reference count the shape_featureset in a custom way (destructors are not called immediately from node.js) to watch for thread unsafe usage of the resource

plugins/input/shape/shape_datasource.cpp

@@ -61,7 +61,8 @@ shape_datasource::shape_datasource(const parameters &params, bool bind)
       file_length_(0),
       indexed_(false),
       row_limit_(*params_.get<int>("row_limit",0)),
-      desc_(*params.get<std::string>("type"), *params.get<std::string>("encoding","utf-8"))
+      desc_(*params.get<std::string>("type"), *params.get<std::string>("encoding","utf-8")),
+      in_use_(0)
 {
     boost::optional<std::string> file = params.get<std::string>("file");
     if (!file) throw datasource_exception("Shape Plugin: missing <file> parameter");
@@ -273,7 +274,8 @@ featureset_ptr shape_datasource::features(const query& q) const
                                                        q.property_names(),
                                                        desc_.get_encoding(),
                                                        shape_name_,
-                                                       row_limit_));
+                                                       row_limit_,
+                                                       in_use_));
     }
     else
     {
@@ -317,7 +319,8 @@ featureset_ptr shape_datasource::features_at_point(coord2d const& pt) const
                                                          names,
                                                          desc_.get_encoding(),
                                                          shape_name_,
-                                                         row_limit_));
+                                                         row_limit_,
+                                                         in_use_));
     }
     else
     {

plugins/input/shape/shape_datasource.hpp

@@ -65,6 +65,10 @@ private:
     mutable bool indexed_;
     const int row_limit_;
     mutable layer_descriptor desc_;
+    // shape_datasource should not be used between threads
+    // because for indexed files shape_ is kept open for speed
+    // since this condition is so hard to debug we'll track and warn
+    mutable int in_use_;
 };
 
 #endif //SHAPE_HPP

plugins/input/shape/shape_index_featureset.cpp

@@ -43,14 +43,17 @@ shape_index_featureset<filterT>::shape_index_featureset(filterT const& filter,
                                                         std::set<std::string> const& attribute_names,
                                                         std::string const& encoding,
                                                         std::string const& shape_name,
-                                                        int row_limit)
+                                                        int row_limit,
+                                                        int & in_use)
     : filter_(filter),
       ctx_(boost::make_shared<mapnik::context_type>()),
       shape_(shape),
       tr_(new transcoder(encoding)),
       row_limit_(row_limit),
-      count_(0)
+      count_(0),
+      in_use_(in_use)
 {
+    in_use_++;
     shape_.shp().skip(100);
     setup_attributes(ctx_, attribute_names, shape_name, shape_,attr_ids_);
     
@@ -75,8 +78,12 @@ shape_index_featureset<filterT>::shape_index_featureset(filterT const& filter,
 template <typename filterT>
 feature_ptr shape_index_featureset<filterT>::next()
 {
+    if (in_use_ > 1)
+        MAPNIK_LOG_ERROR(shape) << "*** Warning: indexed shape_featureset appears to be being used by more than one thread\n";
+
     if (row_limit_ && count_ > row_limit_)
     {
+        in_use_--;
         return feature_ptr();
     }
 
@@ -135,6 +142,7 @@ feature_ptr shape_index_featureset<filterT>::next()
                 }
                 else
                 {
+                    in_use_--;
                     return feature_ptr();
                 }
             }
@@ -201,7 +209,7 @@ feature_ptr shape_index_featureset<filterT>::next()
     else
     {
         MAPNIK_LOG_DEBUG(shape) << "shape_index_featureset: " << count_ << " features";
-
+        in_use_--;
         return feature_ptr();
     }
 }

plugins/input/shape/shape_index_featureset.hpp

@@ -51,7 +51,8 @@ public:
                            std::set<std::string> const& attribute_names,
                            std::string const& encoding,
                            std::string const& shape_name,
-                           int row_limit);
+                           int row_limit,
+                           int & in_use);
     virtual ~shape_index_featureset();
     feature_ptr next();
 
@@ -65,6 +66,7 @@ private:
     std::vector<int> attr_ids_;
     const int row_limit_;
     mutable int count_;
+    int & in_use_;
 };
 
 #endif // SHAPE_INDEX_FEATURESET_HPP