[FEATURE][needs-doc] Support QGIS Server logs to stderr

This commit makes it possible to configure QGIS Server to make it
write its logs to stderr.

This is done by setting the QGIS_SERVER_LOG_FILE env var to the
special value "stderr".

python/core/auto_generated/qgsmessagelog.sip.in

@@ -117,6 +117,17 @@ be the right choice for apps without GUI.
   public:
     QgsMessageLogConsole();
 
+  protected:
+
+    QString formatLogMessage( const QString &message, const QString &tag, Qgis::MessageLevel level = Qgis::Info ) const;
+%Docstring
+Format a log message. Used by child classes.
+
+:param message: the message to format
+:param tag: the tag of the message
+:param level: the log level of the message
+%End
+
   public slots:
     void logMessage( const QString &message, const QString &tag, Qgis::MessageLevel level );
 };

src/core/qgsmessagelog.cpp

@@ -18,7 +18,9 @@
 #include "qgslogger.h"
 #include <QDateTime>
 #include <QMetaType>
+#include <QTextStream>
 #include <iostream>
+#include <stdio.h>
 
 class QgsMessageLogConsole;
 
@@ -47,12 +49,19 @@ QgsMessageLogConsole::QgsMessageLogConsole()
 
 void QgsMessageLogConsole::logMessage( const QString &message, const QString &tag, Qgis::MessageLevel level )
 {
-  std::cerr
-      << tag.toLocal8Bit().data() << "[" <<
-      ( level == Qgis::Info ? "INFO"
-        : level == Qgis::Warning ? "WARNING"
-        : "CRITICAL" )
-      << "]: " << message.toLocal8Bit().data() << std::endl;
+  QString formattedMessage = formatLogMessage( message, tag, level );
+  QTextStream cerr( stderr );
+  cerr << formattedMessage;
+}
+
+QString QgsMessageLogConsole::formatLogMessage( const QString &message, const QString &tag, Qgis::MessageLevel level ) const
+{
+  const QString time = QTime::currentTime().toString();
+  const QString levelStr = level == Qgis::Info ? QStringLiteral( "INFO" ) :
+                           level == Qgis::Warning ? QStringLiteral( "WARNING" ) :
+                           QStringLiteral( "CRITICAL" );
+  const QString pid = QString::number( QCoreApplication::applicationPid() );
+  return QStringLiteral( "%1 %2 %3[%4]: %5\n" ).arg( time, levelStr, tag, pid, message );
 }
 
 //

src/core/qgsmessagelog.h

@@ -140,6 +140,17 @@ class CORE_EXPORT QgsMessageLogConsole : public QObject
   public:
     QgsMessageLogConsole();
 
+  protected:
+
+    /**
+     * Format a log message. Used by child classes.
+     *
+     * \param message the message to format
+     * \param tag the tag of the message
+     * \param level the log level of the message
+     */
+    QString formatLogMessage( const QString &message, const QString &tag, Qgis::MessageLevel level = Qgis::Info ) const;
+
   public slots:
     void logMessage( const QString &message, const QString &tag, Qgis::MessageLevel level );
 };

src/server/qgsserverlogger.cpp

@@ -36,44 +36,44 @@ QgsServerLogger *QgsServerLogger::instance()
 }
 
 QgsServerLogger::QgsServerLogger()
-  : mLogFile( nullptr )
+  : QgsMessageLogConsole()
 {
-  connect( QgsApplication::messageLog(), static_cast<void ( QgsMessageLog::* )( const QString &, const QString &, Qgis::MessageLevel )>( &QgsMessageLog::messageReceived ), this,
-           &QgsServerLogger::logMessage );
 }
 
-void QgsServerLogger::setLogLevel( Qgis::MessageLevel level )
+void QgsServerLogger::logMessage( const QString &message, const QString &tag, Qgis::MessageLevel level )
+{
+  if ( mLogLevel > level )
+  {
+    return;
+  }
+  if ( mLogFile.isOpen() )
+  {
+    QString formattedMessage = formatLogMessage( message, tag, level );
+    mTextStream << formattedMessage;
+    mTextStream.flush();
+  }
+  else if ( QString::compare( mLogFile.fileName(), QStringLiteral( "stderr" ), Qt::CaseInsensitive ) == 0 )
+  {
+    QgsMessageLogConsole::logMessage( message, tag, level );
+  }
+}
+
+void QgsServerLogger::setLogLevel( const Qgis::MessageLevel level )
 {
   mLogLevel = level;
 }
 
 void QgsServerLogger::setLogFile( const QString &f )
 {
-  if ( ! f.isEmpty() )
-  {
-    if ( mLogFile.exists() )
-    {
-      mTextStream.flush();
-      mLogFile.close();
-    }
+  mTextStream.flush();
+  mLogFile.close();
 
-    mLogFile.setFileName( f );
-    if ( mLogFile.open( QIODevice::Append ) )
-    {
-      mTextStream.setDevice( &mLogFile );
-    }
-  }
-}
+  mLogFile.setFileName( f );
 
-void QgsServerLogger::logMessage( const QString &message, const QString &tag, Qgis::MessageLevel level )
-{
-  Q_UNUSED( tag );
-  if ( !mLogFile.isOpen() || mLogLevel > level )
+  if ( ( ! f.isEmpty() ) &&
+       QString::compare( f, QStringLiteral( "stderr" ), Qt::CaseInsensitive ) != 0 &&
+       mLogFile.open( QIODevice::Append ) )
   {
-    return;
+    mTextStream.setDevice( &mLogFile );
   }
-
-  mTextStream << ( "[" + QString::number( qlonglong( QCoreApplication::applicationPid() ) ) + "]["
-                   + QTime::currentTime().toString() + "] " + message + "\n" );
-  mTextStream.flush();
 }

src/server/qgsserverlogger.h

@@ -33,7 +33,7 @@
  * \brief Writes message log into server logfile
  * \since QGIS 2.8
  */
-class QgsServerLogger: public QObject
+class QgsServerLogger: public QgsMessageLogConsole
 {
     Q_OBJECT
   public: