Optionally print from where logging rules are loaded

Tell the user from where logging configurations are loaded from if
the QT_LOGGING_DEBUG environment variable is set. This allows 'debugging'
of the logging rules database, because it's very simple to e.g. silence
all debug messages by adding a logging configuration file somewhere, and
forget about it.

Change-Id: Iee34031d531462060b5603e2210e01fd40952c63
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qloggingcategory.cpp

@@ -137,6 +137,9 @@ Q_GLOBAL_STATIC_WITH_ARGS(QLoggingCategory, qtDefaultCategory,
         QCoreApplication::applicationDirPath() + \c "/data"
     \endlist
 
+    Set the \c QT_LOGGING_DEBUG environment variable to see from where
+    logging rules are loaded.
+
     \section2 Installing a Custom Filter
 
     As a lower-level alternative to the text rules you can also implement a

src/corelib/io/qloggingregistry.cpp

@@ -45,6 +45,13 @@
 #include <QtCore/qfile.h>
 #include <QtCore/qstandardpaths.h>
 #include <QtCore/qtextstream.h>
+#include <QtCore/qdir.h>
+
+// We can't use the default macros because this would lead to recursion.
+// Instead let's define our own one that unconditionally logs...
+#define debugMsg QMessageLogger(__FILE__, __LINE__, __FUNCTION__, "qt.core.logging").debug
+#define warnMsg QMessageLogger(__FILE__, __LINE__, __FUNCTION__, "qt.core.logging").warning
+
 
 QT_BEGIN_NAMESPACE
 
@@ -232,6 +239,12 @@ QLoggingRegistry::QLoggingRegistry()
 {
 }
 
+static bool qtLoggingDebug()
+{
+    static const bool debugEnv = qEnvironmentVariableIsSet("QT_LOGGING_DEBUG");
+    return debugEnv;
+}
+
 /*!
     \internal
     Initializes the rules database by loading
@@ -247,6 +260,9 @@ void QLoggingRegistry::init()
             QTextStream stream(&file);
             QLoggingSettingsParser parser;
             parser.setContent(stream);
+            if (qtLoggingDebug())
+                debugMsg("Loading \"%s\" ...",
+                         QDir::toNativeSeparators(file.fileName()).toUtf8().constData());
             envRules = parser.rules();
         }
     }
@@ -260,6 +276,9 @@ void QLoggingRegistry::init()
             QTextStream stream(&file);
             QLoggingSettingsParser parser;
             parser.setContent(stream);
+            if (qtLoggingDebug())
+                debugMsg("Loading \"%s\" ...",
+                         QDir::toNativeSeparators(envPath).toUtf8().constData());
             configRules = parser.rules();
         }
     }
@@ -308,6 +327,10 @@ void QLoggingRegistry::setApiRules(const QString &content)
     parser.setContent(content);
 
     QMutexLocker locker(&registryMutex);
+
+    if (qtLoggingDebug())
+        debugMsg("Loading logging rules set by Qt API ...");
+
     apiRules = parser.rules();
 
     updateRules();