When deleting histories, go to default history (unnamed, no datasets) if available and create new history only if necessary.

lib/galaxy/model/__init__.py

@@ -666,10 +666,11 @@ class History( object, UsesAnnotations ):
 
     api_collection_visible_keys = ( 'id', 'name', 'published', 'deleted' )
     api_element_visible_keys = ( 'id', 'name', 'published', 'deleted', 'genome_build', 'purged' )
+    default_name = 'Unnamed history'
 
     def __init__( self, id=None, name=None, user=None ):
         self.id = id
-        self.name = name or "Unnamed history"
+        self.name = name or History.default_name
         self.deleted = False
         self.purged = False
         self.importing = False

lib/galaxy/web/framework/__init__.py

@@ -832,6 +832,39 @@ class GalaxyWebTransaction( base.DefaultWebTransaction ):
 
     history = property( get_history, set_history )
 
+    def get_or_create_default_history( self ):
+        """
+        Gets or creates a default history and associates it with the current
+        session.
+        """
+
+        # There must be a user to fetch a default history.
+        if not self.galaxy_session.user:
+            return self.new_history()
+
+        # Look for default history that (a) has default name + is not deleted and 
+        # (b) has no datasets. If suitable history found, use it; otherwise, create 
+        # new history.
+        unnamed_histories = self.sa_session.query( self.app.model.History ).filter_by( 
+                                user=self.galaxy_session.user,
+                                name=self.app.model.History.default_name,
+                                deleted=False )
+        default_history = None
+        for history in unnamed_histories:
+            if len( history.datasets ) == 0:
+                # Found suitable default history.
+                default_history = history
+                break
+            
+        # Set or create hsitory.
+        if default_history:
+            history = default_history
+            self.set_history( history )
+        else:
+            history = self.new_history()
+
+        return history
+
     def new_history( self, name=None ):
         """
         Create a new history and associate it with the current session and

lib/galaxy/webapps/galaxy/controllers/history.py

@@ -309,7 +309,7 @@ class HistoryController( BaseUIController, SharableMixin, UsesAnnotations, UsesI
                 # If deleting the current history, make a new current.
                 if history == trans.get_history():
                     deleted_current = True
-                    trans.new_history()
+                    trans.get_or_create_default_history()
                 trans.log_event( "History (%s) marked as deleted" % history.name )
                 n_deleted += 1
             if purge and trans.app.config.allow_user_dataset_purge:
@@ -571,8 +571,8 @@ class HistoryController( BaseUIController, SharableMixin, UsesAnnotations, UsesI
                     # No need to check other outputs since the job's parent history is this history
                     job.mark_deleted( trans.app.config.track_jobs_in_database )
                     trans.app.job_manager.job_stop_queue.put( job.id )
-        # Regardless of whether it was previously deleted, we make a new history active
-        trans.new_history()
+        # Regardless of whether it was previously deleted, get or create default history.
+        trans.get_or_create_default_history()
         return trans.show_ok_message( "History deleted, a new history is active", refresh_frames=['history'] )
 
     @web.expose