allocate on the heap in Sky_ClipPoly if MAX_CLIP_VERTS limit is reached

Fixes a Sys_Error in lim_daviddg map from Liminal Spaces Jam

Quake/gl_sky.c

@@ -495,14 +495,16 @@ static void Sky_ClipPoly (int nump, vec3_t vecs, int stage)
 	float	*v;
 	qboolean	front, back;
 	float	d, e;
-	float	dists[MAX_CLIP_VERTS];
-	int		sides[MAX_CLIP_VERTS];
-	vec3_t	newv[2][MAX_CLIP_VERTS];
 	int		newc[2];
 	int		i, j;
 
-	if (nump > MAX_CLIP_VERTS-2)
-		Sys_Error ("Sky_ClipPoly: MAX_CLIP_VERTS");
+	const int max_clip_verts = nump + 2;
+	const int on_heap = max_clip_verts > MAX_CLIP_VERTS;
+	int	*sides;
+	float	*dists;
+	vec3_t	*newv_0;
+	vec3_t	*newv_1;
+
 	if (stage == 6) // fully clipped
 	{
 		Sky_ProjectPoly (nump, vecs);
@@ -511,6 +513,10 @@ static void Sky_ClipPoly (int nump, vec3_t vecs, int stage)
 
 	front = back = false;
 	norm = skyclip[stage];
+
+	sides = (int *) (on_heap ? malloc(max_clip_verts * sizeof(int)) : alloca(max_clip_verts * sizeof(int)));
+	dists = (float *) (on_heap ? malloc(max_clip_verts * sizeof(float)) : alloca(max_clip_verts * sizeof(float)));
+
 	for (i=0, v = vecs ; i<nump ; i++, v+=3)
 	{
 		d = DotProduct (v, norm);
@@ -532,6 +538,10 @@ static void Sky_ClipPoly (int nump, vec3_t vecs, int stage)
 	if (!front || !back)
 	{	// not clipped
 		Sky_ClipPoly (nump, vecs, stage+1);
+		if (on_heap) {
+			free(dists);
+			free(sides);
+		}
 		return;
 	}
 
@@ -541,22 +551,26 @@ static void Sky_ClipPoly (int nump, vec3_t vecs, int stage)
 	VectorCopy (vecs, (vecs+(i*3)) );
 	newc[0] = newc[1] = 0;
 
+	// 2-dim vec3_t	 newv[2][MAX_CLIP_VERTS]; as 2 arrays
+	newv_0 = (vec3_t *) (on_heap ? malloc(max_clip_verts * sizeof(vec3_t)) : alloca(max_clip_verts * sizeof(vec3_t)));
+	newv_1 = (vec3_t *) (on_heap ? malloc(max_clip_verts * sizeof(vec3_t)) : alloca(max_clip_verts * sizeof(vec3_t)));
+
 	for (i=0, v = vecs ; i<nump ; i++, v+=3)
 	{
 		switch (sides[i])
 		{
 		case SIDE_FRONT:
-			VectorCopy (v, newv[0][newc[0]]);
+			VectorCopy (v, newv_0[newc[0]]);
 			newc[0]++;
 			break;
 		case SIDE_BACK:
-			VectorCopy (v, newv[1][newc[1]]);
+			VectorCopy (v, newv_1[newc[1]]);
 			newc[1]++;
 			break;
 		case SIDE_ON:
-			VectorCopy (v, newv[0][newc[0]]);
+			VectorCopy (v, newv_0[newc[0]]);
 			newc[0]++;
-			VectorCopy (v, newv[1][newc[1]]);
+			VectorCopy (v, newv_1[newc[1]]);
 			newc[1]++;
 			break;
 		}
@@ -568,16 +582,24 @@ static void Sky_ClipPoly (int nump, vec3_t vecs, int stage)
 		for (j=0 ; j<3 ; j++)
 		{
 			e = v[j] + d*(v[j+3] - v[j]);
-			newv[0][newc[0]][j] = e;
-			newv[1][newc[1]][j] = e;
+			newv_0[newc[0]][j] = e;
+			newv_1[newc[1]][j] = e;
 		}
 		newc[0]++;
 		newc[1]++;
 	}
 
 	// continue
-	Sky_ClipPoly (newc[0], newv[0][0], stage+1);
-	Sky_ClipPoly (newc[1], newv[1][0], stage+1);
+	Sky_ClipPoly (newc[0], newv_0[0], stage+1);
+	Sky_ClipPoly (newc[1], newv_1[0], stage+1);
+
+	if (on_heap)
+	{
+		free(dists);
+		free(sides);
+		free(newv_0);
+		free(newv_1);
+	}
 }
 
 /*
@@ -587,19 +609,27 @@ Sky_ProcessPoly
 */
 void Sky_ProcessPoly (glpoly_t	*p)
 {
-	int			i;
-	vec3_t		verts[MAX_CLIP_VERTS];
-
 	//draw it
 	DrawGLPoly(p);
 	rs_brushpasses++;
 
 	//update sky bounds
 	if (!r_fastsky.value)
 	{
-		for (i=0 ; i<p->numverts ; i++)
+		const int max_clip_verts = p->numverts + 2;
+		const int num_verts = p->numverts;
+		const int on_heap = max_clip_verts > MAX_CLIP_VERTS;
+		vec3_t *verts = (vec3_t *) (on_heap ?
+				 malloc(max_clip_verts * sizeof(vec3_t)) :
+				 alloca(max_clip_verts * sizeof(vec3_t)));
+		int i = 0;
+
+		for ( ; i < num_verts; i++) {
 			VectorSubtract (p->verts[i], r_origin, verts[i]);
-		Sky_ClipPoly (p->numverts, verts[0], 0);
+		}
+		Sky_ClipPoly (num_verts, verts[0], 0);
+
+		if (on_heap) free(verts);
 	}
 }