Log.prototype[severity] functions cannot deal with circular references

[ofrobots]

Right now the PR adds a test that shows this. What are your thoughts on the fix? The simplest fix would be to fix Log.assignSeverityToEntries_ but am concerned that the problem may exist in other places too.

[stephenplusplus]

We chose to throw the error instead of hiding it and removing part of the user's object. My thoughts on that were explained in this issue. Please discuss in that issue if you have reasons that's a bad idea on our part.

1) Logging logs should write a single entry to a log:
     Error: The JSON data for this entry has a circular reference.

[ofrobots]

Either we should test with is-circular uniformly, or not test is-circular at all.. Elsewhere in the repo we use is-circular to test for circular objects and report a nice error.

A large stack overflow in extend is not as useful to users.

I think this is a distinct issue.

[stephenplusplus]

We throw a small error as I pasted above. Maybe I'm not seeing what you're seeing. Can you paste the output you're seeing?

[ofrobots]

I guess it depends on test ordering. I bypassed some of the tests because the system tests aren't working for me (#1982).

Try mocha system-test/*.js --no-timeouts --bail -g 'logs' with my test.

[stephenplusplus]

Okay, I reproduced. We definitely should check with is-circular the same was we do in Entry#toJSON.

[ofrobots]

Here's a better fix. @stephenplusplus PTAL.

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[stephenplusplus]

Cool, thanks!