Panic with setunion() on variable input

[bondsbw]

Terraform Version

Terraform v0.13.0-beta2

Terraform Configuration Files

variable "x" {
  default = []
}

locals {
  a = setunion(var.x, [])
}

Debug Output

https://gist.github.com/bondsbw/8526cd1ed6ff1df0f4aadc2532b45bf2

Expected Behavior

No error.

Actual Behavior

Error: Error in function call

  on main.tf line 6, in locals:
   6:   a = setunion(var.x, [])

Call to function "setunion" failed: panic in function implementation: value is
not known
goroutine 84 [running]:
runtime/debug.Stack(0xc000223b08, 0x225ee00, 0x2c7ff20)
        /usr/local/go/src/runtime/debug/stack.go:24 +0xa4
github.com/zclconf/go-cty/cty/function.errorForPanic(...)
        /home/circleci/project/project/vendor/github.com/zclconf/go-cty/cty/function/error.go:44

...

Steps to Reproduce

1. terraform init
2. terraform plan

Additional Context

Running on Windows 10 version 10.0.17763.0.

[apparentlymart]

Thanks for reporting this, @bondsbw!

I've both reproduced this on Terraform 0.13.0 and seen that it's a regression compared to Terraform 0.12, where this doesn't panic.

I think this is caused by the upstream change zclconf/go-cty#52, which addressed some other misbehavior in these set functions but has introduced this bug because it's calling LengthInt on a value without checking if it's known first. Unknown collections don't have a known length, so it'll need to do something special in that case.

---

In case it's useful in some way, here's my version of the stack trace from my local reproduction on a build from current Terraform v0.13 (the master branch) at the time I'm posting this:

Error: Error in function call

  on setunion-panic.tf line 6, in locals:
   6:   a = setunion(var.x, [])

Call to function "setunion" failed: panic in function implementation: value is
not known
goroutine 56 [running]:
runtime/debug.Stack(0xc000361b08, 0x2210780, 0x2c36b80)
	/home/user/.goenv/versions/1.14.2/src/runtime/debug/stack.go:24 +0x9d
github.com/zclconf/go-cty/cty/function.errorForPanic(...)
	/home/user/dev/terraform/terraform/vendor/github.com/zclconf/go-cty/cty/function/error.go:44
github.com/zclconf/go-cty/cty/function.Function.ReturnTypeForValues.func1(0xc000361fb8,
0xc000361fc8)
	/home/user/dev/terraform/terraform/vendor/github.com/zclconf/go-cty/cty/function/function.go:217
+0x78
panic(0x2210780, 0x2c36b80)
	/home/user/.goenv/versions/1.14.2/src/runtime/panic.go:969 +0x166
github.com/zclconf/go-cty/cty.Value.LengthInt(0x2ce0340, 0xc000081fe0,
0x22cdc20, 0x4115aa0, 0xc00033e180)
	/home/user/dev/terraform/terraform/vendor/github.com/zclconf/go-cty/cty/value_ops.go:994
+0x285
github.com/zclconf/go-cty/cty/function/stdlib.setOperationReturnType(0xc00034a440,
0x2, 0x2, 0xc000081ff0, 0xc000361df0, 0x0, 0x0)
	/home/user/dev/terraform/terraform/vendor/github.com/zclconf/go-cty/cty/function/stdlib/set.go:170
+0x109
github.com/zclconf/go-cty/cty/function.Function.ReturnTypeForValues(0xc0001927e0,
0xc00034a440, 0x2, 0x2, 0x0, 0x0, 0x0, 0x0)
	/home/user/dev/terraform/terraform/vendor/github.com/zclconf/go-cty/cty/function/function.go:221
+0x433
github.com/zclconf/go-cty/cty/function.Function.Call(0xc0001927e0,
0xc00034a440, 0x2, 0x2, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/home/user/dev/terraform/terraform/vendor/github.com/zclconf/go-cty/cty/function/function.go:228
+0xb5
github.com/hashicorp/hcl/v2/hclsyntax.(*FunctionCallExpr).Value(0xc000312000,
0xc000346b60, 0x0, 0xc000363700, 0x1, 0x1, 0x0, 0x0, 0x0)
	/home/user/dev/terraform/terraform/vendor/github.com/hashicorp/hcl/v2/hclsyntax/expression.go:412
+0x109d
github.com/hashicorp/terraform/lang.(*Scope).EvalExpr(0xc0001ec870, 0x2cdf040,
0xc000312000, 0x2ce0240, 0x4115ee8, 0x0, 0x0, 0xc0003638d0, 0x1, 0x1, ...)
	/home/user/dev/terraform/terraform/lang/eval.go:93 +0x1b3
github.com/hashicorp/terraform/terraform.(*BuiltinEvalContext).EvaluateExpr(0xc000350340,
0x2cdf040, 0xc000312000, 0x2ce0240, 0x4115ee8, 0x0, 0x0, 0x0, 0x2,
0xc0001b2a00, ...)
	/home/user/dev/terraform/terraform/terraform/eval_context_builtin.go:286
+0x1e7
github.com/hashicorp/terraform/terraform.(*EvalLocal).Eval(0xc000346540,
0x2d21b60, 0xc000350340, 0x1, 0x1, 0xc00032dc00, 0x500000000000000)
	/home/user/dev/terraform/terraform/terraform/eval_local.go:43 +0x6d7
github.com/hashicorp/terraform/terraform.EvalRaw(0x2c77d00, 0xc000346540,
0x2d21b60, 0xc000350340, 0x22b08c0, 0x3fa58e5, 0x2210780, 0xc00034c260)
	/home/user/dev/terraform/terraform/terraform/eval.go:49 +0xc7
github.com/hashicorp/terraform/terraform.Eval(0x2c77d00, 0xc000346540,
0x2d21b60, 0xc000350340, 0xc000346540, 0x2c77d00, 0xc000346540, 0x0)
	/home/user/dev/terraform/terraform/terraform/eval.go:35 +0x4d
github.com/hashicorp/terraform/terraform.(*Graph).walk.func1(0x24f69e0,
0xc00033d170, 0x0, 0x0, 0x0)
	/home/user/dev/terraform/terraform/terraform/graph.go:73 +0xc77
github.com/hashicorp/terraform/dag.(*Walker).walkVertex(0xc00035a660,
0x24f69e0, 0xc00033d170, 0xc00034a380)
	/home/user/dev/terraform/terraform/dag/walk.go:387 +0x357
created by github.com/hashicorp/terraform/dag.(*Walker).Update
	/home/user/dev/terraform/terraform/dag/walk.go:309 +0x11d7
.

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