jsonplan: Improve performance for deep objects

[alisdair]

When calculating the unknown values for JSON plan output, we would previously recursively call the unknownAsBool function on the current sub-tree twice, if any values were unknown. This was wasteful, but not noticeable for normal Terraform resource shapes.

However for deeper nested object values, such as Kubernetes manifests, this was a severe performance problem, causing terraform show -json to take several hours to render a plan.

This commit reuses the already calculated unknown value for the subtree, and adds benchmark coverage to demonstrate the improvement.

Fixes #30548.

Benchmark results

For these benchmarks, we construct a fairly narrow object value of variable depth, then compute its unknown value. The execution time difference for this change is:

Object Depth	Speedup
2	4x
3	7x
5	28x
7	116x
9	422x

Before:

$ go test -bench=. ./internal/command/jsonplan/
goos: darwin
goarch: amd64
pkg: github.com/hashicorp/terraform/internal/command/jsonplan
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
BenchmarkUnknownAsBool_2-16    	   10000	    101835 ns/op
BenchmarkUnknownAsBool_3-16    	    1947	    610288 ns/op
BenchmarkUnknownAsBool_5-16    	      48	  22955366 ns/op
BenchmarkUnknownAsBool_7-16    	       2	 836706698 ns/op
BenchmarkUnknownAsBool_9-16    	       1	27352356756 ns/op
PASS
ok  	github.com/hashicorp/terraform/internal/command/jsonplan	33.781s

After:

$ go test -bench=. ./internal/command/jsonplan/
goos: darwin
goarch: amd64
pkg: github.com/hashicorp/terraform/internal/command/jsonplan
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
BenchmarkUnknownAsBool_2-16    	   33733	     30257 ns/op
BenchmarkUnknownAsBool_3-16    	   13136	     90776 ns/op
BenchmarkUnknownAsBool_5-16    	    1458	    815009 ns/op
BenchmarkUnknownAsBool_7-16    	     162	   7209663 ns/op
BenchmarkUnknownAsBool_9-16    	      19	  64790729 ns/op
PASS
ok  	github.com/hashicorp/terraform/internal/command/jsonplan	9.800s

[apparentlymart]

Makes sense to me! Thanks for tracking this down.

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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