src, test: node internals' postmortem metadata

Before these changes, only V8 added postmortem metadata to Node's
binary, limiting the possibilities for debugger's developers to add some
features that rely on investigating Node's internal structures.

These changes are first steps towards empowering debug tools to
navigate Node's internal structures. One example of what can be
achieved with this is shown at nodejs/llnode#122 (a command which prints
information about handles and requests on the queue for a core dump
file). Node postmortem metadata are prefixed with nodedbg_.

This also adds tests to validate if all postmortem metadata are
calculated correctly, plus some documentation on what is postmortem
metadata and a few care to be taken to avoid breaking it.

Ref: https://github.com/nodejs/llnode/pull/122
Ref: https://github.com/nodejs/post-mortem/issues/46

PR-URL: https://github.com/nodejs/node/pull/14901
Refs: https://github.com/nodejs/post-mortem/issues/46
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

6 files changed, 149 insertions, 7 deletions

diff --git a/src/base_object.h b/src/base_object.h
index 0998920f49d..5852f764066 100644
--- a/src/base_object.h
+++ b/src/base_object.h
@@ -65,6 +65,11 @@ class BaseObject {
   static inline void WeakCallback(
       const v8::WeakCallbackInfo<Type>& data);
 
+  // persistent_handle_ needs to be at a fixed offset from the start of the
+  // class because it is used by src/node_postmortem_metadata.cc to calculate
+  // offsets and generate debug symbols for BaseObject, which assumes that the
+  // position of members in memory are predictable. For more information please
+  // refer to `doc/guides/node-postmortem-support.md`
   v8::Persistent<v8::Object> persistent_handle_;
   Environment* env_;
 };
diff --git a/src/env.h b/src/env.h
index 198cda1d52c..6e0b74d620a 100644
--- a/src/env.h
+++ b/src/env.h
@@ -765,6 +765,12 @@ class Environment {
   std::unique_ptr<inspector::Agent> inspector_agent_;
 #endif
 
+  // handle_wrap_queue_ and req_wrap_queue_ needs to be at a fixed offset from
+  // the start of the class because it is used by
+  // src/node_postmortem_metadata.cc to calculate offsets and generate debug
+  // symbols for Environment, which assumes that the position of members in
+  // memory are predictable. For more information please refer to
+  // `doc/guides/node-postmortem-support.md`
   HandleWrapQueue handle_wrap_queue_;
   ReqWrapQueue req_wrap_queue_;
   ListHead<HandleCleanup,
diff --git a/src/handle_wrap.h b/src/handle_wrap.h
index 29e4f364603..1cfe3e2ebb4 100644
--- a/src/handle_wrap.h
+++ b/src/handle_wrap.h
@@ -81,6 +81,11 @@ class HandleWrap : public AsyncWrap {
   friend class Environment;
   friend void GetActiveHandles(const v8::FunctionCallbackInfo<v8::Value>&);
   static void OnClose(uv_handle_t* handle);
+  // handle_wrap_queue_ needs to be at a fixed offset from the start of the
+  // class because it is used by src/node_postmortem_metadata.cc to calculate
+  // offsets and generate debug symbols for HandleWrap, which assumes that the
+  // position of members in memory are predictable. For more information please
+  // refer to `doc/guides/node-postmortem-support.md`
   ListNode<HandleWrap> handle_wrap_queue_;
   enum { kInitialized, kClosing, kClosingWithCallback, kClosed } state_;
   uv_handle_t* const handle_;
diff --git a/src/node_postmortem_metadata.cc b/src/node_postmortem_metadata.cc
new file mode 100644
index 00000000000..4a463958f54
--- /dev/null
+++ b/src/node_postmortem_metadata.cc
@@ -0,0 +1,118 @@
+// Need to import standard headers before redefining private, otherwise it
+// won't compile.
+#include <algorithm>
+#include <array>
+#include <atomic>
+#include <bitset>
+#include <cctype>
+#include <climits>
+#include <cmath>
+#include <cstdarg>
+#include <cstddef>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <ctime>
+#include <deque>
+#include <exception>
+#include <forward_list>
+#include <fstream>
+#include <functional>
+#include <iomanip>
+#include <iosfwd>
+#include <iostream>
+#include <istream>
+#include <iterator>
+#include <limits>
+#include <list>
+#include <map>
+#include <memory>
+#include <new>
+#include <ostream>
+#include <queue>
+#include <set>
+#include <sstream>
+#include <stack>
+#include <streambuf>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <typeinfo>
+#include <unordered_map>
+#include <unordered_set>
+#include <utility>
+#include <vector>
+
+namespace node {
+// Forward declaration needed before redefining private.
+int GenDebugSymbols();
+}  // namespace node
+
+
+#define private friend int GenDebugSymbols(); private
+
+#include "env.h"
+#include "base_object-inl.h"
+#include "handle_wrap.h"
+#include "util-inl.h"
+#include "req_wrap.h"
+#include "v8abbr.h"
+
+#define NODEDBG_SYMBOL(Name)  nodedbg_ ## Name
+
+// nodedbg_offset_CLASS__MEMBER__TYPE: Describes the offset to a class member.
+#define NODEDBG_OFFSET(Class, Member, Type) \
+    NODEDBG_SYMBOL(offset_ ## Class ## __ ## Member ## __ ## Type)
+
+// These are the constants describing Node internal structures. Every constant
+// should use the format described above.  These constants are declared as
+// global integers so that they'll be present in the generated node binary. They
+// also need to be declared outside any namespace to avoid C++ name-mangling.
+#define NODE_OFFSET_POSTMORTEM_METADATA(V)                                    \
+  V(BaseObject, persistent_handle_, v8_Persistent_v8_Object,                  \
+    BaseObject::persistent_handle_)                                           \
+  V(Environment, handle_wrap_queue_, Environment_HandleWrapQueue,             \
+    Environment::handle_wrap_queue_)                                          \
+  V(Environment, req_wrap_queue_, Environment_ReqWrapQueue,                   \
+    Environment::req_wrap_queue_)                                             \
+  V(HandleWrap, handle_wrap_queue_, ListNode_HandleWrap,                      \
+    HandleWrap::handle_wrap_queue_)                                           \
+  V(Environment_HandleWrapQueue, head_, ListNode_HandleWrap,                  \
+    Environment::HandleWrapQueue::head_)                                      \
+  V(ListNode_HandleWrap, next_, uintptr_t, ListNode<HandleWrap>::next_)       \
+  V(ReqWrap, req_wrap_queue_, ListNode_ReqWrapQueue,                          \
+    ReqWrap<uv_req_t>::req_wrap_queue_)                                       \
+  V(Environment_ReqWrapQueue, head_, ListNode_ReqWrapQueue,                   \
+    Environment::ReqWrapQueue::head_)                                         \
+  V(ListNode_ReqWrap, next_, uintptr_t, ListNode<ReqWrap<uv_req_t>>::next_)
+
+extern "C" {
+int nodedbg_const_Environment__kContextEmbedderDataIndex__int;
+uintptr_t nodedbg_offset_ExternalString__data__uintptr_t;
+
+#define V(Class, Member, Type, Accessor)                                      \
+  NODE_EXTERN uintptr_t NODEDBG_OFFSET(Class, Member, Type);
+  NODE_OFFSET_POSTMORTEM_METADATA(V)
+#undef V
+}
+
+namespace node {
+
+int GenDebugSymbols() {
+  nodedbg_const_Environment__kContextEmbedderDataIndex__int =
+      Environment::kContextEmbedderDataIndex;
+
+  nodedbg_offset_ExternalString__data__uintptr_t = NODE_OFF_EXTSTR_DATA;
+
+  #define V(Class, Member, Type, Accessor)                                    \
+    NODEDBG_OFFSET(Class, Member, Type) = OffsetOf(&Accessor);
+    NODE_OFFSET_POSTMORTEM_METADATA(V)
+  #undef V
+
+  return 1;
+}
+
+int debug_symbols_generated = GenDebugSymbols();
+
+}  // namespace node
diff --git a/src/req_wrap.h b/src/req_wrap.h
index 83baf9d2a35..05bc558570a 100644
--- a/src/req_wrap.h
+++ b/src/req_wrap.h
@@ -27,9 +27,13 @@ class ReqWrap : public AsyncWrap {
  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_.
+  // ReqWrap. ContainerOf compiles down to simple, fixed pointer arithmetic. It
+  // is also used by src/node_postmortem_metadata.cc to calculate offsets and
+  // generate debug symbols for ReqWrap, which assumes that the position of
+  // members in memory are predictable. 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_. For more information please refer to
+  // `doc/guides/node-postmortem-support.md`
   T req_;
 };
 
diff --git a/src/util-inl.h b/src/util-inl.h
index 558a0ab2b42..c5a25c91ffb 100644
--- a/src/util-inl.h
+++ b/src/util-inl.h
@@ -142,12 +142,16 @@ typename ListHead<T, M>::Iterator ListHead<T, M>::end() const {
 }
 
 template <typename Inner, typename Outer>
+constexpr uintptr_t OffsetOf(Inner Outer::*field) {
+  return reinterpret_cast<uintptr_t>(&(static_cast<Outer*>(0)->*field));
+}
+
+template <typename Inner, typename Outer>
 ContainerOfHelper<Inner, Outer>::ContainerOfHelper(Inner Outer::*field,
                                                    Inner* pointer)
-    : pointer_(reinterpret_cast<Outer*>(
-          reinterpret_cast<uintptr_t>(pointer) -
-          reinterpret_cast<uintptr_t>(&(static_cast<Outer*>(0)->*field)))) {
-}
+    : pointer_(
+        reinterpret_cast<Outer*>(
+            reinterpret_cast<uintptr_t>(pointer) - OffsetOf(field))) {}
 
 template <typename Inner, typename Outer>
 template <typename TypeName>