How to debug Terraform external providers with concurrency issues?

I use Terraform 0.14.7 to deploy my infrastructure. Since I use AWS Lambda as a deploy target of my Whook API, I have a lot of scripts (7 different but one that is ran for all lambdas ~200) to retrieve the various informations about the API and each lambdas (see this example : https://github.com/nfroidure/whook/pull/108/files#diff-47625134d02e23a98ccff7918d11baa19c2ac409f4c90d76520b031b613b555cR74-R78).

To do so, I use the external provider that expects some JSON in return. My various commands indeed return JSON but the fact is that my terraform plan -out=terraform.plan failes with a message saying: "Error: command "env" produced invalid JSON: unexpected end of JSON input".

What I tried so far :

• run TF_LOG=TRACE terraform plan -out=terraform.plan 2>&1: the plan is executed correctly. What I guess of that unexpected success is that in someway, there is a concurrency issue on some resource (file descriptor, memory ?) since the back-pressure of the fact to write heavy trace logs may reduce concurrency. That said, I'm not sure, maybe that the trace/debug mode reduce concurrency by itself?
• run TF_LOG=TRACE terraform plan -out=terraform.plan 2>&1 | grep terraform-provider-external > out.txt: it reproduces the error, but, nothing abnormal into logs... not even the above error which seems legit. Maybe you have a grep in mind that would catch more infos while reducing logs volumetry to avoid the above mentioned back-pressure
• create aliases for env to at least know which command was failing, but terraform seems to dislike aliases
• log the JSON output of the command via a debug log service in the injector but everything ooks right.

I am currently in the process of creating external scripts using the tee command to have the output at the bash level but I suspect that it will take more file descriptors so not sure it ise the good way to go.

Except that, I really have no clue of how I could get rid of that problem, so I try here before creating an issue to get more verbose output for the external provider in Terraform.

Thanks for your help :)

Update: I managed to find out the bad JSON by doing so:

data "external" "lambdas" {
  program     = [
    "bash", "-ec",
    <<-EOF
    echo \"env NODE_ENV=${terraform.workspace} npx whook terraformValues --type='lambdas'\" >> lambdas-stderr.log
    env NODE_ENV=${terraform.workspace} npx whook terraformValues --type='lambdas' 2>> lambdas-stderr.log | tee lambdas-stdout.log
    EOF
  ]
  working_dir = ".."
}

It basically write stderr in a file and stdout in another while still proxying stdout to the command output.

With it I could iterate over all saved files and find out that the bad JSON were a JSON truncated at 65536 bytes (2^16) so I think there is some limitation somewhere. Will keep you up to date if I manage to find out what is limiting the JSON size.

To reproduce the problem everytime, I had to turn the parallelism option to 100. It think it could have a link to the pipe buffer size (see https://unix.stackexchange.com/questions/11946/how-big-is-the-pipe-buffer) and that in some conditions, the output read blocks and no backpressure mecanism takes this in count.