Fix crash when resurfacing SVG canvases

Fixes Automattic#1380

CHANGELOG.md

@@ -12,6 +12,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 * Fix crash when changing canvas width/height while `fillStyle` or `strokeStyle`
   was set to a `CanvasPattern` or `CanvasGradient` (#1357).
+* Fix crash when changing width/height of SVG canvases (#1380).
 
 2.5.0
 ==================

src/backend/SvgBackend.cc

@@ -3,6 +3,7 @@
 #include <cairo-svg.h>
 #include "../Canvas.h"
 #include "../closure.h"
+#include <cassert>
 
 using namespace v8;
 
@@ -13,7 +14,10 @@ SvgBackend::SvgBackend(int width, int height)
 
 SvgBackend::~SvgBackend() {
   cairo_surface_finish(surface);
-  if (_closure) delete _closure;
+  if (_closure) {
+    delete _closure;
+    _closure = nullptr;
+  }
   destroySurface();
 }
 
@@ -22,14 +26,16 @@ Backend *SvgBackend::construct(int width, int height){
 }
 
 cairo_surface_t* SvgBackend::createSurface() {
-  if (!_closure) _closure = new PdfSvgClosure(canvas);
+  assert(!_closure);
+  _closure = new PdfSvgClosure(canvas);
   surface = cairo_svg_surface_create_for_stream(PdfSvgClosure::writeVec, _closure, width, height);
   return surface;
 }
 
 cairo_surface_t* SvgBackend::recreateSurface() {
   cairo_surface_finish(surface);
   delete _closure;
+  _closure = nullptr;
   cairo_surface_destroy(surface);
 
   return createSurface();

test/canvas.test.js

@@ -392,6 +392,11 @@ describe('Canvas', function () {
     })
   });
 
+  it('SVG Canvas#width changes don\'t crash (#1380)', function () {
+    const myCanvas = createCanvas(100, 100, 'svg')
+    myCanvas.width = 120;
+  });
+
   it('Canvas#stride', function() {
     var canvas = createCanvas(24, 10);
     assert.ok(canvas.stride >= 24, 'canvas.stride is too short');