src: dispose of V8 platform in process.exit()

[addaleax]

Calling process.exit() calls the C exit() function, which in turn
calls the destructors of static C++ objects. This can lead to race
conditions with other concurrently executing threads; disposing of the
V8 platform instance helps with this (although it might not be a full
solution for all problems of this kind).

Fixes: #24403

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[nodejs-github-bot]

@addaleax build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/1824/pipeline

[addaleax]

Beginning to wonder though … does this interfere with Workers? They might be attempting to access the platform while we are shutting it down here, right? :/

[bnoordhuis]

Isn't the proper solution to join all threads first?

[Trott]

If I'm not mistaken, it looks like this doesn't fix test-cli-syntax. https://ci.nodejs.org/job/node-test-commit-linux/23744/nodes=centos7-64-gcc6/testReport/ is a CI run for this PR.

assert.js:86
  throw new AssertionError(obj);
  ^

AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:

null !== 1

    at common.mustCall (/home/iojs/build/workspace/node-test-commit-linux/nodes/centos7-64-gcc6/test/parallel/test-cli-syntax.js:97:14)
    at /home/iojs/build/workspace/node-test-commit-linux/nodes/centos7-64-gcc6/test/common/index.js:346:15
    at ChildProcess.exithandler (child_process.js:301:5)
    at ChildProcess.emit (events.js:189:13)
    at maybeClose (internal/child_process.js:977:16)
    at Socket.stream.socket.on (internal/child_process.js:395:11)
    at Socket.emit (events.js:189:13)
    at Pipe._handle.close (net.js:612:12)

[addaleax]

Yeah, I’m closing this, will have to look into it a bit more…