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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>
73 lines
2.5 KiB
Markdown
73 lines
2.5 KiB
Markdown
# Postmortem Support
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Postmortem metadata are constants present in the final build which can be used
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by debuggers and other tools to navigate through internal structures of software
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when analyzing its memory (either on a running process or a core dump). Node
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provides this metadata in its builds for V8 and Node internal structures.
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### V8 Postmortem metadata
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V8 prefixes all postmortem constants with `v8dbg_`, and they allow inspection of
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objects on the heap as well as object properties and references. V8 generates
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those symbols with a script (`deps/v8/tools/gen-postmortem-metadata.py`), and
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Node always includes these constants in the final build.
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### Node Debug Symbols
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Node prefixes all postmortem constants with `nodedbg_`, and they complement V8
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constants by providing ways to inspect Node-specific structures, like
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`node::Environment`, `node::BaseObject` and its descendants, classes from
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`src/utils.h` and others. Those constants are declared in
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`src/node_postmortem_metadata.cc`, and most of them are calculated at compile
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time.
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#### Calculating offset of class members
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Node constants referring to the offset of class members in memory are calculated
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at compile time. Because of that, those class members must be at a fixed offset
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from the start of the class. That's not a problem in most cases, but it also
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means that those members should always come after any templated member on the
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class definition.
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For example, if we want to add a constant with the offset for
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`ReqWrap::req_wrap_queue_`, it should be defined after `ReqWrap::req_`, because
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`sizeof(req_)` depends on the type of T, which means the class definition should
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be like this:
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```c++
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template <typename T>
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class ReqWrap : public AsyncWrap {
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private:
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// req_wrap_queue_ comes before any templated member, which places it in a
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// fixed offset from the start of the class
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ListNode<ReqWrap> req_wrap_queue_;
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T req_;
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};
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```
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instead of:
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```c++
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template <typename T>
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class ReqWrap : public AsyncWrap {
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private:
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T req_;
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// req_wrap_queue_ comes after a templated member, which means it won't be in
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// a fixed offset from the start of the class
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ListNode<ReqWrap> req_wrap_queue_;
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};
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```
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There are also tests on `test/cctest/test_node_postmortem_metadata.cc` to make
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sure all Node postmortem metadata are calculated correctly.
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## Tools and References
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* [llnode](https://github.com/nodejs/llnode): LLDB plugin
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* [`mdb_v8`](https://github.com/joyent/mdb_v8): mdb plugin
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* [nodejs/post-mortem](https://github.com/nodejs/post-mortem): Node.js
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post-mortem working group
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