Errors swallowed in synchronous render (2018)

[AlJohri]

hi @fdintino, just raising the priority of this bug since it seems fairly serious. in some conditions, when there is a template syntax error, the synchronous version of env.render simply returns null instead of throwing the error. there are several issues describing this such as:

• Errors in included templates are thrown asynchronously #678
• nunjucks do not throw error when syntax error or none existing filter #1116
• Errors seem to be swallowed. #1127

It appears the bug has something to do with async as the callback version of this function seems to pass down the error object.

@carljm requested a failing a test that showcases this bug which can be found here: #689 thanks to @nicolasartman

@oleics did some initial debugging back in 2016 #678 (comment) and found that there is a line var forceAsync = true; which could be contributing although this code has changed quite a bit since then.

This bug makes it very hard to write templates with nunjucks and it would be great to resolve this soon.

[fdintino]

Agreed this is serious. I got bit by this myself recently. I'll prioritize finding a fix.

[rdmurphy]

We're hitting this too, and it's led to a lot of baffling debugging sessions and burnt time.

[AlJohri]

Thanks @fdintino!