gg: add explicit bounds checking for image caching methods in gg (fix #25590) (#25591)

Author: mike-ward

vlib/gg/image.v

@@ -46,11 +46,17 @@ pub fn (mut ctx Context) cache_image(img Image) int {
 	return image_idx
 }
 
+const missing_image = Image{}
+
 // get_cached_image_by_idx returns a cached `Image` identified by `image_idx`.
+// If image not found, returns `Image{ok: false}`
 //
 // See also: cache_image
 // See also: remove_cached_image_by_idx
 pub fn (mut ctx Context) get_cached_image_by_idx(image_idx int) &Image {
+	if image_idx < 0 || image_idx > ctx.image_cache.len - 1 {
+		return &missing_image
+	}
 	return &ctx.image_cache[image_idx]
 }
 
@@ -60,7 +66,10 @@ pub fn (mut ctx Context) get_cached_image_by_idx(image_idx int) &Image {
 // See also: cache_image
 // See also: get_cached_image_by_idx
 pub fn (mut ctx Context) remove_cached_image_by_idx(image_idx int) {
-	ctx.image_cache.delete(image_idx)
+	if image_idx < 0 || image_idx > ctx.image_cache.len - 1 {
+		return
+	}
+	ctx.image_cache[image_idx] = &missing_image
 }
 
 // Draw part of an image using uv coordinates

vlib/gg/image_test.v

@@ -0,0 +1,19 @@
+// vtest build: !docker-ubuntu-musl // needs GL/gl.h
+import gg
+
+fn test_get_cached_image_idx_bounds_checking() {
+	mut ctx := gg.new_context(width: 100)
+	assert ctx.get_cached_image_by_idx(-1).ok == false
+	assert ctx.get_cached_image_by_idx(1).ok == false
+}
+
+fn test_remove_cached_image_remove_and_get() {
+	mut ctx := gg.new_context(width: 100)
+	image := gg.Image{
+		ok: true
+	}
+	idx := ctx.cache_image(image)
+	assert ctx.get_cached_image_by_idx(idx).ok == true
+	ctx.remove_cached_image_by_idx(idx)
+	assert ctx.get_cached_image_by_idx(idx).ok == false
+}