Add limit to the grid-generating functions

This adds an optinal maxcells parameter to the rectangle and hexagonal grids,
with a default value of 262144 (2^18).
An error occurs when the grids would need more cells.

scripts-available/CDB_Hexagon.sql

@@ -10,6 +10,10 @@ AS $$
     WHERE path[1] % 2 != 0
 $$ LANGUAGE 'sql' IMMUTABLE STRICT PARALLEL SAFE;
 
+
+-- In older versions of the extension, CDB_HexagonGrid had a different signature
+DROP FUNCTION IF EXISTS cartodb.CDB_HexagonGrid(GEOMETRY, FLOAT8, GEOMETRY);
+
 --
 -- Fill given extent with an hexagonal coverage
 --
@@ -25,10 +29,13 @@ $$ LANGUAGE 'sql' IMMUTABLE STRICT PARALLEL SAFE;
 --               If omitted the origin will be 0,0.
 --               The parameter is checked for having the same SRID
 --               as the extent.
---  
 --
+-- @param maxcells Optional maximum number of grid cells to generate;
+--                 if the grid requires more cells to cover the extent
+--                 and exception will occur.
+----
 -- DROP FUNCTION IF EXISTS CDB_HexagonGrid(ext GEOMETRY, side FLOAT8);
-CREATE OR REPLACE FUNCTION CDB_HexagonGrid(ext GEOMETRY, side FLOAT8, origin GEOMETRY DEFAULT NULL)
+CREATE OR REPLACE FUNCTION CDB_HexagonGrid(ext GEOMETRY, side FLOAT8, origin GEOMETRY DEFAULT NULL, maxcells INTEGER DEFAULT 512*512)
 RETURNS SETOF GEOMETRY
 AS $$
 DECLARE
@@ -107,6 +114,12 @@ BEGIN
     vstartary := ARRAY[ vstart - (vstep/2.0), vstart ];
   END IF;
 
+  If maxcells IS NOT NULL AND maxcells > 0 THEN
+    IF CEIL((CEIL((vend-vstart)/(vstep/2.0)) * CEIL((hend-hstart)/(hstep*2.0/3.0)))/3.0)::integer > maxcells THEN
+      RAISE EXCEPTION 'The requested grid is too big to be rendered';
+    END IF;
+  END IF;
+
   vstartidx := abs(hskip)%2;
 
   RAISE DEBUG 'vstartary: % : %', vstartary[1], vstartary[2];

scripts-available/CDB_RectangleGrid.sql

@@ -1,3 +1,6 @@
+-- In older versions of the extension, CDB_RectangleGrid had a different signature
+DROP FUNCTION IF EXISTS cartodb.CDB_RectangleGrid(GEOMETRY, FLOAT8, FLOAT8, GEOMETRY);
+
 --
 -- Fill given extent with a rectangular coverage
 --
@@ -14,9 +17,12 @@
 --               If omitted the origin will be 0,0.
 --               The parameter is checked for having the same SRID
 --               as the extent.
---  
 --
-CREATE OR REPLACE FUNCTION CDB_RectangleGrid(ext GEOMETRY, width FLOAT8, height FLOAT8, origin GEOMETRY DEFAULT NULL)
+-- @param maxcells Optional maximum number of grid cells to generate;
+--                 if the grid requires more cells to cover the extent
+--                 and exception will occur.
+--
+CREATE OR REPLACE FUNCTION CDB_RectangleGrid(ext GEOMETRY, width FLOAT8, height FLOAT8, origin GEOMETRY DEFAULT NULL, maxcells INTEGER DEFAULT 512*512)
 RETURNS SETOF GEOMETRY
 AS $$
 DECLARE
@@ -79,6 +85,12 @@ BEGIN
   --RAISE DEBUG 'hend: %', hend;
   --RAISE DEBUG 'vend: %', vend;
 
+  If maxcells IS NOT NULL AND maxcells > 0 THEN
+    IF ((hend - hstart)/hstep * (vend - vstart)/vstep)::integer > maxcells THEN
+        RAISE EXCEPTION 'The requested grid is too big to be rendered';
+    END IF;
+  END IF;
+
   x := hstart;
   WHILE x < hend LOOP -- over X
     y := vstart;

test/CDB_HexagonTest.sql

@@ -45,3 +45,9 @@ WITH
  0.002 as radius ),
  grid AS ( SELECT CDB_HexagonGrid(env, radius) AS cell from params)
 SELECT '#160', count(cell) > 23000 from grid;
+
+-- Check small grids are generated...
+SELECT COUNT(*) FROM cartodb.CDB_HexagonGrid(ST_MakeEnvelope(0,0,1000,1000,3857), 10);
+
+-- But large grids produce an error
+SELECT COUNT(*) FROM cartodb.CDB_HexagonGrid(ST_MakeEnvelope(0,0,1000,1000,3857), 1);

test/CDB_HexagonTest_expect

@@ -1,2 +1,5 @@
 9|63|count / npoints
 #160|t
+3886
+ERROR:  The requested grid is too big to be rendered
+CONTEXT:  PL/pgSQL function cdb_hexagongrid(geometry,double precision,geometry,integer) line 80 at RAISE

test/CDB_RectangleTest.sql

@@ -0,0 +1,5 @@
+-- Check small grids are generated...
+SELECT COUNT(*) FROM cartodb.CDB_RectangleGrid(ST_MakeEnvelope(0,0,1000,1000,3857), 10, 10);
+
+-- But large grids produce an error
+SELECT COUNT(*) FROM cartodb.CDB_RectangleGrid(ST_MakeEnvelope(0,0,1000,1000,3857), 1, 1);

test/CDB_RectangleTest_expect

@@ -0,0 +1,3 @@
+10000
+ERROR:  The requested grid is too big to be rendered
+CONTEXT:  PL/pgSQL function cdb_rectanglegrid(geometry,double precision,double precision,geometry,integer) line 64 at RAISE