FIX: release heap snapshot buffers

Free native buffers after heap snapshots and low memory notifications
to avoid leaking memory.
Delete the V8 heap snapshot once it has been serialized.

Author: SamSaffron

CHANGELOG

@@ -1,5 +1,6 @@
 - (Unreleased)
   - Add `Context#perform_microtask_checkpoint` to synchronously drain the V8 microtask queue, useful for spec-compliant `dispatchEvent` sequencing inside Ruby callbacks
+  - Fix native memory leaks in `Context#heap_snapshot`/`Context#write_heap_snapshot`; thanks to Pranjali Thakur from depthfrist.com
 
 - 0.21.1 - 25-05-2026
   - Run `:single_threaded` V8 dispatches on a reusable mini_racer-owned native thread so V8 does not execute on Ruby-owned threads

ext/mini_racer_extension/mini_racer_extension.c

@@ -1458,6 +1458,20 @@ static VALUE context_heap_stats(VALUE self)
     return h;
 }
 
+static VALUE buf_reset_ensure(VALUE arg)
+{
+    buf_reset((Buf *)arg);
+    return Qnil;
+}
+
+static VALUE heap_snapshot_to_str(VALUE arg)
+{
+    Buf *res;
+
+    res = (Buf *)arg;
+    return rb_utf8_str_new((char *)res->buf, res->len);
+}
+
 static VALUE context_heap_snapshot(VALUE self)
 {
     Buf req, res;
@@ -1467,7 +1481,8 @@ static VALUE context_heap_snapshot(VALUE self)
     buf_init(&req);
     buf_putc(&req, 'H');              // (H)eap snapshot, returns plain bytes
     rendezvous_no_des(c, &req, &res); // takes ownership of |req|
-    return rb_utf8_str_new((char *)res.buf, res.len);
+    return rb_ensure(heap_snapshot_to_str, (VALUE)&res,
+                     buf_reset_ensure, (VALUE)&res);
 }
 
 static VALUE context_perform_microtask_checkpoint(VALUE self)
@@ -1501,6 +1516,7 @@ static VALUE context_low_memory_notification(VALUE self)
     buf_init(&req);
     buf_putc(&req, 'L');              // (L)ow memory notification, returns nothing
     rendezvous_no_des(c, &req, &res); // takes ownership of |req|
+    buf_reset(&res);
     return Qnil;
 }
 

ext/mini_racer_extension/mini_racer_v8.cc

@@ -659,6 +659,7 @@ extern "C" void v8_heap_snapshot(State *pst)
     auto snapshot = st.isolate->GetHeapProfiler()->TakeHeapSnapshot();
     OutputStream os;
     snapshot->Serialize(&os, v8::HeapSnapshot::kJSON);
+    const_cast<v8::HeapSnapshot*>(snapshot)->Delete();
     v8_reply(st.ruby_context, os.buf.data(), os.buf.size()); // not serialized because big
 }