Mocha: Node stack traces truncated

[jamietre]

I think this is a Mocha problem but it's hard to say, since I don't have anything to compare to, though it could be a general environmental problem. We are getting out of memory errors on a large test suite. I've researched this a lot, and it seems it could be one of several things, but at this point it's very difficult to troubleshoot because I can't get a full stack trace.

[email protected], [email protected]

<--- Last few GCs --->

   82518 ms: Mark-sweep 807.1 (1039.7) -> 802.3 (1038.7) MB, 149.2 / 0.0 ms [allocation failure] [GC in old space requested].
   82668 ms: Mark-sweep 802.3 (1038.7) -> 802.3 (1036.7) MB, 150.6 / 0.0 ms [allocation failure] [GC in old space requested].
   82838 ms: Mark-sweep 802.3 (1036.7) -> 802.2 (993.7) MB, 169.7 / 0.0 ms [last resort gc].
   82989 ms: Mark-sweep 802.2 (993.7) -> 802.2 (982.7) MB, 150.6 / 0.0 ms [last resort gc].


<--- JS stacktrace --->

==== JS stack trace =========================================

Security context: 0000024EE58CFB61 <JS Object>
    1: SparseJoinWithSeparatorJS(aka SparseJoinWithSeparatorJS) [native array.js:~75] [pc=000002B8298FC057] (this=0000024EE5804381 <undefined>,w=0000011715C4D061 <JS Array[7440]>,F=000003681BBC8B19 <JS Array[7440]>,x=7440,I=0000024EE58B46F1 <JS Function ConvertToString (SharedFunctionInfo 0000024EE5852DC9)>,J=000003681BBC8AD9 <String[4]\: ,\n  >)
    2: DoJoin(aka DoJoin) [native array.js:137...

FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory

As you can see it's truncating it. I've tried everything I can think of, but none of the node command line options have any effect, e.g.

node --stack_trace_limit=1000 --max_stack_trace_source_length=1000 ./node_modules/mocha/bin/_mocha ./test --full-trace

I looked at all the v8 options with stack here:

node --v8-options | grep -B0 -A1 stack

.. and didn't see any others that might be related. Thoughts?

[jamietre]

Followup: this is not a mocha bug. This is a babel bug -- or at a minimum an area that really could use some improvement in Babel.

One thing I didn't mention here is my mocha.opts which includes this: --compilers js:babel-core/register -- which is the source of the problem.

After some painful debugging I found the entry point of the v8 crash in babel-register/lib/cache.js ... v8 blows up when trying to serialize a 200 megabyte JSON object in the save() method. Why do I have a 200 megabyte JSON object? Well...

'babel-register' apparently caches data about files it compiles by default, and saves them to a file. From looking at cache.js, it also appears to never worry about how big the cache gets and never expires anything. It basically just reads a JSO from a file when it starts, adds stuff to it, and writes it when it finishes.

My .babel.json was almost 200 megabytes. I blew it away and everything started working again. You can disable the cache entirely, see babel-register doc, though wiping it once in a while seems to be the best for performance. I have never touched this in two years, so I assume it was full of all kinds of nonsense. My tests ran about 10 times faster after I blew the cache away (on the 2nd run anyway, after the cache was rebuilt with my current state).

I still don't know why the stack traces get truncated.

Bug reportl: https://github.com/babel/babel/issues/5667

[dasilvacontin]

Thanks for sharing your findings! :)

[openjr]

Was happening on our CI server and failing all the builds, thanks for sharing this