ENH: release GIL during STRtree creation

[brendan-ward]

We aren't currently releasing the GIL during the creation of STRtree; this might be possible and could yield potential performance improvements in a multi-threaded environment.

In particular, the initial query against the tree after construction toward the end of STRtree_new should not need to hold the GIL.

[jorisvandenbossche]

In particular, the initial query against the tree after construction toward the end of STRtree_new should not need to hold the GIL.

Ah good point, it might be that the most expensive part is actually here? So releasing the GIL here might give most of the benefit.

The rest of the code in STRtree_New is inserting the geometries in the tree, and adding them to a vector, which needs an INCREF, which I think cannot be done without the GIL?

pygeos/src/strtree.c

Lines 151 to 200 in ea76ac9

	GEOS_INIT;
	tree = GEOSSTRtree_create_r(ctx, (size_t)node_capacity);
	if (tree == NULL) {
	errstate = PGERR_GEOS_EXCEPTION;
	return NULL;
	GEOS_FINISH;
	}
	n = PyArray_SIZE((PyArrayObject*)arr);
	_geoms = (GeometryObject**)malloc(n * sizeof(GeometryObject*));
	for (i = 0; i < n; i++) {
	/* get the geometry */
	ptr = PyArray_GETPTR1((PyArrayObject*)arr, i);
	obj = *(GeometryObject**)ptr;
	/* fail and cleanup incase obj was no geometry */
	if (!get_geom(obj, &geom)) {
	errstate = PGERR_NOT_A_GEOMETRY;
	GEOSSTRtree_destroy_r(ctx, tree);
	// free the geometries
	for (i = 0; i < counter; i++) {
	Py_XDECREF(_geoms[i]);
	}
	free(_geoms);
	GEOS_FINISH;
	return NULL;
	}
	/* If geometry is None or empty, do not add it to the tree or count.
	* Set it as NULL for the internal geometries used for predicate tests.
	*/
	if (geom == NULL || GEOSisEmpty_r(ctx, geom)) {
	_geoms[i] = NULL;
	} else {
	// NOTE: we must keep a reference to the GeometryObject added to the tree in order
	// to avoid segfaults later. See: https://github.com/pygeos/pygeos/pull/100.
	Py_INCREF(obj);
	_geoms[i] = obj;
	count_indexed++;
	// Store the address of this geometry within _geoms array as the item data in the
	// tree. This address is used to calculate the original index of the geometry in
	// the input array.
	// NOTE: the type of item data we store is GeometryObject**.
	GEOSSTRtree_insert_r(ctx, tree, geom, &(_geoms[i]));
	}
	counter++;
	}