feat(metrics/family): add Family::get_or_create_clone()

fixes prometheus#231.

this introduces a new method to `Family<S, M, C>` to help alleviate the
risk of deadlocks when accessing multiple series within a given metrics
family.

this returns a plain `M`, rather than the `MappedRwLockReadGuard<M>`
RAII guard returned by `get_or_create()`.

a test case is introduced in this commit to demonstrate that structures
accessing multiple series within a single expression will not
accidentally create a deadlock.

Signed-off-by: katelyn martin <[email protected]>

src/metrics/family.rs

@@ -207,6 +207,37 @@ impl<S: Clone + std::hash::Hash + Eq, M, C> Family<S, M, C> {
     }
 }
 
+impl<S: Clone + std::hash::Hash + Eq, M: Clone, C: MetricConstructor<M>> Family<S, M, C>
+where
+    S: Clone + std::hash::Hash + Eq,
+    M: Clone,
+    C: MetricConstructor<M>,
+{
+    /// Access a metric with the given label set, creating it if one does not yet exist.
+    ///
+    /// ```
+    /// # use prometheus_client::metrics::counter::{Atomic, Counter};
+    /// # use prometheus_client::metrics::family::Family;
+    /// #
+    /// let family = Family::<Vec<(String, String)>, Counter>::default();
+    ///
+    /// // Will create and return the metric with label `method="GET"` when first called.
+    /// family.get_or_create_clone(&vec![("method".to_owned(), "GET".to_owned())]).inc();
+    ///
+    /// // Will return a clone of the existing metric on all subsequent calls.
+    /// family.get_or_create_clone(&vec![("method".to_owned(), "GET".to_owned())]).inc();
+    /// ```
+    pub fn get_or_create_clone(&self, label_set: &S) -> M {
+        use std::ops::Deref;
+
+        let guard = self.get_or_create(label_set);
+        let metric = guard.deref().to_owned();
+        drop(guard);
+
+        metric
+    }
+}
+
 impl<S: Clone + std::hash::Hash + Eq, M, C: MetricConstructor<M>> Family<S, M, C> {
     /// Access a metric with the given label set, creating it if one does not
     /// yet exist.
@@ -452,4 +483,23 @@ mod tests {
                 .get()
         );
     }
+
+    /// Tests that [`Family::get_or_create_clone()`] does not cause deadlocks.
+    #[test]
+    fn counter_family_does_not_deadlock() {
+        /// A structure we'll place two counters into, within a single expression.
+        struct S {
+            apples: Counter,
+            oranges: Counter,
+        }
+
+        let family = Family::<(&str, &str), Counter>::default();
+        let s = S {
+            apples: family.get_or_create_clone(&("kind", "apple")),
+            oranges: family.get_or_create_clone(&("kind", "orange")),
+        };
+
+        s.apples.inc();
+        s.oranges.inc_by(2);
+    }
 }