Go serverless function always returns 400 status code after first 400 #1482

denk0403 · 2023-02-09T09:32:28Z

denk0403
Feb 9, 2023

Summary

I have a serverless function written in Go (GitHub Link) that converts well-formatted DOT syntax into an image. Bad syntax returns a 400 error.

Locally, using vercel dev, this function works perfectly. However, when I deploy my function to Vercel, it only works until there is a 400 Bad Request, and then every subsequent response returns a 400 Bad Request, even if the DOT syntax is changed to be well-formatted again. This shouldn't seem possible considering the function is stateless/has no side effects.

What do I need to do to make the function behave as it does locally? Any ideas as to what's causing this strange behavior?

Additional information

When looking at the response headers, x-vercel-cache is MISS, and x-vercel-id is something different each time. So I don't think the function is somehow returning duplicate response headers. Moreover, different syntax errors/error messages are reported correctly, so the response body doesn't seem to be duplicated either.

If you go to this minimally reproducible example, submitting the following well-formatted syntax:

graph test { a -- b }

should return the following image:
,
which it does initially.

On the other hand, the following bad syntax:

graph test { a -- }

should return the following error message:

syntax error in line 1 near '}'

And the following bad syntax:

graph test { a - b }

should return the following error message:

syntax error in line 1 near '-'

After trying one of the bad syntaxes, the good syntax no longer returns a 200 OK, but rather always returns the error message of the most recent error. Switching between the two bad syntaxes behaves correctly. Refreshing the web page in the example does not solve the problem. The function behaves incorrectly for around 5-10 minutes, after which I assume the server enters an idle state and can properly "restart".

Here is the GitHub code for the minimal example.

vercel.go.behavior2.mp4