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>

1 files changed, 5 insertions, 0 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_;
 };