Add a report-only mode to the BalanceCapacity middleware

This adds a single environment variable that can be quickly set or
removed to toggle capacity enforcement. If this variable is set to any
value (including the empty string) then no requests will be rejected due
to load.

This will allow us to evolve the heuristics used in this middleware to
find the right balance before turning on enforcement.

This also increases the default logging percentage to a higher default.

Author: jtgeibel

src/middleware/balance_capacity.rs

@@ -8,6 +8,7 @@
 //! we should avoid dropping download requests even if that means rejecting some legitimate
 //! requests to other endpoints.
 
+use std::env;
 use std::sync::atomic::{AtomicUsize, Ordering};
 
 use super::prelude::*;
@@ -18,6 +19,7 @@ pub(super) struct BalanceCapacity {
     handler: Option<Box<dyn Handler>>,
     capacity: usize,
     in_flight_requests: AtomicUsize,
+    report_only: bool,
     log_at_percentage: usize,
     throttle_at_percentage: usize,
     dl_only_at_percentage: usize,
@@ -29,7 +31,8 @@ impl BalanceCapacity {
             handler: None,
             capacity,
             in_flight_requests: AtomicUsize::new(0),
-            log_at_percentage: read_env_percentage("WEB_CAPACITY_LOG_PCT", 20),
+            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),
             dl_only_at_percentage: read_env_percentage("WEB_CAPACITY_DL_ONLY_PCT", 80),
         }
@@ -54,6 +57,11 @@ impl Handler for BalanceCapacity {
             super::log_request::add_custom_metadata(request, "in_flight_requests", count);
         }
 
+        // In report-only mode we serve all requests and only enforce the logging limit above
+        if self.report_only {
+            return handler.call(request);
+        }
+
         // Download requests are always accepted
         if request.path().starts_with("/api/v1/crates/") && request.path().ends_with("/download") {
             return handler.call(request);
@@ -86,7 +94,7 @@ fn over_capacity_response(request: &mut dyn RequestExt) -> AfterResult {
 }
 
 fn read_env_percentage(name: &str, default: usize) -> usize {
-    if let Ok(value) = std::env::var(name) {
+    if let Ok(value) = env::var(name) {
         value.parse().unwrap_or(default)
     } else {
         default