deps: backport 56a0a79 from V8 upstream

[misterdjules]

Backport 56a0a797f210e04746f2888116365d29a4bb6afc from V8 upstream to
include post-mortem metadata used by mdb_v8 to support V8 4.6.

Original commit message:

Update post-mortem metadata generation

mdb_v8, a post-mortem debugger for Node.js, now uses JSArrayBuffer's
backing_store property and JSArrayBufferView's byte_offset property to
get access to the content of Buffer instances in node (which are
Uint8Array instances). This change adds post-mortem metadata for these
two properties.

This change also fixes a typo in
inobject_properties_of_constructor_function_index_offset that was added
to gen-postmortem-metadata in a previous change. It should be named
inobject_properties_or_constructor_function_index instead.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1363403003

Cr-Commit-Position: refs/heads/master@{#30926}

/cc @targos @ofrobots @nodejs/post-mortem

See related mdb_v8 change here: misterdjules/mdb_v8@853e337.

[ofrobots]

LGTM.

BTW, is there a way to somehow test the metadata file generated by gen-postmortem-metadata.py?

[targos]

LGTM

[jasnell]

LGTM

[misterdjules]

CI tests fail on a lot of platforms on test-stringbytes-external.js, even though the vee-eight-4.6 branch seems to have a change that landed to make that test faster.

Also, one of the FreeBSD Jenkins agents is down, and thus the Jenkins job cannot complete.

Landing anyway, as I can't imagine how this change would cause these regressions.

[misterdjules]

Landed in 7ffbfd8.

[misterdjules]

@ofrobots Regarding testing generated post-mortem data: that's a very good and important question.

Currently, this is done my mdb_v8's tests suite, because the only way to test if the post-mortem metadata is generated properly is to run post-mortem debugging tools on known programs and check that inspected data structures match what we expect.

We could run mdb_v8's tests suite as part of node's tests suite before a release, but:

• mdb_v8 was removed from node's code base with io.js, and hasn't been put back yet. I don't know if it will be put back in the near future, because I'm not sure yet if there's a benefit in doing that.
• mdb_v8 only works on SmartOS, because it's a "plug-in" for mdb (what we call dmods). So that restricts the platforms on which we would be able to test the generated post-mortem metadata.

There's also the question of whether these tests should be ran by node's tests suite, or V8's tests suite. Ultimately, this post-mortem debugging data is generated by V8 and would be usable by any V8 embedder. That being said, mdb_v8 for instance has debugging commands that are specific to node, like ::nodebuffer, so it's a grey area.

Finally, @davepacheco started some work to allow parts of mdb_v8 that are not platform specific to be abstracted out in a reusable library. We could imagine building a small multi-platform binary that uses this library to run post-mortem debugging tests.

This is only me thinking out loud though, and I believe this would be a great topic to discuss with @nodejs/post-mortem, maybe during our next meeting?

Anyway, thank you for bringing that up! 👍