Tweak retry counter

src/lib.rs

@@ -986,7 +986,9 @@ impl Rng for ThreadRng {
 /// `EntropyRng` uses the interface for random numbers provided by the operating
 /// system ([`OsRng`]). If that returns an error, it will fall back to the
 /// [`JitterRng`] entropy collector. Occasionally it will then check if `OsRng`
-/// is still not available, and switch back if possible.
+/// is still not available, and switch back if possible. Once every 8 calls to
+/// `try_fill_bytes` if `OsRng` failed with `ErrorKind:Unavailable`, otherwise
+/// with every call.
 ///
 /// [`OsRng`]: os/struct.OsRng.html
 /// [`JitterRng`]: jitter/struct.JitterRng.html
@@ -1074,22 +1076,28 @@ impl Rng for EntropyRng {
                 }
             }
             EntropySource::Jitter(ref mut rng) => {
-                if self.counter >= 8 {
-                    if let Ok(os_rng) = try_os_new(dest) {
-                        switch_rng = Some(EntropySource::Os(os_rng));
-                    } else {
-                        self.counter = (self.counter + 1) % 8;
-                        return rng.try_fill_bytes(dest); // use JitterRng
+                if self.counter == 0 {
+                    match try_os_new(dest) {
+                        Ok(os_rng) => {
+                            switch_rng = Some(EntropySource::Os(os_rng));
+                        }
+                        Err(e) => {
+                            if e.kind() == ErrorKind::Unavailable {
+                                self.counter = 8; // Wait 8 calls before retry
+                            } else {
+                                self.counter = 0; // Retry on next call
+                            }
+                            return rng.try_fill_bytes(dest); // use JitterRng
+                        }
                     }
                 } else {
-                    self.counter = (self.counter + 1) % 8;
+                    self.counter -= 1;
                     return rng.try_fill_bytes(dest); // use JitterRng
                 }
             }
         }
         if let Some(rng) = switch_rng {
             self.rng = rng;
-            self.counter = 0;
         }
         Ok(())
     }