Fix issue where sync render would not raise errors in included templates

[fdintino]

Summary

The fix introduced in PR #1049, preventing multiple calls to the render callback, was a bit too aggressive and prevented some template errors from being surfaced when render or renderString were called synchronously.

The real bug here lies in the compiler, which should not be calling the same callback function more than once. But the way that waterfall tasks are chained makes it difficult to properly halt execution in a template. We ought to fix the underlying issue, but for now this should (hopefully) ensure that thrown errors always surface when rendering synchronously, while not causing a regression for #1029.

Closes #1272 . I suspect this also fixes #1246, but I can't confirm because the reporter of that issue hasn't provided a test case I can use.

Checklist

• Proposed change helps towards purpose of this project.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).
• Documentation is added / updated to describe proposed change. (N/A)

[codecov-commenter]

Codecov Report

Merging #1301 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1301      +/-   ##
==========================================
- Coverage   89.61%   89.60%   -0.01%     
==========================================
  Files          22       22              
  Lines        3043     3041       -2     
==========================================
- Hits         2727     2725       -2     
  Misses        316      316              

Impacted Files	Coverage Δ
nunjucks/src/environment.js	85.82% <100.00%> (-0.11%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 63c4baf...bbcbaf3. Read the comment docs.

[fdintino]

The second call to the root render callback in issue ##1029 happened here in compileRoot:

nunjucks/nunjucks/src/compiler.js

Lines 1129 to 1133 in 63c4baf

	this._emitLine('if(parentTemplate) {');
	this._emitLine('parentTemplate.rootRenderFunc(env, context, frame, runtime, cb);');
	this._emitLine('} else {');
	this._emitLine(`cb(null, ${this.buffer});`);
	this._emitLine('}');

If an error occurs inside an included template, the error will be passed up the callback chain, and the user's callback will get called with the correct error argument. But it won't cause a return from the root compiled template function, so it will still call cb(null, this.buffer) at the very end. this.buffer is always a string, even if empty, so by confirming that res is not undefined we can check specifically for the situation described above.