fix(wasm): realloc should handle smaller new size

crates/language/wasm/src/stdlib.c

@@ -14,6 +14,7 @@ extern void tree_sitter_debug_message(const char *, size_t);
 
 #define PAGESIZE 0x10000
 #define MAX_HEAP_SIZE (4 * 1024 * 1024)
+#define MIN(a, b) ((a) < (b) ? (a) : (b))
 
 typedef struct {
   size_t size;
@@ -118,18 +119,41 @@ void *realloc(void *ptr, size_t new_size) {
     return malloc(new_size);
   }
 
+  // Per the C standard, realloc(ptr, 0) acts like free(ptr) and returns NULL.
+  if (new_size == 0) {
+    free(ptr);
+    return NULL;
+  }
+
   Region *region = region_for_ptr(ptr);
   Region *region_end = region_after(region, region->size);
 
   // When reallocating the last allocated region, return
   // the same pointer, and skip copying the data.
   if (region_end == next) {
-    next = region;
-    return malloc(new_size);
+    // grow the heap and update last region's size so we do not need to copy data
+    Region *new_region_end = region_after(region, new_size);
+    if (new_region_end > heap_end) {
+      if ((char *)new_region_end - (char *)heap_start > MAX_HEAP_SIZE) {
+        return NULL;
+      }
+      if (!grow_heap(new_size)) return NULL;
+      heap_end = get_heap_end();
+    }
+
+    region->size = new_size;
+    next = new_region_end;
+    return ptr;
   }
 
   void *result = malloc(new_size);
-  memcpy(result, &region->data, region->size);
+  if (result == NULL) {
+    return NULL;
+  }
+
+  size_t copy_size = MIN(region->size, new_size);
+  memcpy(result, &region->data, copy_size);
+  free(ptr);
   return result;
 }