WIP: implements aws_instances

[artburkart]

I'm not sure if it's acceptable to create an unfinished PR, but I hit a wall and am unsure what to do to resolve it. This PR is in response to the request for an "aws_instances" data source in Issue #12376. My approach was to make the returned data have a field called "instances", which would be a list of instances with the same format as the one returned in the "aws_instance" data source.

I wanted to be able to do something like this: https://gist.github.com/artburkart/16f88b5856861d164dae4b1aeefc0724

Because I wanted it to be similar to the "aws_route_table" data source which lets you do this: https://gist.github.com/artburkart/d5dd81302179493b2ab2c23fbfcb84b8

Everything seems to work. The multiple instances are getting queried and the data sources are getting populated, but for some reason, I can't get them to attach to the "aws_instances" ResourceData object. I have a feeling the problem is in the code here: https://github.com/artburkart/terraform/blob/e7f03452ffd58fc953c0915d77e5b8551e1f70f5/builtin/providers/aws/data_source_aws_instance.go#L315-L334

In this code I tried to follow the pattern laid out in data_source_aws_route_table, but something has gone awry. Whenever I perform a terraform apply with the configs I've provided in this PR, I get this:

data.aws_instances.boxes: Refreshing state...
data.aws_instances.boxes: Refreshing state...
Error applying plan:

1 error(s) occurred:

* data.template_file.boxes: Resource 'data.aws_instances.boxes' does not have attribute 'instances.0.subnet_id' for variable 'data.aws_instances.boxes.instances.0.subnet_id'

For some reason, data.aws_instances.boxes.instances does not exist, and I'm not exactly sure why not. I'm going to keep pecking at it, but if someone could offer some guidance, it would be much appreciated. Once I figure this bug out, I'll add all the tests and docs that generally accompany a new data source.

Cheers

[artburkart]

Well, I've noticed an indication of a bug. Some code is getting double-executed. I think, once I get to the bottom of that, I may have a working solution.

[cemo]

eager to see :) What is the state of this issue?

[artburkart]

I'm still debugging my code, will keep you posted.

[artburkart]

I've figured out my problem. Everything I had was more-or-less correct, except I didn't realize resources and data sources must have at least one invocation of d.SetId(). I didn't do this, and so every time it got to the state writing portion of execution, it would see that no id was set and throw a fit.

I should have a working copy with tests and docs soon.

[artburkart]

I'm closing this PR in favor of a different one I made. You can find it here:
#12856

[ghost]

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