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, 7 insertions, 3 deletions

diff --git a/src/req_wrap.h b/src/req_wrap.h
index 83baf9d2a3..05bc558570 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_;
 };