Fix #10127 (crash on parallel rendering of spatialite layers)

There was a run condition in connection pool singleton: at the beginning,
the singleton got initialized twice. Now the singleton is handled
as a static function variable which should be initialized in thread-safe way.

src/providers/postgres/qgspostgresconnpool.cpp

@@ -17,12 +17,9 @@
 
 #include "qgspostgresconn.h"
 
-QgsPostgresConnPool* QgsPostgresConnPool::mInstance = 0;
-
 
 QgsPostgresConnPool* QgsPostgresConnPool::instance()
 {
-  if ( !mInstance )
-    mInstance = new QgsPostgresConnPool;
-  return mInstance;
+  static QgsPostgresConnPool sInstance;
+  return &sInstance;
 }

src/providers/postgres/qgspostgresconnpool.h

@@ -60,8 +60,6 @@ class QgsPostgresConnPool : public QgsConnectionPool<QgsPostgresConn*, QgsPostgr
   public:
     static QgsPostgresConnPool* instance();
 
-  protected:
-    static QgsPostgresConnPool* mInstance;
 };
 
 

src/providers/spatialite/qgsspatialiteconnpool.cpp

@@ -15,12 +15,9 @@
 
 #include "qgsspatialiteconnpool.h"
 
-QgsSpatiaLiteConnPool* QgsSpatiaLiteConnPool::mInstance = 0;
-
 
 QgsSpatiaLiteConnPool* QgsSpatiaLiteConnPool::instance()
 {
-  if ( !mInstance )
-    mInstance = new QgsSpatiaLiteConnPool;
-  return mInstance;
+  static QgsSpatiaLiteConnPool sInstance;
+  return &sInstance;
 }

src/providers/spatialite/qgsspatialiteconnpool.h

@@ -59,8 +59,6 @@ class QgsSpatiaLiteConnPool : public QgsConnectionPool<QgsSqliteHandle*, QgsSpat
   public:
     static QgsSpatiaLiteConnPool* instance();
 
-  protected:
-    static QgsSpatiaLiteConnPool* mInstance;
 };