Add syslog logger implementation

This implements RFC5424 version of the protocol.

I didn't use https://github.com/arcao/Syslog since the protocol itself
is trivial and most of the libraries functionality is not needed here.
The library also doesn't support setting the PROCID field, which is set
to a random id to indicate a reboot here.

include/Configuration.h

@@ -178,7 +178,7 @@ struct CONFIG_T {
     struct {
         bool Enabled;
         char Hostname[SYSLOG_MAX_HOSTNAME_STRLEN + 1];
-        uint32_t Port;
+        uint16_t Port;
     } Syslog;
 
     struct {

include/SyslogLogger.h

@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#pragma once
+#include <WiFiUdp.h>
+#include <TaskSchedulerDeclarations.h>
+#include <mutex>
+
+class SyslogLogger {
+public:
+    SyslogLogger();
+    void init(Scheduler& scheduler);
+    void updateSettings(const String&& hostname);
+    void write(const uint8_t *buffer, size_t size);
+
+private:
+    void loop();
+    void disable();
+    void enable();
+    bool resolveAndStart();
+    bool isResolved() const {
+        return _address != INADDR_NONE;
+    }
+
+    Task _loopTask;
+    std::mutex _mutex;
+    WiFiUDP _udp;
+    IPAddress _address;
+    String _syslog_hostname;
+    String _proc_id;
+    String _header;
+    uint16_t _port;
+    bool _enabled;
+};
+
+extern SyslogLogger Syslog;

src/MessageOutput.cpp

@@ -4,6 +4,7 @@
  */
 #include <HardwareSerial.h>
 #include "MessageOutput.h"
+#include "SyslogLogger.h"
 
 MessageOutputClass MessageOutput;
 
@@ -102,12 +103,14 @@ void MessageOutputClass::loop()
 
     if (!_ws) {
         while (!_lines.empty()) {
+            Syslog.write(_lines.front().data(), _lines.front().size());
             _lines.pop(); // do not hog memory
         }
         return;
     }
 
     while (!_lines.empty() && _ws->availableForWriteAll()) {
+        Syslog.write(_lines.front().data(), _lines.front().size());
         _ws->textAll(std::make_shared<message_t>(std::move(_lines.front())));
         _lines.pop();
     }

src/NetworkSettings.cpp

@@ -5,6 +5,7 @@
 #include "NetworkSettings.h"
 #include "Configuration.h"
 #include "MessageOutput.h"
+#include "SyslogLogger.h"
 #include "PinMapping.h"
 #include "Utils.h"
 #include "SPIPortManager.h"
@@ -53,6 +54,8 @@ void NetworkSettingsClass::init(Scheduler& scheduler)
 
     scheduler.addTask(_loopTask);
     _loopTask.enable();
+
+    Syslog.init(scheduler);
 }
 
 void NetworkSettingsClass::NetworkEvent(const WiFiEvent_t event, WiFiEventInfo_t info)
@@ -294,6 +297,8 @@ void NetworkSettingsClass::applyConfig()
     }
     MessageOutput.println("done");
     setStaticIp();
+
+    Syslog.updateSettings(getHostname());
 }
 
 void NetworkSettingsClass::setHostname()

src/SyslogLogger.cpp

@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2022-2024 Thomas Basler and others
+ */
+#include <HardwareSerial.h>
+#include <ESPmDNS.h>
+#include "defaults.h"
+#include "SyslogLogger.h"
+#include "Configuration.h"
+#include "MessageOutput.h"
+#include "NetworkSettings.h"
+
+SyslogLogger::SyslogLogger()
+    : _loopTask(TASK_IMMEDIATE, TASK_FOREVER, std::bind(&SyslogLogger::loop, this))
+{
+}
+
+void SyslogLogger::init(Scheduler& scheduler)
+{
+    // PROCID change indicates a restart.
+    _proc_id = String(esp_random(), HEX);
+
+    scheduler.addTask(_loopTask);
+    _loopTask.enable();
+}
+
+void SyslogLogger::updateSettings(const String&& hostname)
+{
+    auto& config = Configuration.get().Syslog;
+
+    // Disable logger while it is reconfigured.
+    disable();
+
+    if (!config.Enabled) {
+        MessageOutput.println("[SyslogLogger] Syslog not enabled");
+        return;
+    }
+
+    _port = config.Port;
+    _syslog_hostname = config.Hostname;
+    if (_syslog_hostname.isEmpty()) {
+        MessageOutput.println("[SyslogLogger] Hostname not configured");
+        return;
+    }
+
+    MessageOutput.printf("[SyslogLogger] Logging to %s!\r\n", _syslog_hostname.c_str());
+
+    _header = "<7>1 - ";  // RFC5424: Facility KERNEL, severity DEBUG, version 1, NIL timestamp.
+    _header += hostname;
+    _header += " OpenDTU ";
+    _header += _proc_id;
+    // NIL values for message id and structured // data; utf-8 BOM.
+    _header += " - - \xEF\xBB\xBF";
+
+    // Enable logger.
+    enable();
+}
+
+void SyslogLogger::write(const uint8_t *buffer, size_t size)
+{
+    std::lock_guard<std::mutex> lock(_mutex);
+    if (!_enabled || !isResolved()) {
+        return;
+    }
+    for (int i = 0; i < size; i++) {
+        uint8_t c = buffer[i];
+        bool overflow = false;
+        if (c != '\r' && c != '\n') {
+            // Replace control and non-ASCII characters with '?'.
+            overflow = !_udp.write(c >= 0x20 && c < 0x7f ? c : '?');
+        }
+        if (c == '\n' || overflow) {
+            _udp.endPacket();
+            _udp.beginPacket(_address, _port);
+            _udp.print(_header);
+        }
+    }
+}
+
+void SyslogLogger::disable()
+{
+    MessageOutput.println("[SyslogLogger] Disable");
+    std::lock_guard<std::mutex> lock(_mutex);
+    if (_enabled) {
+        _enabled = false;
+        _address = INADDR_NONE;
+        _udp.stop();
+    }
+}
+
+void SyslogLogger::enable()
+{
+    // Bind random source port.
+    if (!_udp.begin(0)) {
+        MessageOutput.println("[SyslogLogger] No sockets available");
+        return;
+    }
+
+    std::lock_guard<std::mutex> lock(_mutex);
+    _enabled = true;
+}
+
+bool SyslogLogger::resolveAndStart()
+{
+    if (Configuration.get().Mdns.Enabled) {
+        _address = MDNS.queryHost(_syslog_hostname); // INADDR_NONE if failed
+    }
+    if (_address != INADDR_NONE) {
+        if (!_udp.beginPacket(_address, _port)) {
+            return false;
+        }
+    } else {
+        if (!_udp.beginPacket(_syslog_hostname.c_str(), _port)) {
+            return false;
+        }
+        _address = _udp.remoteIP();  // Store resolved address.
+    }
+    _udp.print(_header);
+    _udp.print("[SyslogLogger] Logging to ");
+    _udp.print(_syslog_hostname);
+    _udp.endPacket();
+    _udp.beginPacket(_address, _port);
+    _udp.print(_header);
+    return true;
+}
+
+void SyslogLogger::loop()
+{
+    std::lock_guard<std::mutex> lock(_mutex);
+    if (!_enabled || !NetworkSettings.isConnected() || isResolved()) {
+        return;
+    }
+    if (!resolveAndStart()) {
+        _enabled = false;
+    }
+}
+
+SyslogLogger Syslog;