Support importing partial JSON files

[loganek]

Partial files are allowed in many specs, e.g. Trace Event Format,
so the viewer should be able to load partial files as well.

[coveralls]

Coverage increased (+0.1%) to 46.754% when pulling 325f0fa on loganek:master into ddc6130 on jlfwong:master.

[loganek]

Hey @jlfwong , could you please review that change? Thanks!

[jlfwong]

@loganek Hi! My home internet has been down for the last week, but I'll try to get to this soon!

I'm generally wary of adding third party dependencies, but I took a look at partial-json-parser, and it looks like it has no dependencies and is enough code that I'd rather not just do it myself, so I'm happy to accept this.

I'll try to take a closer look later this week, merge if I don't find anything concerning, then do a release that picks this up.

Out of curiousity, were you motivated to do this because you have Chrome trace profiles that are not terminated? What was generating them?

[loganek]

@jlfwong So sorry, I didn't want to rush you at all! Just thought that you missed the PR, so wanted to ping you, but please take your time, it's not super urgent for me.

Yeah, I wasn't sure what's your opinion about third party dependencies, but since it wasn't very time consuming, I decided to give a try.

Yes, I did this PR exactly because my files are not terminated; those files are generated by hawktracer-converter (https://github.com/loganek/hawktracer-converter/) which is a client app of my profiler (https://github.com/amzn/hawktracer). I guess I could work-around this in the converter to close the file properly, but since the format specification allow to have non-terminated files, I thought it'd be more correct to fix your viewer instead.

Btw. great tool, it feels much faster than the one embedded to chrome, and flamegraphs is a nice feature too. Good job, and thanks for doing that :)

[jlfwong]

This is great as is :) Thanks for taking the time to carefully craft this PR!

[jlfwong]

@loganek Ah... sorry I spoke too soon. I had to revert the commit because partial-json-parser doesn't guarantee termination: gov-ind/partial-json-parser#3.

The repository now depends upon babel too as a non-dev dependency, so at this point I think I would seriously consider writing my own partial JSON parser to avoid taking on too many additional transitive dependencies.

The specific cases of partial json files supported by the Trace Event Format is also a small subset of what that module tries to support. For example, it doesn't support unterminated strings. In fact, as far as I can tell, the only allowance it specifies is this:

When provided as a string to the importer the ] at the end of the JSON Array Format is optional. The Trace Viewer importer will automatically add a ] if needed to turn the string into valid JSON.. This is to support tracing systems that can not cleanly finish writing the trace. For example, when tracing the exit of a program.

(From https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/preview#).

So I think retrying import after appending a ] would be pretty straightforward.