🐛 fix WASM memory allocation and add ops buffer overflow check

Size memory dynamically based on clayterm_size() instead of hardcoding
256 pages, which was too small for large terminals.

Prevent silent memory corruption with a bounds check in pack() to make
sure we do not pack more operations that can fit in the buffer.

1MB should fit thousands of elements.

Author: cowboyd

ops.ts

@@ -62,8 +62,14 @@ function packString(view: DataView, bytes: Uint8Array, o: number): number {
   return o;
 }
 
-export function pack(ops: Op[], mem: ArrayBufferLike, offset: number): number {
+export function pack(
+  ops: Op[],
+  mem: ArrayBufferLike,
+  offset: number,
+  limit?: number,
+): number {
   let view = new DataView(mem);
+  let end = limit ?? mem.byteLength;
   let o = offset;
 
   for (let op of ops) {
@@ -192,6 +198,13 @@ export function pack(ops: Op[], mem: ArrayBufferLike, offset: number): number {
         break;
       }
     }
+    if (o > end) {
+      throw new RangeError(
+        `ops exceed buffer capacity (${o - offset} bytes packed, ${
+          end - offset
+        } available)`,
+      );
+    }
   }
 
   return (o - offset) / 4;

term-native.ts

@@ -10,7 +10,7 @@ export interface Native {
 import { compiled } from "./wasm.ts";
 
 export async function createTermNative(w: number, h: number): Promise<Native> {
-  let memory = new WebAssembly.Memory({ initial: 256 });
+  let memory = new WebAssembly.Memory({ initial: 2 });
   let exports: Record<string, CallableFunction> = {};
 
   let instance = await WebAssembly.instantiate(compiled, {
@@ -49,6 +49,15 @@ export async function createTermNative(w: number, h: number): Promise<Native> {
 
   let heap = ct.__heap_base.value as number;
   let size = ct.clayterm_size(w, h);
+
+  // grow memory to fit heap + state + ops buffer (1MB headroom for ops)
+  let needed = heap + size + 1024 * 1024;
+  let pages = Math.ceil(needed / 65536);
+  let current = memory.buffer.byteLength / 65536;
+  if (pages > current) {
+    memory.grow(pages - current);
+  }
+
   let statePtr = ct.init(heap, w, h);
   let opsBuf = (heap + size + 3) & ~3;
 

term.ts

@@ -20,7 +20,7 @@ export async function createTerm(options: TermOptions): Promise<Term> {
 
   return {
     render(ops: Op[]): Uint8Array {
-      let len = pack(ops, memory.buffer, opsBuf);
+      let len = pack(ops, memory.buffer, opsBuf, memory.buffer.byteLength);
       reduce(statePtr, opsBuf, len);
       return new Uint8Array(
         memory.buffer,