worker: Fix ownership of the ArrayBuffer backing storage in the go->js path (ry#29)

We witnessed double frees on buffers passed to the JS recv() function:

  jkcfg/jk#71
  jkcfg/jk#91

With a bit of intrumentation (https://github.com/dlespiau/debug-allocations),
we traced this down to both, the go code and the JS garbage collection freeing
the buffer. And indeed we call C.free and create the AB telling the JS garbage
collector it owns the storage with ArrayBufferCreationMode::kInternalized.

One of the two has to go. Given C.CBytes() allocates the buffer on the C stack
using malloc and the v8 allocator is the libc malloc/free we can safely pass
this buffer to v8. This will allow recv() users to not have to care about
copying content of that AB before recv() returns.

I traced the malloc/free of that buffer and confirmed they are properly freed
(in this case a 340 bytes buffer with address 0x7fc914000a40):

malloc 340   0x7fc914000a40 ./log-malloc.so(+0xb4e)[0x7fc9219dab4e]
./log-malloc.so(malloc+0xa3)[0x7fc9219dac6b]
jk(_cgo_9d48446f0bab_Cfunc__Cmalloc+0x14)[0x9ccd54]
jk[0x6dcc00]

[...]

free   0x7fc914000a40 ./log-malloc.so(+0xb4e)[0x7fc9219dab4e]
./log-malloc.so(free+0x8f)[0x7fc9219dae1f]
jk(_ZN2v88internal20ArrayBufferCollector15FreeAllocationsEv+0x8e)[0xf9807e]
jk(_ZN2v88internal20ArrayBufferCollector11FreeingTask11RunInternalEv+0x146)[0xf98326]
jk(_ZN2v88platform12WorkerThread3RunEv+0x36)[0x9d38f6]
jk[0xe0b260]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x76ba)[0x7fc9217c46ba]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7fc920c5941d]

Author: dlespiau

worker.go

@@ -244,8 +244,11 @@ func (w *Worker) LoadModule(scriptName string, code string, resolve ModuleResolv
 // Same as Send but for []byte. $recv callback will get an ArrayBuffer.
 func (w *Worker) SendBytes(msg []byte) error {
 	msg_p := C.CBytes(msg)
-	defer C.free(msg_p)
 
+	// C.CBytes allocates memory on the C heap that is used as the backing
+	// storage for the ArrayBuffer given to javascript. v8 will free the buffer
+	// as part of the ArrayBuffer garbage collection as we create the AB with
+	// ArrayBufferCreationMode::kInternalized in worker_send_bytes.
 	r := C.worker_send_bytes(w.worker.cWorker, msg_p, C.size_t(len(msg)))
 	if r != 0 {
 		errStr := C.GoString(C.worker_last_exception(w.worker.cWorker))