fix: Improve logging of the reason to refuse a peer connection (#5664)

Currently, all peer connection rejections are logged with the reason "slots full". This is inaccurate, as the PeerFinder can also reject connections if they are a duplicate. This change updates the logging logic to correctly report the specific reason (full or duplicate) for a rejected peer connection, providing more accurate diagnostic information.

src/xrpld/overlay/detail/ConnectAttempt.cpp

@@ -379,7 +379,7 @@ ConnectAttempt::processResponse()
         auto const result = overlay_.peerFinder().activate(
             slot_, publicKey, static_cast<bool>(member));
         if (result != PeerFinder::Result::success)
-            return fail("Outbound slots full");
+            return fail("Outbound " + std::string(to_string(result)));
 
         auto const peer = std::make_shared<PeerImp>(
             app_,

src/xrpld/overlay/detail/OverlayImpl.cpp

@@ -26,6 +26,7 @@
 #include <xrpld/overlay/Cluster.h>
 #include <xrpld/overlay/detail/ConnectAttempt.h>
 #include <xrpld/overlay/detail/PeerImp.h>
+#include <xrpld/overlay/detail/TrafficCount.h>
 #include <xrpld/overlay/detail/Tuning.h>
 #include <xrpld/overlay/predicates.h>
 #include <xrpld/peerfinder/make_Manager.h>
@@ -41,8 +42,6 @@
 
 #include <boost/algorithm/string/predicate.hpp>
 
-#include "xrpld/overlay/detail/TrafficCount.h"
-
 namespace ripple {
 
 namespace CrawlOptions {
@@ -269,8 +268,8 @@ OverlayImpl::onHandoff(
             if (result != PeerFinder::Result::success)
             {
                 m_peerFinder->on_closed(slot);
-                JLOG(journal.debug())
-                    << "Peer " << remote_endpoint << " redirected, slots full";
+                JLOG(journal.debug()) << "Peer " << remote_endpoint
+                                      << " redirected, " << to_string(result);
                 handoff.moved = false;
                 handoff.response = makeRedirectResponse(
                     slot, request, remote_endpoint.address());

src/xrpld/peerfinder/PeerfinderManager.h

@@ -28,6 +28,8 @@
 
 #include <boost/asio/ip/tcp.hpp>
 
+#include <string_view>
+
 namespace ripple {
 namespace PeerFinder {
 
@@ -136,6 +138,36 @@ using Endpoints = std::vector<Endpoint>;
 /** Possible results from activating a slot. */
 enum class Result { duplicate, full, success };
 
+/**
+ * @brief Converts a `Result` enum value to its string representation.
+ *
+ * This function provides a human-readable string for a given `Result` enum,
+ * which is useful for logging, debugging, or displaying status messages.
+ *
+ * @param result The `Result` enum value to convert.
+ * @return A `std::string_view` representing the enum value. Returns "unknown"
+ * if the enum value is not explicitly handled.
+ *
+ * @note This function returns a `std::string_view` for performance.
+ * A `std::string` would need to allocate memory on the heap and copy the
+ * string literal into it every time the function is called.
+ */
+inline std::string_view
+to_string(Result result) noexcept
+{
+    switch (result)
+    {
+        case Result::success:
+            return "success";
+        case Result::duplicate:
+            return "duplicate connection";
+        case Result::full:
+            return "slots full";
+    }
+
+    return "unknown";
+}
+
 /** Maintains a set of IP addresses used for getting into the network. */
 class Manager : public beast::PropertyStream::Source
 {