Seemingly random incorrect evaluation of the can() function

[durnik-ivo]

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Overview of the Issue

I am afraid I am bringing the worst type of bug. I am getting different result for calling can() function between different executions of Packer. I was not able to determine what makes the difference between correct and faulty evaluation. Sometimes it is mostly right, sometimes 50:50 and for some of my templates it is always wrong. It sometimes seems that the more complex the template is the more often I get the wrong result.

I am noticing something similar with try() function. However, its result does not seem to change with different executions but with different arguments provided, so I might create a separate bug report for that later on.

Reproduction Steps

Repeatedly run provided Packer template with packer console . and print local object.

Packer version

From 1.11.0 and 1.9.4

Simplified Packer Template

locals {
  test_data = [
    { key = "value" }
  ]

  test_local = can(local.test_data) ? local.test_data : []
}

Operating system and Environment details

Tested in two environments for two different versions of Packer:

• Apple MacBook M3 Pro with MacOS 14.5 Sonoma (1.11.0)
• Ubuntu 20.04 on AWS EC2 instance (1.9.4)

Log Fragments and crash.log files

$ packer console .
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
> exit
$ packer console . # In this case, for the second try, can() is evaluated to false and thus [] is assigned. 
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = []
}
> can(local.test_data) # Individual can() executed in the console evaluates correctly
true

[gitbisect]

My trials on intel Mac. Packer version 1.8.5

➜  tmp ls
j.pkr.hcl
➜  tmp cat j.pkr.hcl
locals {
  test_data = [
    { key = "value" }
  ]

  test_local = can(local.test_data) ? local.test_data : []
}
➜  tmp packer console .
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
>
>
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
>
>
> exit
➜  tmp packer console .
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
>

> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
>

> ^C
➜  tmp packer console .
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
> ^C
➜  tmp packer console .
> can(local.test_data)
> true
>
>
> can(local.test_data)
> true
> ^C
➜  tmp packer console .
> can(local.test_data)
> true
> ^C
➜  tmp packer console .
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
>
>
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
>
>
> ^C
➜  tmp packer console .
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
>
>
> exit
➜  tmp packer console .
> can(local.test_data)
> true
> exit
➜  tmp packer console .
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
>
>
> can(local.test_data)
> true
>
>
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
> exit
➜  tmp packer console .
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
>
>
> exit
➜  tmp packer console .
> can(local.test_data)
> true
>
>
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
>
>
> ^C
➜  tmp packer console .
> local
> {
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
> ^C
➜  tmp

[durnik-ivo]

I have tried 1.8.5 amd64 and arm64 binaries on Mac M3 and it shows the same error. The same on Ubuntu 22.04, it just took a bit more tries (I recommend trying with echo local | packer console .).

[gitbisect]

False once. True twice.
(Heads up to avoid confusion... in the snippet below ➜ tmp is the prompt on my Mac.)

➜  tmp echo local | packer console .
{
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = []
}
➜  tmp echo local | packer console .
{
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}
➜  tmp echo local | packer console .
{
  "test_data" = [
    {
      "key" = "value"
    },
  ]
  "test_local" = [
    {
      "key" = "value"
    },
  ]
}

[lbajolet-hashicorp]

Interesting, I too am able to replicate this behaviour.

Are you experiencing this only with packer console, or is it a generalised issue?
I'll see if I can debug this, thanks for reporting it with a minimal test case, appreciate it!

[durnik-ivo]

Thanks!

I observe this also in regular builds, but I was not able to prepare a simple test case, which would run as is, so I went down with packer console.

I have finally tried to prepare the test case for try() and it seems to do something similar. I guess these might be related, so probably not worth creating a separate issue, but let me know.

Code:

locals {
  test_data = [
    { key = "value" }
  ]

  test1 = try(local.test_data)
  test2 = try(local.test_data, false)
}

Output:

$ echo local | packer console .
{
  "test1" = [
    {
      "key" = "value"
    },
  ]
  "test2" = [
    {
      "key" = "value"
    },
  ]
  "test_data" = [
    {
      "key" = "value"
    },
  ]
}
$ echo local | packer console .
{
  "test1" = [
    {
      "key" = "value"
    },
  ]
  "test2" = false
  "test_data" = [
    {
      "key" = "value"
    },
  ]
}

[lbajolet-hashicorp]

Yeah it's likely the same problem.

Looking at the code it seems to be an order of evaluation problem, not sure if the order in which the variables are defined an aggravating factor (I suspect it is by experience with a similar issue in the past), but definitely we should be more resilient to this, and we should build a solid order of evaluation rather than the optimistic one we've adopted here.

I'll see to open a PR soon for this.

[github-actions]

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