Add RefCellValueObserver (#1363)

* add RefCellValueObserver

* appease the clippy gods

* Update libafl/src/observers/value.rs

Co-authored-by: Langston Barrett <langston.barrett@gmail.com>

---------

Co-authored-by: Langston Barrett <langston.barrett@gmail.com>

libafl/src/observers/value.rs

@@ -5,8 +5,10 @@ use alloc::{
     string::{String, ToString},
 };
 use core::{
+    cell::{Ref, RefCell},
     fmt::Debug,
     hash::{BuildHasher, Hash, Hasher},
+    ops::Deref,
 };
 
 use ahash::RandomState;
@@ -22,9 +24,8 @@ use crate::{
 
 /// A simple observer with a single value.
 ///
-/// The intent is that the value is something with interior mutability (e.g., a
-/// `RefCell`), which the target could write to even though this observer has a
-/// reference to it.
+/// The intent is that the value is something with interior mutability which the target could write to even though this
+/// observer has a reference to it. Use [`RefCellValueObserver`] if using a [`RefCell`] around the value.
 #[derive(Serialize, Deserialize, Debug)]
 #[serde(bound = "T: serde::de::DeserializeOwned")]
 pub struct ValueObserver<'a, T>
@@ -104,3 +105,87 @@ where
         Some(s.finish())
     }
 }
+
+/// A simple observer with a single [`RefCell`]'d value.
+#[derive(Serialize, Deserialize, Debug)]
+#[serde(bound = "T: serde::de::DeserializeOwned")]
+pub struct RefCellValueObserver<'a, T>
+where
+    T: Debug + Serialize,
+{
+    /// The name of this observer.
+    name: String,
+    /// The value.
+    pub value: OwnedRef<'a, RefCell<T>>,
+}
+
+impl<'a, T> RefCellValueObserver<'a, T>
+where
+    T: Debug + Serialize + serde::de::DeserializeOwned,
+{
+    /// Creates a new [`RefCellValueObserver`] with the given name.
+    #[must_use]
+    pub fn new(name: &'static str, value: &'a RefCell<T>) -> Self {
+        Self {
+            name: name.to_string(),
+            value: OwnedRef::Ref(value),
+        }
+    }
+
+    /// Get a reference to the underlying value.
+    #[must_use]
+    pub fn get_ref<'b>(&'b self) -> Ref<'a, T>
+    where
+        'b: 'a,
+    {
+        self.value.as_ref().borrow()
+    }
+
+    /// Set the value.
+    pub fn set(&mut self, new_value: T) {
+        self.value.as_ref().replace(new_value);
+    }
+
+    /// Clone or move the current value out of this object.
+    #[must_use]
+    pub fn take(self) -> T
+    where
+        T: Clone,
+    {
+        match self.value {
+            OwnedRef::Ref(r) => r.borrow().deref().clone(),
+            OwnedRef::Owned(v) => v.borrow().clone(),
+        }
+    }
+}
+
+/// This *does not* reset the value inside the observer.
+impl<'a, S, T> Observer<S> for RefCellValueObserver<'a, T>
+where
+    S: UsesInput,
+    T: Debug + Serialize + serde::de::DeserializeOwned,
+{
+    fn pre_exec(&mut self, _state: &mut S, _input: &S::Input) -> Result<(), Error> {
+        Ok(())
+    }
+}
+
+impl<'a, T> Named for RefCellValueObserver<'a, T>
+where
+    T: Debug + Serialize + serde::de::DeserializeOwned,
+{
+    fn name(&self) -> &str {
+        &self.name
+    }
+}
+
+impl<'a, T: Hash> ObserverWithHashField for RefCellValueObserver<'a, T>
+where
+    T: Debug + Serialize + serde::de::DeserializeOwned,
+{
+    fn hash(&self) -> Option<u64> {
+        let mut s = RandomState::with_seeds(1, 2, 3, 4).build_hasher();
+        self.value.as_ref().borrow().hash(&mut s);
+        Some(s.finish())
+    }
+}