diff --git a/python/core/qgsfeatureiterator.sip b/python/core/qgsfeatureiterator.sip
index e7a61697a7f..ce2950a88bf 100644
--- a/python/core/qgsfeatureiterator.sip
+++ b/python/core/qgsfeatureiterator.sip
@@ -63,6 +63,21 @@ end of iterating: free the resources / lock
  :rtype: CompileStatus
 %End
 
+    virtual bool isValid() const;
+%Docstring
+ Returns if this iterator is valid.
+ An invalid feature iterator is not able to provide a reliable source for data.
+ If an iterator is invalid, either give up or try to send the request again (preferably
+ after a timeout to give the system some time to stay responsive).
+
+ If you want to check if the iterator successfully completed, better use QgsFeatureIterator.isClosed().
+
+.. note::
+
+   Added in QGIS 3.0
+ :rtype: bool
+%End
+
   protected:
 
     virtual bool fetchFeature( QgsFeature &f ) = 0;
@@ -145,6 +160,7 @@ Setup the simplification of geometries to fetch using the specified simplify met
  :rtype: bool
 %End
 
+
 };
 
 
@@ -233,6 +249,20 @@ destructor deletes the iterator if it has no more references
  :rtype: bool
 %End
 
+    virtual bool isValid() const;
+%Docstring
+ Will return if this iterator is valid.
+ An invalid iterator was probably introduced by a failed attempt to acquire a connection
+ or is a default constructed iterator.
+
+.. seealso:: isClosed to check if the iterator successfully completed and returned all the features.
+
+.. note::
+
+   Added in QGIS 3.0
+ :rtype: bool
+%End
+
     bool isClosed() const;
 %Docstring
 find out whether the iterator is still valid or closed already
diff --git a/python/core/qgsfeaturerequest.sip b/python/core/qgsfeaturerequest.sip
index 57b8d5c33e3..e4ea9f52f80 100644
--- a/python/core/qgsfeaturerequest.sip
+++ b/python/core/qgsfeaturerequest.sip
@@ -625,6 +625,19 @@ Set a subset of attributes by names that will be fetched
  :rtype: bool
 %End
 
+    int connectionTimeout() const;
+%Docstring
+ The timeout for how long we should wait for a connection if none is available from the pool
+ at this moment. A negative value (which is set by default) will wait forever.
+ :rtype: int
+%End
+
+    void setConnectionTimeout( int connectionTimeout );
+%Docstring
+ The timeout for how long we should wait for a connection if none is available from the pool
+ at this moment. A negative value (which is set by default) will wait forever.
+%End
+
   protected:
 };
 
diff --git a/python/core/qgsvectorlayerfeatureiterator.sip b/python/core/qgsvectorlayerfeatureiterator.sip
index 6446e8eda77..b6b53d0dee9 100644
--- a/python/core/qgsvectorlayerfeatureiterator.sip
+++ b/python/core/qgsvectorlayerfeatureiterator.sip
@@ -107,6 +107,8 @@ end of iterating: free the resources / lock
     };
 
 
+    virtual bool isValid() const;
+
   protected:
     virtual bool fetchFeature( QgsFeature &feature );
 %Docstring
diff --git a/src/core/qgsconnectionpool.h b/src/core/qgsconnectionpool.h
index 5cc3c4e47a2..e8bd1897b65 100644
--- a/src/core/qgsconnectionpool.h
+++ b/src/core/qgsconnectionpool.h
@@ -85,10 +85,11 @@ class QgsConnectionPoolGroup
     //! QgsConnectionPoolGroup cannot be copied
     QgsConnectionPoolGroup &operator=( const QgsConnectionPoolGroup &other ) = delete;
 
-    T acquire()
+    T acquire( int timeout )
     {
       // we are going to acquire a resource - if no resource is available, we will block here
-      sem.acquire();
+      if ( !sem.tryAcquire( 1, timeout ) )
+        return nullptr;
 
       // quick (preferred) way - use cached connection
       {
@@ -259,10 +260,13 @@ class QgsConnectionPool
     }
 
     /**
-     * Try to acquire a connection: if no connections are available, the thread will get blocked.
-     * \returns initialized connection or null on error
+     * Try to acquire a connection for a maximum of \a timeout milliseconds.
+     * 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 )
+    T acquireConnection( const QString &connInfo, int timeout = -1 )
     {
       mMutex.lock();
       typename T_Groups::iterator it = mGroups.find( connInfo );
@@ -273,7 +277,7 @@ class QgsConnectionPool
       T_Group *group = *it;
       mMutex.unlock();
 
-      return group->acquire();
+      return group->acquire( timeout );
     }
 
     //! Release an existing connection so it will get back into the pool and can be reused
diff --git a/src/core/qgsfeatureiterator.cpp b/src/core/qgsfeatureiterator.cpp
index 01b75f6412d..e96094cae7a 100644
--- a/src/core/qgsfeatureiterator.cpp
+++ b/src/core/qgsfeatureiterator.cpp
@@ -235,3 +235,8 @@ QgsFeatureIterator &QgsFeatureIterator::operator=( const QgsFeatureIterator &oth
   }
   return *this;
 }
+
+bool QgsFeatureIterator::isValid() const
+{
+  return mIter && mIter->isValid();
+}
diff --git a/src/core/qgsfeatureiterator.h b/src/core/qgsfeatureiterator.h
index f38d2a2204f..a3188cff4d7 100644
--- a/src/core/qgsfeatureiterator.h
+++ b/src/core/qgsfeatureiterator.h
@@ -82,6 +82,21 @@ class CORE_EXPORT QgsAbstractFeatureIterator
      */
     CompileStatus compileStatus() const { return mCompileStatus; }
 
+    /**
+     * Returns if this iterator is valid.
+     * An invalid feature iterator is not able to provide a reliable source for data.
+     * If an iterator is invalid, either give up or try to send the request again (preferably
+     * after a timeout to give the system some time to stay responsive).
+     *
+     * If you want to check if the iterator successfully completed, better use QgsFeatureIterator::isClosed().
+     *
+     * @note Added in QGIS 3.0
+     */
+    virtual bool isValid() const
+    {
+      return mValid;
+    }
+
   protected:
 
     /**
@@ -175,6 +190,16 @@ class CORE_EXPORT QgsAbstractFeatureIterator
     //! Setup the simplification of geometries to fetch using the specified simplify method
     virtual bool prepareSimplification( const QgsSimplifyMethod &simplifyMethod );
 
+    /**
+     * An invalid state of a feature iterator indicates that there was a problem with
+     * even getting it up and running.
+     * This should be set to false by subclasses if they have problems connecting to
+     * the provider.
+     * Do NOT set this to false when the feature iterator closes or has no features but
+     * we are sure, that it's just an empty dataset.
+     */
+    bool mValid = true;
+
   private:
     bool mUseCachedFeatures;
     QList<QgsIndexedFeature> mCachedFeatures;
@@ -279,6 +304,17 @@ class CORE_EXPORT QgsFeatureIterator
     bool rewind();
     bool close();
 
+    /**
+     * Will return if this iterator is valid.
+     * An invalid iterator was probably introduced by a failed attempt to acquire a connection
+     * or is a default constructed iterator.
+     *
+     * \see isClosed to check if the iterator successfully completed and returned all the features.
+     *
+     * @note Added in QGIS 3.0
+     */
+    virtual bool isValid() const;
+
     //! find out whether the iterator is still valid or closed already
     bool isClosed() const;
 
diff --git a/src/core/qgsfeaturerequest.cpp b/src/core/qgsfeaturerequest.cpp
index a5e5ed2da1c..ba44d80f50e 100644
--- a/src/core/qgsfeaturerequest.cpp
+++ b/src/core/qgsfeaturerequest.cpp
@@ -85,6 +85,7 @@ QgsFeatureRequest &QgsFeatureRequest::operator=( const QgsFeatureRequest &rh )
   mOrderBy = rh.mOrderBy;
   mCrs = rh.mCrs;
   mTransformErrorCallback = rh.mTransformErrorCallback;
+  mConnectionTimeout = rh.mConnectionTimeout;
   return *this;
 }
 
@@ -281,6 +282,16 @@ bool QgsFeatureRequest::acceptFeature( const QgsFeature &feature )
   return true;
 }
 
+int QgsFeatureRequest::connectionTimeout() const
+{
+  return mConnectionTimeout;
+}
+
+void QgsFeatureRequest::setConnectionTimeout( int connectionTimeout )
+{
+  mConnectionTimeout = connectionTimeout;
+}
+
 
 #include "qgsfeatureiterator.h"
 #include "qgslogger.h"
diff --git a/src/core/qgsfeaturerequest.h b/src/core/qgsfeaturerequest.h
index 043e2f68b3a..7c7e1a27043 100644
--- a/src/core/qgsfeaturerequest.h
+++ b/src/core/qgsfeaturerequest.h
@@ -601,6 +601,18 @@ class CORE_EXPORT QgsFeatureRequest
      */
     bool acceptFeature( const QgsFeature &feature );
 
+    /**
+     * The timeout for how long we should wait for a connection if none is available from the pool
+     * at this moment. A negative value (which is set by default) will wait forever.
+     */
+    int connectionTimeout() const;
+
+    /**
+     * The timeout for how long we should wait for a connection if none is available from the pool
+     * at this moment. A negative value (which is set by default) will wait forever.
+     */
+    void setConnectionTimeout( int connectionTimeout );
+
   protected:
     FilterType mFilter = FilterNone;
     QgsRectangle mFilterRect;
@@ -617,6 +629,7 @@ class CORE_EXPORT QgsFeatureRequest
     std::function< void( const QgsFeature & ) > mInvalidGeometryCallback;
     std::function< void( const QgsFeature & ) > mTransformErrorCallback;
     QgsCoordinateReferenceSystem mCrs;
+    int mConnectionTimeout = -1;
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS( QgsFeatureRequest::Flags )
diff --git a/src/core/qgsvectorlayerfeatureiterator.cpp b/src/core/qgsvectorlayerfeatureiterator.cpp
index f27f19678c7..99e67e23274 100644
--- a/src/core/qgsvectorlayerfeatureiterator.cpp
+++ b/src/core/qgsvectorlayerfeatureiterator.cpp
@@ -211,12 +211,8 @@ QgsVectorLayerFeatureIterator::QgsVectorLayerFeatureIterator( QgsVectorLayerFeat
   if ( mSource->mHasEditBuffer )
   {
     mChangedFeaturesRequest = mProviderRequest;
-    QgsFeatureIds changedIds;
-    QgsChangedAttributesMap::const_iterator attIt = mSource->mChangedAttributeValues.constBegin();
-    for ( ; attIt != mSource->mChangedAttributeValues.constEnd(); ++attIt )
-    {
-      changedIds << attIt.key();
-    }
+    const QgsFeatureIds changedIds = mSource->mChangedAttributeValues.keys().toSet();
+
     mChangedFeaturesRequest.setFilterFids( changedIds );
 
     if ( mChangedFeaturesRequest.limit() > 0 )
@@ -408,6 +404,11 @@ void QgsVectorLayerFeatureIterator::setInterruptionChecker( QgsInterruptionCheck
   mInterruptionChecker = interruptionChecker;
 }
 
+bool QgsVectorLayerFeatureIterator::isValid() const
+{
+  return mProviderIterator.isValid();
+}
+
 bool QgsVectorLayerFeatureIterator::fetchNextAddedFeature( QgsFeature &f )
 {
   while ( mFetchAddedFeaturesIt-- != mSource->mAddedFeatures.constBegin() )
diff --git a/src/core/qgsvectorlayerfeatureiterator.h b/src/core/qgsvectorlayerfeatureiterator.h
index cd4da95434c..70c0118c299 100644
--- a/src/core/qgsvectorlayerfeatureiterator.h
+++ b/src/core/qgsvectorlayerfeatureiterator.h
@@ -139,6 +139,8 @@ class CORE_EXPORT QgsVectorLayerFeatureIterator : public QgsAbstractFeatureItera
     };
 
 
+    virtual bool isValid() const override;
+
   protected:
     //! fetch next feature, return true on success
     virtual bool fetchFeature( QgsFeature &feature ) override;