feat: callback_with_output

This changeset introduces callback_with_output, which is a way to use a callback at runtime to determine what parameter to pass to a potential failpoint.

Signed-off-by: o0Ignition0o <[email protected]>

Author: o0Ignition0o

src/lib.rs

@@ -260,6 +260,34 @@ impl SyncCallback {
     }
 }
 
+#[derive(Clone)]
+struct SyncCallbackWithOutput(Arc<dyn Fn() -> Option<Option<String>> + Send + Sync>);
+
+impl Debug for SyncCallbackWithOutput {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("SyncCallbackWithOutput()")
+    }
+}
+
+impl PartialEq for SyncCallbackWithOutput {
+    fn eq(&self, other: &Self) -> bool {
+        Arc::ptr_eq(&self.0, &other.0)
+    }
+}
+
+impl SyncCallbackWithOutput {
+    fn new(
+        f: impl Fn() -> Option<Option<String>> + Send + Sync + 'static,
+    ) -> SyncCallbackWithOutput {
+        SyncCallbackWithOutput(Arc::new(f))
+    }
+
+    fn run(&self) -> Option<Option<String>> {
+        let callback = &self.0;
+        callback()
+    }
+}
+
 /// Supported tasks.
 #[derive(Clone, Debug, PartialEq)]
 enum Task {
@@ -281,6 +309,8 @@ enum Task {
     Delay(u64),
     /// Call callback function.
     Callback(SyncCallback),
+    /// Call callback function, and pass output to the fail_point expression.
+    CallbackWithOutput(SyncCallbackWithOutput),
 }
 
 #[derive(Debug)]
@@ -324,6 +354,17 @@ impl Action {
         }
     }
 
+    fn from_callback_with_output(
+        f: impl Fn() -> Option<Option<String>> + Send + Sync + 'static,
+    ) -> Action {
+        let task = Task::CallbackWithOutput(SyncCallbackWithOutput::new(f));
+        Action {
+            task,
+            freq: 1.0,
+            count: None,
+        }
+    }
+
     fn get_task(&self) -> Option<Task> {
         use rand::Rng;
 
@@ -508,6 +549,7 @@ impl FailPoint {
             Task::Callback(f) => {
                 f.run();
             }
+            Task::CallbackWithOutput(f) => return f.run(),
         }
         None
     }
@@ -696,6 +738,28 @@ where
     Ok(())
 }
 
+/// Configure the actions for a fail point at runtime.
+///
+/// Each fail point can be configured by a callback. Process will call this callback function
+/// when it meet this fail-point.
+/// Its output will be used as the expression parameter for the `fail_point!` macro.
+///
+/// Refer to the `test_callback_with_output` test for more information.
+pub fn cfg_callback_with_output<S, F>(name: S, f: F) -> Result<(), String>
+where
+    S: Into<String>,
+    F: Fn() -> Option<Option<String>> + Send + Sync + 'static,
+{
+    let mut registry = REGISTRY.registry.write().unwrap();
+    let p = registry
+        .entry(name.into())
+        .or_insert_with(|| Arc::new(FailPoint::new()));
+    let action = Action::from_callback_with_output(f);
+    let actions = vec![action];
+    p.set_actions("callback_with_output", actions);
+    Ok(())
+}
+
 /// Remove a fail point.
 ///
 /// If the fail point doesn't exist, nothing will happen.

tests/tests.rs

@@ -160,6 +160,70 @@ fn test_callback() {
     assert_eq!(2, counter.load(Ordering::SeqCst));
 }
 
+#[test]
+#[cfg_attr(not(feature = "failpoints"), ignore)]
+fn test_callback_with_output() {
+    let return_42 = || {
+        fail_point!("cb_with_output", |output_from_callback: Option<String>| {
+            if let Some(output) = output_from_callback {
+                let as_i32 = output.parse().expect("invalid integer");
+                // return the number that was passed
+                as_i32
+            } else {
+                // No output was passed by the callback,
+                // return an arbitrary number
+                84
+            }
+        });
+        // default function behavior
+        42
+    };
+
+    let counter = Arc::new(AtomicUsize::new(0));
+    let counter2 = counter.clone();
+
+    // We haven't set up the callback yet,
+    // the function will behave as usual
+    assert_eq!(42, return_42());
+
+    // Configure the callback to return the counter,
+    // Only if counter can be divided by two
+    fail::cfg_callback_with_output("cb_with_output", move || {
+        let prev = counter2.fetch_add(1, Ordering::SeqCst);
+
+        if prev == 0 {
+            // First call, we decide to not return anything
+            return None;
+        }
+
+        if prev % 2 == 0 {
+            Some(Some(prev.to_string()))
+        } else {
+            Some(None)
+        }
+    })
+    .unwrap();
+
+    // Fist call via the callback,
+    // The callback must have passed `None`
+    // to the fail point,
+    // Thus not triggering it.
+    assert_eq!(42, return_42());
+    // Second call via the callback,
+    // which returned Some(None)
+    // We entered the "arbitrary number" branch
+    assert_eq!(84, return_42());
+    // Third call via the callback,
+    // which returned Some(Some("2".to_string()))
+    // We entered the "return passed number" branch
+    assert_eq!(2, return_42());
+
+    // The callback has always been called,
+    // it was responsible for determining the parameter
+    // passed to the failpoint
+    assert_eq!(3, counter.load(Ordering::SeqCst));
+}
+
 #[test]
 #[cfg_attr(not(feature = "failpoints"), ignore)]
 fn test_delay() {