api: Add all in-flight requests /reverse_proxy/upstreams (Fixes #7277)

This refactors the initial approach in PR #7281, replacing the UsagePool
with a dedicated package-level sync.Map and atomic.Int64 to track
in-flight requests without global lock contention.

It also introduces a lookup map in the admin API to fix a potential
O(n^2) iteration over upstreams, ensuring that draining upstreams
are correctly exposed across config reloads without leaking memory.

Co-authored-by: Y.Horie <u5.horie@gmail.com>

reverseproxy: optimize in-flight tracking and admin API

- Replaced sync.RWMutex with sync.Map and atomic.Int64 to avoid lock contention under high RPS.
- Introduced a lookup map in the admin API to fix a potential O(n^2) iteration over upstreams.

Author: pauloappbr

modules/caddyhttp/reverseproxy/admin.go

@@ -73,6 +73,7 @@ func (adminUpstreams) handleUpstreams(w http.ResponseWriter, r *http.Request) er
 
 	// Collect the results to respond with
 	results := []upstreamStatus{}
+	knownHosts := make(map[string]struct{})
 
 	// Iterate over the upstream pool (needs to be fast)
 	var rangeErr error
@@ -95,6 +96,8 @@ func (adminUpstreams) handleUpstreams(w http.ResponseWriter, r *http.Request) er
 			return false
 		}
 
+		knownHosts[address] = struct{}{}
+
 		results = append(results, upstreamStatus{
 			Address:     address,
 			NumRequests: upstream.NumRequests(),
@@ -103,7 +106,17 @@ func (adminUpstreams) handleUpstreams(w http.ResponseWriter, r *http.Request) er
 		return true
 	})
 
-	// If an error happened during the range, return it
+	currentInFlight := getInFlightRequests()
+	for address, count := range currentInFlight {
+		if _, exists := knownHosts[address]; !exists && count > 0 {
+			results = append(results, upstreamStatus{
+				Address:     address,
+				NumRequests: int(count),
+				Fails:       0,
+			})
+		}
+	}
+
 	if rangeErr != nil {
 		return rangeErr
 	}

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -32,6 +32,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"go.uber.org/zap"
@@ -46,6 +47,31 @@ import (
 	"github.com/caddyserver/caddy/v2/modules/caddyhttp/rewrite"
 )
 
+// inFlightRequests uses sync.Map with atomic.Int64 for lock-free updates on the hot path
+var inFlightRequests sync.Map
+
+func incInFlightRequest(address string) {
+	v, _ := inFlightRequests.LoadOrStore(address, new(atomic.Int64))
+	v.(*atomic.Int64).Add(1)
+}
+
+func decInFlightRequest(address string) {
+	if v, ok := inFlightRequests.Load(address); ok {
+		if v.(*atomic.Int64).Add(-1) <= 0 {
+			inFlightRequests.Delete(address)
+		}
+	}
+}
+
+func getInFlightRequests() map[string]int64 {
+	copyMap := make(map[string]int64)
+	inFlightRequests.Range(func(key, value any) bool {
+		copyMap[key.(string)] = value.(*atomic.Int64).Load()
+		return true
+	})
+	return copyMap
+}
+
 func init() {
 	caddy.RegisterModule(Handler{})
 }
@@ -904,8 +930,16 @@ func (h Handler) addForwardedHeaders(req *http.Request) error {
 // Go standard library which was used as the foundation.)
 func (h *Handler) reverseProxy(rw http.ResponseWriter, req *http.Request, origReq *http.Request, repl *caddy.Replacer, di DialInfo, next caddyhttp.Handler) error {
 	_ = di.Upstream.Host.countRequest(1)
+
+	// Increment the in-flight request count
+	incInFlightRequest(di.Address)
+
 	//nolint:errcheck
-	defer di.Upstream.Host.countRequest(-1)
+	defer func() {
+		di.Upstream.Host.countRequest(-1)
+		// Decrement the in-flight request count
+		decInFlightRequest(di.Address)
+	}()
 
 	// point the request to this upstream
 	h.directRequest(req, di)