Merge pull request #528 from carljm/fix-book-cycle-fallback

[book] fix the cycle fallback section to match current code

book/src/cycles/fallback.md

@@ -2,17 +2,20 @@
 
 Panicking when a cycle occurs is ok for situations where you believe a cycle is impossible. But sometimes cycles can result from illegal user input and cannot be statically prevented. In these cases, you might prefer to gracefully recover from a cycle rather than panicking the entire query. Salsa supports that with the idea of *cycle recovery*.
 
-To use cycle recovery, you annotate potential participants in the cycle with a `#[salsa::cycle(my_recover_fn)]` attribute. When a cycle occurs, if any participant P has recovery information, then no panic occurs. Instead, the execution of P is aborted and P will execute the recovery function to generate its result. Participants in the cycle that do not have recovery information continue executing as normal, using this recovery result.
+To use cycle recovery, you annotate potential participants in the cycle with the `recovery_fn` argument to `#[salsa::tracked]`, e.g. `#[salsa::tracked(recovery_fn=my_recovery_fn)]`. When a cycle occurs, if any participant P has recovery information, then no panic occurs. Instead, the execution of P is aborted and P will execute the recovery function to generate its result. Participants in the cycle that do not have recovery information continue executing as normal, using this recovery result.
 
-The recovery function has a similar signature to a query function. It is given a reference to your database along with a `salsa::Cycle` describing the cycle that occurred; it returns the result of the query. Example:
+The recovery function has a similar signature to a query function. It is given a reference to your database along with a `salsa::Cycle` describing the cycle that occurred and the arguments to the tracked function that caused the cycle; it returns the result of the query. Example:
 
 ```rust
 fn my_recover_fn(
     db: &dyn MyDatabase,
     cycle: &salsa::Cycle,
+    arg1: T1,
+    ...
+    argN: TN,
 ) -> MyResultValue
 ```
 
-The `db` and `cycle` argument can be used to prepare a useful error message for your users.
+See [the tests](https://github.com/salsa-rs/salsa/blob/cd339fc1c9a6ea0ffb1d09bd3bffb5633f776ef3/tests/cycles.rs#L132-L141) for an example.
 
 **Important:** Although the recovery function is given a `db` handle, you should be careful to avoid creating a cycle from within recovery or invoking queries that may be participating in the current cycle. Attempting to do so can result in inconsistent results.