format package unexpectedly mutating AST nodes

[tsandall]

The format.Ast function may mutate AST nodes to un-mangle wildcard variables. The problem is that this mutation/un-mangling is not applied consistently across multiple calls to format.Ast if the the inputs to format.Ast contain pointers to the same term strurcts. This problem can be reproduced by running the new opa build command with optimization enabled and multiple entrypoints specified:

x.rego

package test

p = x {
	input.b = x
}

q = y {
	input.a = y
}

run the build:

opa build x.rego -O=1 -e test/p -e test/-q -o bundle.tar.gz

inspect the output:

torin:~$ tar xzvf bundle.tar.gz
tar: Removing leading '/' from member names
x data.json
x optimized/test.1.rego
x optimized/test.rego
x .manifest
torin:~$ cat optimized/test.rego
package test

p = __wildcard0__ {
	input.b = _
}
torin:~$ cat optimized/test.1.rego
package test

q = __wildcard0__ {
	input.a = __wildcard0__
}

We run into this with opa build because the compiler only generates a single result term for all entrypoint PE runs--e.g., the compiler outputs rules like this:

p = $result { input.b = $result }

and

q = $result { input.a = $result }

Under the hood, the $result variable in the rule heads point to the same memory. The $result variable in the bodies point to different memory due to rewriting that happens on build. When the format package runs on one of the modules the $result variables are un-mangled to __wildcard0__ consistently--however, when the $result in the head is un-mangled, the term is mutated in place--this causes the term in the other module to get updated. When the format package runs on the other module, it only sees one occurrence of $result so it doesn't do any unmangling.