fix a crash in getImageData if the rectangle is outside the canvas

Author: Philippe Plantier

CHANGELOG.md

@@ -15,6 +15,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 * Support for accessibility and links in PDFs
 
 ### Fixed
+* Fix a crash in `getImageData` when the rectangle is entirely outside the canvas. ([#2024](https://github.com/Automattic/node-canvas/issues/2024))
 
 3.0.1
 ==================

src/CanvasRenderingContext2d.cc

@@ -1014,10 +1014,6 @@ Context2d::GetImageData(const Napi::CallbackInfo& info) {
   if (sx + sw > width) sw = width - sx;
   if (sy + sh > height) sh = height - sy;
 
-  // WebKit/moz functionality. node-canvas used to return in either case.
-  if (sw <= 0) sw = 1;
-  if (sh <= 0) sh = 1;
-
   // Non-compliant. "Pixels outside the canvas must be returned as transparent
   // black." This instead clips the returned array to the canvas area.
   if (sx < 0) {
@@ -1029,6 +1025,10 @@ Context2d::GetImageData(const Napi::CallbackInfo& info) {
     sy = 0;
   }
 
+  // WebKit/moz functionality. node-canvas used to return in either case.
+  if (sw <= 0) sw = 1;
+  if (sh <= 0) sh = 1;
+
   int srcStride = canvas->stride();
   int bpp = srcStride / width;
   int size = sw * sh * bpp;

test/canvas.test.js

@@ -1274,6 +1274,18 @@ describe('Canvas', function () {
         ctx.getImageData(0, 0, 3, 6)
       })
     })
+
+    it('does not throw if rectangle is outside the canvas (#2024)', () => {
+      const canvas = createCanvas(200, 200)
+      const ctx = canvas.getContext('2d')
+
+      ctx.rect(0, 0, 100, 100);
+      ctx.fill();
+
+      const imageData = ctx.getImageData(0, -11, 10, 10);
+      assert.equal(10, imageData.width)
+      assert.equal(1, imageData.height)
+    })
   })
 
   it('Context2d#createPattern(Canvas)', function () {