Make cache_set_lifespan the inverse of cache_get_lifespan

This makes it possible to temporarily change a cache lifespan, or to
remove a cache lifespan once it's set. See jaemk#197 for context.

src/lib.rs

@@ -419,7 +419,9 @@ pub trait IOCached<K, V> {
     }
 
     /// Set the lifespan of cached values, returns the old value
-    fn cache_set_lifespan(&mut self, _seconds: u64) -> Option<u64> {
+    ///
+    /// If the lifespan is set to `None`, cached values do not expire.
+    fn cache_set_lifespan(&mut self, _seconds: Option<u64>) -> Option<u64> {
         None
     }
 }

src/stores/disk.rs

@@ -273,9 +273,9 @@ where
         self.seconds
     }
 
-    fn cache_set_lifespan(&mut self, seconds: u64) -> Option<u64> {
+    fn cache_set_lifespan(&mut self, seconds: Option<u64>) -> Option<u64> {
         let old = self.seconds;
-        self.seconds = Some(seconds);
+        self.seconds = seconds;
         old
     }
 
@@ -346,15 +346,15 @@ mod tests {
         sleep(Duration::new(2, 500000));
         assert!(c.cache_get(&1).unwrap().is_none());
 
-        let old = c.cache_set_lifespan(1).unwrap();
+        let old = c.cache_set_lifespan(Some(1)).unwrap();
         assert_eq!(2, old);
         assert!(c.cache_set(1, 100).unwrap().is_none());
         assert!(c.cache_get(&1).unwrap().is_some());
 
         sleep(Duration::new(1, 600000));
         assert!(c.cache_get(&1).unwrap().is_none());
 
-        c.cache_set_lifespan(10).unwrap();
+        c.cache_set_lifespan(Some(10)).unwrap();
         assert!(c.cache_set(1, 100).unwrap().is_none());
         assert!(c.cache_set(2, 100).unwrap().is_none());
         assert_eq!(c.cache_get(&1).unwrap().unwrap(), 100);

src/stores/redis.rs

@@ -345,9 +345,11 @@ where
         Some(self.seconds)
     }
 
-    fn cache_set_lifespan(&mut self, seconds: u64) -> Option<u64> {
+    fn cache_set_lifespan(&mut self, seconds: Option<u64>) -> Option<u64> {
         let old = self.seconds;
-        self.seconds = seconds;
+        // Redis must have a cache TTL, so get as close to an unbounded cache as we can if
+        // the user wants values to never expire.
+        self.seconds = seconds.unwrap_or(u64::MAX);
         Some(old)
     }
 
@@ -736,15 +738,15 @@ mod tests {
         sleep(Duration::new(2, 500_000));
         assert!(c.cache_get(&1).unwrap().is_none());
 
-        let old = c.cache_set_lifespan(1).unwrap();
+        let old = c.cache_set_lifespan(Some(1)).unwrap();
         assert_eq!(2, old);
         assert!(c.cache_set(1, 100).unwrap().is_none());
         assert!(c.cache_get(&1).unwrap().is_some());
 
         sleep(Duration::new(1, 600_000));
         assert!(c.cache_get(&1).unwrap().is_none());
 
-        c.cache_set_lifespan(10).unwrap();
+        c.cache_set_lifespan(Some(10)).unwrap();
         assert!(c.cache_set(1, 100).unwrap().is_none());
         assert!(c.cache_set(2, 100).unwrap().is_none());
         assert_eq!(c.cache_get(&1).unwrap().unwrap(), 100);