Add try_fail_point!

In my project I have a ton of code which uses `anyhow::Result`.  I
want a convenient way to force a function to return a stock error
from a failpoint.  Today this requires something like:

```
    fail::fail_point!("main", true, |msg| {
        let msg = msg.as_deref().unwrap_or("synthetic error");
        Err(anyhow::anyhow!("{msg}"))
    });
```
which is cumbersome to copy around.

Now, I conservatively made this a new macro.  I am not sure how
often the use case of a fail point for an infallible (i.e. non-`Result`)
function occurs.  It may make sense to require those to take
a distinct `inject_point!` or something?

Author: cgwalters

Cargo.toml

@@ -18,6 +18,9 @@ log = { version = "0.4", features = ["std"] }
 once_cell = "1.9.0"
 rand = "0.8"
 
+[dev-dependencies]
+anyhow = "1.0"
+
 [features]
 failpoints = []
 

src/lib.rs

@@ -227,7 +227,8 @@
 
 use std::collections::HashMap;
 use std::env::VarError;
-use std::fmt::Debug;
+use std::error::Error;
+use std::fmt::{Debug, Display};
 use std::str::FromStr;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, Condvar, Mutex, MutexGuard, RwLock, TryLockError};
@@ -428,6 +429,33 @@ impl FromStr for Action {
     }
 }
 
+/// A synthetic error created as part of [`try_fail_point!`].
+#[doc(hidden)]
+#[derive(Debug)]
+pub struct ReturnError(pub String);
+
+impl ReturnError {
+    const SYNTHETIC: &str = "synthetic failpoint error";
+}
+
+impl From<Option<String>> for ReturnError {
+    fn from(msg: Option<String>) -> Self {
+        Self(msg.unwrap_or_else(|| Self::SYNTHETIC.to_string()))
+    }
+}
+
+impl Display for ReturnError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(&self.0)
+    }
+}
+
+impl Error for ReturnError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        None
+    }
+}
+
 #[cfg_attr(feature = "cargo-clippy", allow(clippy::mutex_atomic))]
 #[derive(Debug)]
 struct FailPoint {
@@ -843,6 +871,32 @@ macro_rules! fail_point {
     }};
 }
 
+/// A variant of [`fail_point`] designed for a function that returns [`std::result::Result`].
+///
+/// ```rust
+/// # #[macro_use] extern crate fail;
+/// fn fallible_function() -> Result<u32, Box<dyn std::error::Error>> {
+///     try_fail_point!("a-fail-point");
+///     Ok(42)
+/// }
+/// ```
+///
+/// This
+#[macro_export]
+#[cfg(feature = "failpoints")]
+macro_rules! try_fail_point {
+    ($name:expr) => {{
+        if let Some(e) = $crate::eval($name, |msg| $crate::ReturnError::from(msg)) {
+            return Err(From::from(e));
+        }
+    }};
+    ($name:expr, $cond:expr) => {{
+        if $cond {
+            $crate::try_fail_point!($name);
+        }
+    }};
+}
+
 /// Define a fail point (disabled, see `failpoints` feature).
 #[macro_export]
 #[cfg(not(feature = "failpoints"))]
@@ -852,6 +906,14 @@ macro_rules! fail_point {
     ($name:expr, $cond:expr, $e:expr) => {{}};
 }
 
+/// Define a fail point for a Result-returning function (disabled, see `failpoints` feature).
+#[macro_export]
+#[cfg(not(feature = "failpoints"))]
+macro_rules! try_fail_point {
+    ($name:expr) => {{}};
+    ($name:expr, $cond:expr) => {{}};
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -1032,6 +1094,25 @@ mod tests {
         }
     }
 
+    #[test]
+    #[cfg(feature = "failpoints")]
+    fn test_try_failpoint() -> anyhow::Result<()> {
+        // This is effective a compile-only test.
+        #[allow(dead_code)]
+        fn test_anyhow() -> anyhow::Result<()> {
+            try_fail_point!("tryfail-with-result");
+            Ok(())
+        }
+        #[allow(dead_code)]
+        fn test_stderr() -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
+            try_fail_point!("tryfail-with-result-2");
+            Ok(())
+        }
+        test_anyhow()?;
+        test_stderr().map_err(anyhow::Error::msg)?;
+        Ok(())
+    }
+
     // This case should be tested as integration case, but when calling `teardown` other cases
     // like `test_pause` maybe also affected, so it's better keep it here.
     #[test]