src: make ReqWrap req_ member private

This commit attempts to address one of the items in
https://github.com/nodejs/node/issues/4641 which is
related to src/req-wrap.h and making the req_ member private.

PR-URL: https://github.com/nodejs/node/pull/8532
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

src/cares_wrap.cc

@@ -1146,7 +1146,7 @@ static void GetAddrInfo(const FunctionCallbackInfo<Value>& args) {
   hints.ai_flags = flags;
 
   int err = uv_getaddrinfo(env->event_loop(),
-                           &req_wrap->req_,
+                           req_wrap->req(),
                            AfterGetAddrInfo,
                            *hostname,
                            nullptr,
@@ -1176,7 +1176,7 @@ static void GetNameInfo(const FunctionCallbackInfo<Value>& args) {
   GetNameInfoReqWrap* req_wrap = new GetNameInfoReqWrap(env, req_wrap_obj);
 
   int err = uv_getnameinfo(env->event_loop(),
-                           &req_wrap->req_,
+                           req_wrap->req(),
                            AfterGetNameInfo,
                            (struct sockaddr*)&addr,
                            NI_NAMEREQD);

src/node_file.cc

@@ -137,7 +137,7 @@ static inline bool IsInt64(double x) {
 
 static void After(uv_fs_t *req) {
   FSReqWrap* req_wrap = static_cast<FSReqWrap*>(req->data);
-  CHECK_EQ(&req_wrap->req_, req);
+  CHECK_EQ(req_wrap->req(), req);
   req_wrap->ReleaseEarly();  // Free memory that's no longer used now.
 
   Environment* env = req_wrap->env();
@@ -320,7 +320,7 @@ static void After(uv_fs_t *req) {
 
   req_wrap->MakeCallback(env->oncomplete_string(), argc, argv);
 
-  uv_fs_req_cleanup(&req_wrap->req_);
+  uv_fs_req_cleanup(req_wrap->req());
   req_wrap->Dispose();
 }
 
@@ -337,18 +337,18 @@ class fs_req_wrap {
 };
 
 
-#define ASYNC_DEST_CALL(func, req, dest, encoding, ...)                       \
+#define ASYNC_DEST_CALL(func, request, dest, encoding, ...)                   \
   Environment* env = Environment::GetCurrent(args);                           \
-  CHECK(req->IsObject());                                                     \
-  FSReqWrap* req_wrap = FSReqWrap::New(env, req.As<Object>(),                 \
+  CHECK(request->IsObject());                                                 \
+  FSReqWrap* req_wrap = FSReqWrap::New(env, request.As<Object>(),             \
                                        #func, dest, encoding);                \
   int err = uv_fs_ ## func(env->event_loop(),                                 \
-                           &req_wrap->req_,                                   \
+                           req_wrap->req(),                                   \
                            __VA_ARGS__,                                       \
                            After);                                            \
   req_wrap->Dispatched();                                                     \
   if (err < 0) {                                                              \
-    uv_fs_t* uv_req = &req_wrap->req_;                                        \
+    uv_fs_t* uv_req = req_wrap->req();                                        \
     uv_req->result = err;                                                     \
     uv_req->path = nullptr;                                                   \
     After(uv_req);                                                            \
@@ -1164,7 +1164,7 @@ static void WriteString(const FunctionCallbackInfo<Value>& args) {
   FSReqWrap* req_wrap =
       FSReqWrap::New(env, req.As<Object>(), "write", buf, UTF8, ownership);
   int err = uv_fs_write(env->event_loop(),
-                        &req_wrap->req_,
+                        req_wrap->req(),
                         fd,
                         &uvbuf,
                         1,
@@ -1172,7 +1172,7 @@ static void WriteString(const FunctionCallbackInfo<Value>& args) {
                         After);
   req_wrap->Dispatched();
   if (err < 0) {
-    uv_fs_t* uv_req = &req_wrap->req_;
+    uv_fs_t* uv_req = req_wrap->req();
     uv_req->result = err;
     uv_req->path = nullptr;
     After(uv_req);

src/pipe_wrap.cc

@@ -168,7 +168,7 @@ void PipeWrap::Connect(const FunctionCallbackInfo<Value>& args) {
 
   ConnectWrap* req_wrap =
       new ConnectWrap(env, req_wrap_obj, AsyncWrap::PROVIDER_PIPECONNECTWRAP);
-  uv_pipe_connect(&req_wrap->req_,
+  uv_pipe_connect(req_wrap->req(),
                   &wrap->handle_,
                   *name,
                   AfterConnect);

src/req-wrap.h

@@ -18,13 +18,19 @@ class ReqWrap : public AsyncWrap {
                  AsyncWrap::ProviderType provider);
   inline ~ReqWrap() override;
   inline void Dispatched();  // Call this after the req has been dispatched.
+  T* req() { return &req_; }
 
  private:
   friend class Environment;
   ListNode<ReqWrap> req_wrap_queue_;
 
- public:
-  T req_;  // Must be last.  TODO(bnoordhuis) Make private.
+ protected:
+  // req_wrap_queue_ needs to be at a fixed offset from the start of the class
+  // because it is used by ContainerOf to calculate the address of the embedding
+  // ReqWrap. ContainerOf compiles down to simple, fixed pointer arithmetic.
+  // sizeof(req_) depends on the type of T, so req_wrap_queue_ would
+  // no longer be at a fixed offset if it came after req_.
+  T req_;
 };
 
 }  // namespace node

src/stream_base.h

@@ -8,6 +8,7 @@
 #include "req-wrap.h"
 #include "req-wrap-inl.h"
 #include "node.h"
+#include "util.h"
 
 #include "v8.h"
 
@@ -56,6 +57,10 @@ class ShutdownWrap : public ReqWrap<uv_shutdown_t>,
     CHECK(args.IsConstructCall());
   }
 
+  static ShutdownWrap* from_req(uv_shutdown_t* req) {
+    return ContainerOf(&ShutdownWrap::req_, req);
+  }
+
   inline StreamBase* wrap() const { return wrap_; }
   size_t self_size() const override { return sizeof(*this); }
 
@@ -82,6 +87,10 @@ class WriteWrap: public ReqWrap<uv_write_t>,
     CHECK(args.IsConstructCall());
   }
 
+  static WriteWrap* from_req(uv_write_t* req) {
+    return ContainerOf(&WriteWrap::req_, req);
+  }
+
   static const size_t kAlignSize = 16;
 
  protected:

src/stream_wrap.cc

@@ -276,14 +276,15 @@ void StreamWrap::SetBlocking(const FunctionCallbackInfo<Value>& args) {
 
 int StreamWrap::DoShutdown(ShutdownWrap* req_wrap) {
   int err;
-  err = uv_shutdown(&req_wrap->req_, stream(), AfterShutdown);
+  err = uv_shutdown(req_wrap->req(), stream(), AfterShutdown);
   req_wrap->Dispatched();
   return err;
 }
 
 
 void StreamWrap::AfterShutdown(uv_shutdown_t* req, int status) {
-  ShutdownWrap* req_wrap = ContainerOf(&ShutdownWrap::req_, req);
+  ShutdownWrap* req_wrap = ShutdownWrap::from_req(req);
+  CHECK_NE(req_wrap, nullptr);
   HandleScope scope(req_wrap->env()->isolate());
   Context::Scope context_scope(req_wrap->env()->context());
   req_wrap->Done(status);
@@ -336,9 +337,9 @@ int StreamWrap::DoWrite(WriteWrap* w,
                         uv_stream_t* send_handle) {
   int r;
   if (send_handle == nullptr) {
-    r = uv_write(&w->req_, stream(), bufs, count, AfterWrite);
+    r = uv_write(w->req(), stream(), bufs, count, AfterWrite);
   } else {
-    r = uv_write2(&w->req_, stream(), bufs, count, send_handle, AfterWrite);
+    r = uv_write2(w->req(), stream(), bufs, count, send_handle, AfterWrite);
   }
 
   if (!r) {
@@ -360,7 +361,8 @@ int StreamWrap::DoWrite(WriteWrap* w,
 
 
 void StreamWrap::AfterWrite(uv_write_t* req, int status) {
-  WriteWrap* req_wrap = ContainerOf(&WriteWrap::req_, req);
+  WriteWrap* req_wrap = WriteWrap::from_req(req);
+  CHECK_NE(req_wrap, nullptr);
   HandleScope scope(req_wrap->env()->isolate());
   Context::Scope context_scope(req_wrap->env()->context());
   req_wrap->Done(status);

src/tcp_wrap.cc

@@ -257,7 +257,7 @@ void TCPWrap::Connect(const FunctionCallbackInfo<Value>& args) {
   if (err == 0) {
     ConnectWrap* req_wrap =
         new ConnectWrap(env, req_wrap_obj, AsyncWrap::PROVIDER_TCPCONNECTWRAP);
-    err = uv_tcp_connect(&req_wrap->req_,
+    err = uv_tcp_connect(req_wrap->req(),
                          &wrap->handle_,
                          reinterpret_cast<const sockaddr*>(&addr),
                          AfterConnect);
@@ -292,7 +292,7 @@ void TCPWrap::Connect6(const FunctionCallbackInfo<Value>& args) {
   if (err == 0) {
     ConnectWrap* req_wrap =
         new ConnectWrap(env, req_wrap_obj, AsyncWrap::PROVIDER_TCPCONNECTWRAP);
-    err = uv_tcp_connect(&req_wrap->req_,
+    err = uv_tcp_connect(req_wrap->req(),
                          &wrap->handle_,
                          reinterpret_cast<const sockaddr*>(&addr),
                          AfterConnect);

src/udp_wrap.cc

@@ -305,7 +305,7 @@ void UDPWrap::DoSend(const FunctionCallbackInfo<Value>& args, int family) {
   }
 
   if (err == 0) {
-    err = uv_udp_send(&req_wrap->req_,
+    err = uv_udp_send(req_wrap->req(),
                       &wrap->handle_,
                       *bufs,
                       count,