Do not count downloads as in-flight

Download requests are no longer counted as in-flight and the variable
name is updated to reflect this. By default 20% of database connections
are still reserved for updating download counts, but other requests will
be rejected less often. Additionally this saves 2 atomic updates when
serving the most popular request.

Author: jtgeibel

src/middleware/balance_capacity.rs

@@ -18,7 +18,7 @@ use conduit::{RequestExt, StatusCode};
 pub(super) struct BalanceCapacity {
     handler: Option<Box<dyn Handler>>,
     capacity: usize,
-    in_flight_requests: AtomicUsize,
+    in_flight_non_dl_requests: AtomicUsize,
     report_only: bool,
     log_at_percentage: usize,
     throttle_at_percentage: usize,
@@ -30,7 +30,7 @@ impl BalanceCapacity {
         Self {
             handler: None,
             capacity,
-            in_flight_requests: AtomicUsize::new(0),
+            in_flight_non_dl_requests: AtomicUsize::new(0),
             report_only: env::var("WEB_CAPACITY_REPORT_ONLY").ok().is_some(),
             log_at_percentage: read_env_percentage("WEB_CAPACITY_LOG_PCT", 50),
             throttle_at_percentage: read_env_percentage("WEB_CAPACITY_THROTTLE_PCT", 70),
@@ -73,18 +73,18 @@ impl AroundMiddleware for BalanceCapacity {
 
 impl Handler for BalanceCapacity {
     fn call(&self, request: &mut dyn RequestExt) -> AfterResult {
+        // Download requests are always accepted and do not affect the request count
+        if request.path().starts_with("/api/v1/crates/") && request.path().ends_with("/download") {
+            return self.handle(request);
+        }
+
         // The _drop_on_exit ensures the counter is decremented for all exit paths (including panics)
-        let (_drop_on_exit, count) = RequestCounter::add_one(&self.in_flight_requests);
+        let (_drop_on_exit, count) = RequestCounter::add_one(&self.in_flight_non_dl_requests);
         let load = 100 * count / self.capacity;
 
         // Begin logging request count so early stages of load increase can be located
         if load >= self.log_at_percentage {
-            super::log_request::add_custom_metadata(request, "in_flight_requests", count);
-        }
-
-        // Download requests are always accepted
-        if request.path().starts_with("/api/v1/crates/") && request.path().ends_with("/download") {
-            return self.handle(request);
+            super::log_request::add_custom_metadata(request, "in_flight_non_dl_requests", count);
         }
 
         // Reject read-only requests as load nears capacity. Bots are likely to send only safe
@@ -110,9 +110,6 @@ fn read_env_percentage(name: &str, default: usize) -> usize {
     }
 }
 
-// FIXME(JTG): I've copied the following from my `conduit-hyper` crate.  Once we transition from
-// `civet`, we could pass the in_flight_request count from `condut-hyper` via a request extension.
-
 /// A struct that stores a reference to an atomic counter so it can be decremented when dropped
 struct RequestCounter<'a> {
     counter: &'a AtomicUsize,