservation. The first step may fail (e.g., the current process ran
+/// out of available slots), but commit and drop never fail (and are mutually exclusive).
+pub struct FileDescriptorReservation {
+    fd: u32,
+}
+
+impl FileDescriptorReservation {
+    /// Creates a new file descriptor reservation.
+    pub fn new(flags: u32) -> Result<Self> {
+        let fd = unsafe { bindings::get_unused_fd_flags(flags) };
+        if fd < 0 {
+            return Err(Error::from_kernel_errno(fd));
+        }
+        Ok(Self { fd: fd as _ })
+    }
+
+    /// Returns the file descriptor number that was reserved.
+    pub fn reserved_fd(&self) -> u32 {
+        self.fd
+    }
+
+    /// Commits the reservation.
+    ///
+    /// The previously reserved file descriptor is bound to `file`.
+    pub fn commit(self, file: File) {
+        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`, and `file.ptr` is
+        // guaranteed to have an owned ref count by its type invariants.
+        unsafe { bindings::fd_install(self.fd, file.ptr) };
+
+        // `fd_install` consumes both the file descriptor and the file reference, so we cannot run
+        // the destructors.
+        core::mem::forget(self);
+        core::mem::forget(file);
+    }
+}
+
+impl Drop for FileDescriptorReservation {
+    fn drop(&mut self) {
+        // SAFETY: `self.fd` was returned by a previous call to `get_unused_fd_flags`.
+        unsafe { bindings::put_unused_fd(self.fd) };
+    }
+}