syslog: implement rate limiting

enabling a lot of logging messages, e.g., by setting verbose as the
default level, can make the system unresponsive if syslogging is
enabled. to protect against this, a rate limiting algorithm is
implemented in the syslogger.

include/SyslogLogger.h

@@ -20,6 +20,7 @@ private:
         return _address != INADDR_NONE;
     }
     static uint8_t calculatePrival(uint8_t facility, char errorCode);
+    bool consumeToken();
 
     Task _loopTask;
     std::mutex _mutex;
@@ -30,6 +31,14 @@ private:
     String _header;
     uint16_t _port;
     bool _enabled;
+
+    static constexpr uint32_t RATE_LIMIT_WINDOW_MS = 1000;
+    static constexpr size_t RATE_LIMIT_MAX_TOKENS = 100;
+    size_t _available_tokens = RATE_LIMIT_MAX_TOKENS;
+    uint32_t _last_token_refill_millis = 0;
+    size_t _rate_limited_packets = 0;
+    uint32_t _last_rate_limit_warning_millis = 0;
+    static constexpr uint32_t RATE_LIMIT_WARNING_INTERVAL_MS = 1000;
 };
 
 extern SyslogLogger Syslog;

src/SyslogLogger.cpp

@@ -8,6 +8,7 @@
 #include "defaults.h"
 #include <ESPmDNS.h>
 #include <HardwareSerial.h>
+#include <algorithm>
 
 #undef TAG
 static const char* TAG = "syslog";
@@ -65,6 +66,27 @@ void SyslogLogger::write(const uint8_t* buffer, size_t size)
         return;
     }
 
+    // Check rate limiting using token bucket
+    if (!consumeToken()) {
+        if (_rate_limited_packets == 0) {
+            _last_rate_limit_warning_millis = millis();
+        }
+        ++_rate_limited_packets;
+        return;
+    }
+
+    if (_rate_limited_packets > 0) {
+        uint32_t elapsed = (millis() - _last_rate_limit_warning_millis);
+        if (elapsed > RATE_LIMIT_WARNING_INTERVAL_MS) {
+            char buffer[128];
+            snprintf(buffer, sizeof(buffer), "Rate limited %d packets in the last %dms",
+                _rate_limited_packets, elapsed);
+            Serial.println(buffer);
+            _rate_limited_packets = 0;
+            _last_rate_limit_warning_millis = millis();
+        }
+    }
+
     String header = "<";
     header += String(calculatePrival(1, buffer[0]));
 
@@ -82,6 +104,26 @@ void SyslogLogger::write(const uint8_t* buffer, size_t size)
     _udp.endPacket();
 }
 
+bool SyslogLogger::consumeToken()
+{
+    uint32_t now = millis();
+
+    uint32_t elapsed = now - _last_token_refill_millis;
+    size_t new_tokens = RATE_LIMIT_MAX_TOKENS * elapsed / RATE_LIMIT_WINDOW_MS;
+
+    if (new_tokens > 0) {
+        _available_tokens = std::min(_available_tokens + new_tokens, RATE_LIMIT_MAX_TOKENS);
+        _last_token_refill_millis = now;
+    }
+
+    if (_available_tokens > 0) {
+        --_available_tokens;
+        return true;
+    }
+
+    return false;
+}
+
 void SyslogLogger::disable()
 {
     ESP_LOGI(TAG, "Disable");
@@ -103,6 +145,8 @@ void SyslogLogger::enable()
 
     std::lock_guard<std::mutex> lock(_mutex);
     _enabled = true;
+    _available_tokens = RATE_LIMIT_MAX_TOKENS;
+    _last_token_refill_millis = millis();
 }
 
 bool SyslogLogger::resolveAndStart()