node: call set_trace_sync_io before bootstrap

[joshgav]

Pull Request check-list

• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with
  this change (including linting)?
• Is the commit message formatted according to [CONTRIBUTING.md][0]?
• If this change fixes a bug (or a performance problem), is a regression
  test (or a benchmark) included?

Affected core subsystem(s)

node --trace-sync-io

Description of change

env->set_trace_sync_io was previously being called after node::LoadEnvironment so it wasn't in effect for the initial script.

[joshgav]

Save the following in a script and call with node --trace-sync-io script.js. Without this fix no traces are output, with fix they are. This will only effect the first turn, prior to any uv_run's, which may be why it doesn't seem to effect node -e.

var fs = require('fs')
console.log(fs.readFileSync(__filename, {encoding:'UTF8'}))

I'll add a test.

/cc @cjihrig

[cjihrig]

@trevnorris --trace-sync-io is supposed to allow sync calls on the first tick, right? I thought it was, but wanted to double check. The reasoning being that the first tick is usually application initialization.

[evanlucas]

Yea, I think that was by design.

[Fishrock123]

@cjihrig correct.

[joshgav]

Okay, I'll add a comment to that effect then.

[joshgav]

On second thought, @trevnorris could you explain why we skip the trace on the initial tick? I can add a comment and check the docs, would be nice to have an explanation.

[trevnorris]

First, yes, it is supposed to run after bootstrap. From node -h:

  --trace-sync-io       show stack trace when use of sync IO
                        is detected after the first tick

Reasons for this:

1. require() is a sync I/O operation. So we would have to special case this in order to not print a warning for every use of require().
2. It is common practice to load resources synchronously during bootstrap so that they are available when/if other modules require them.

Have you tried running ./node --trace-sync-io script.js where the script is an empty file? You'll see that there's a lot of output.

[Fishrock123]

Closing since this does not appear to be an issue, please let me know if this wasn't correct. :)