Allow specifying the number of required connections

for a request. Most requests should reserve more
than a single connection (default: 3) and only
requests that are executed as nested expressions
should specify 1.

python/core/auto_generated/qgsfeaturerequest.sip.in

@@ -657,6 +657,40 @@ at this moment. A negative value (which is set by default) will wait forever.
    Only works if the provider supports this option.
 
 .. versionadded:: 3.0
+%End
+
+    int freeConnectionsRequirement() const;
+%Docstring
+The amount of free connections required to start this request.
+The system will block the request until the specified amount of connections
+is available for usage.
+
+By default this amount is 3. This makes sure, that we have 2 spare connections
+that might be used by "nested" requests which are executed while iterating
+over the results of this request.
+
+This number should be changed to one, when we know that no nested requests happen
+and that this request might happen in a nested way. This is for example given for
+expression functions that do internal requests.
+
+.. versionadded:: 3.4
+%End
+
+    void setFreeConnectionsRequirement( int freeConnectionsRequirement );
+%Docstring
+The amount of free connections required to start this request.
+The system will block the request until the specified amount of connections
+is available for usage.
+
+By default this amount is 3. This makes sure, that we have 2 spare connections
+that might be used by "nested" requests which are executed while iterating
+over the results of this request.
+
+This number should be changed to one, when we know that no nested requests happen
+and that this request might happen in a nested way. This is for example given for
+expression functions that do internal requests.
+
+.. versionadded:: 3.4
 %End
 
   protected:

src/core/qgsconnectionpool.h

@@ -29,7 +29,7 @@
 #include <QThread>
 
 
-#define CONN_POOL_MAX_CONCURRENT_CONNS      4
+#define CONN_POOL_MAX_CONCURRENT_CONNS      6
 #define CONN_POOL_EXPIRATION_TIME           60    // in seconds
 
 
@@ -93,12 +93,12 @@ class QgsConnectionPoolGroup
      *
      * \returns initialized connection or nullptr if unsuccessful
      */
-    T acquire( int timeout )
+    T acquire( int timeout, int freeConnectionRequirement )
     {
       // we are going to acquire a resource - if no resource is available, we will block here
       if ( timeout >= 0 )
       {
-        if ( !sem.tryAcquire( 1, timeout ) )
+        if ( !sem.tryAcquire( freeConnectionRequirement, timeout ) )
           return nullptr;
       }
       else
@@ -107,8 +107,9 @@ class QgsConnectionPoolGroup
         // tryAcquire is broken on Qt > 5.8 with negative timeouts - see
         // https://bugreports.qt.io/browse/QTBUG-64413
         // https://lists.osgeo.org/pipermail/qgis-developer/2017-November/050456.html
-        sem.acquire( 1 );
+        sem.acquire( freeConnectionRequirement );
       }
+      sem.release( freeConnectionRequirement - 1 );
 
       // quick (preferred) way - use cached connection
       {
@@ -283,9 +284,11 @@ class QgsConnectionPool
      * If \a timeout is a negative value the calling thread will be blocked
      * until a connection becomes available. This is the default behavior.
      *
+     *
+     *
      * \returns initialized connection or nullptr if unsuccessful
      */
-    T acquireConnection( const QString &connInfo, int timeout = -1 )
+    T acquireConnection( const QString &connInfo, int timeout = -1, int freeConnectionRequirement = 3 )
     {
       mMutex.lock();
       typename T_Groups::iterator it = mGroups.find( connInfo );
@@ -296,7 +299,7 @@ class QgsConnectionPool
       T_Group *group = *it;
       mMutex.unlock();
 
-      return group->acquire( timeout );
+      return group->acquire( timeout, freeConnectionRequirement );
     }
 
     //! Release an existing connection so it will get back into the pool and can be reused

src/core/qgsfeaturerequest.cpp

@@ -86,6 +86,7 @@ QgsFeatureRequest &QgsFeatureRequest::operator=( const QgsFeatureRequest &rh )
   mCrs = rh.mCrs;
   mTransformErrorCallback = rh.mTransformErrorCallback;
   mConnectionTimeout = rh.mConnectionTimeout;
+  mFreeConnectionsRequirement = rh.mFreeConnectionsRequirement;
   return *this;
 }
 
@@ -298,6 +299,16 @@ void QgsFeatureRequest::setConnectionTimeout( int connectionTimeout )
   mConnectionTimeout = connectionTimeout;
 }
 
+int QgsFeatureRequest::freeConnectionsRequirement() const
+{
+  return mFreeConnectionsRequirement;
+}
+
+void QgsFeatureRequest::setFreeConnectionsRequirement( int freeConnectionsRequirement )
+{
+  mFreeConnectionsRequirement = freeConnectionsRequirement;
+}
+
 
 #include "qgsfeatureiterator.h"
 #include "qgslogger.h"

src/core/qgsfeaturerequest.h

@@ -636,6 +636,40 @@ class CORE_EXPORT QgsFeatureRequest
      */
     void setConnectionTimeout( int connectionTimeout );
 
+    /**
+     * The amount of free connections required to start this request.
+     * The system will block the request until the specified amount of connections
+     * is available for usage.
+     *
+     * By default this amount is 3. This makes sure, that we have 2 spare connections
+     * that might be used by "nested" requests which are executed while iterating
+     * over the results of this request.
+     *
+     * This number should be changed to one, when we know that no nested requests happen
+     * and that this request might happen in a nested way. This is for example given for
+     * expression functions that do internal requests.
+     *
+     * \since QGIS 3.4
+     */
+    int freeConnectionsRequirement() const;
+
+    /**
+     * The amount of free connections required to start this request.
+     * The system will block the request until the specified amount of connections
+     * is available for usage.
+     *
+     * By default this amount is 3. This makes sure, that we have 2 spare connections
+     * that might be used by "nested" requests which are executed while iterating
+     * over the results of this request.
+     *
+     * This number should be changed to one, when we know that no nested requests happen
+     * and that this request might happen in a nested way. This is for example given for
+     * expression functions that do internal requests.
+     *
+     * \since QGIS 3.4
+     */
+    void setFreeConnectionsRequirement( int freeConnectionsRequirement );
+
   protected:
     FilterType mFilter = FilterNone;
     QgsRectangle mFilterRect;
@@ -654,6 +688,7 @@ class CORE_EXPORT QgsFeatureRequest
     QgsCoordinateReferenceSystem mCrs;
     QgsCoordinateTransformContext mTransformContext;
     int mConnectionTimeout = -1;
+    int mFreeConnectionsRequirement = 3;
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS( QgsFeatureRequest::Flags )

src/providers/postgres/qgspostgresfeatureiterator.cpp

@@ -38,7 +38,7 @@ QgsPostgresFeatureIterator::QgsPostgresFeatureIterator( QgsPostgresFeatureSource
 
   if ( !source->mTransactionConnection )
   {
-    mConn = QgsPostgresConnPool::instance()->acquireConnection( mSource->mConnInfo );
+    mConn = QgsPostgresConnPool::instance()->acquireConnection( mSource->mConnInfo, request.connectionTimeout(), request.freeConnectionsRequirement() );
     mIsTransactionConnection = false;
   }
   else