opa build generated WASM binary always returns empty set

[patrick-east]

Expected Behavior

When using the new OPA build command with an example policy like:

package example

default hello = false

hello {
    x := input.message
    x == data.world
}

And then building with a command like:

 build -t wasm -e example/hello ./example.rego

If I load and evaluate it, dumping all the results, I would expect that regardless of data or input it should at least return some value for the default rule, eg:

[
    { "x": false }
]

with older OPA versions where the query was something like x := data.example.hello

Actual Behavior

The result is consistently

[
  {}
]

Steps to Reproduce the Problem

Using the example over at https://github.com/open-policy-agent/npm-opa-wasm/tree/master/examples/nodejs-app

Building with:

opa build -t wasm -e 'example/hello' ./example.rego
tar -xzf ./bundle.tar.gz /policy.wasm

Then run with:

node app.js '{"message": "world"}'

This just dumps the resulting JSON document without any modifications.

Additional Info

You can see in the dumped plan output (not available with the build command? had to modify the source to dump to stdout) there is no "insert" for the result, and the $result variable is never defined in the static section with the other ones:

*ir.Policy Policy
| *ir.Static Static (2 strings)
| | *ir.StringConst &{Value:message}
| | *ir.StringConst &{Value:world}
| *ir.Plan Plan (1 blocks)
| | *ir.Block Block (4 statements)
| | | *ir.CallStmt &{Func:g0.data.example.hello Args:[0 1] Result:2}
| | | *ir.AssignVarStmt &{Source:2 Target:3}
| | | *ir.MakeObjectStmt &{Target:4}
| | | *ir.ResultSetAdd &{Value:4}
| *ir.Funcs Funcs (1 funcs)
| | *ir.Func g0.data.example.hello (2 params: [0 1], 3 blocks)
| | | *ir.Block Block (7 statements)

<snip>

This differs from the older compile if you use an "x" var, you get:

*ir.Policy Policy
| *ir.Static Static (3 strings)
| | *ir.StringConst &{Value:x}
| | *ir.StringConst &{Value:message}
| | *ir.StringConst &{Value:world}
| *ir.Plan Plan (2 blocks)
| | *ir.Block Block (1 statements)
| | | *ir.MakeStringStmt &{Index:0 Target:2}
| | *ir.Block Block (5 statements)
| | | *ir.CallStmt &{Func:g0.data.example.hello Args:[0 1] Result:3}
| | | *ir.AssignVarStmt &{Source:3 Target:4}
| | | *ir.MakeObjectStmt &{Target:5}
| | | *ir.ObjectInsertStmt &{Key:2 Value:4 Object:5}
| | | *ir.ResultSetAdd &{Value:5}
| *ir.Funcs Funcs (1 funcs)
| | *ir.Func g0.data.example.hello (2 params: [0 1], 3 blocks)
| | | *ir.Block Block (7 statements)

<snip>

(the snipped parts are identical)

Still need to debug more but it seems like maybe something being mishandled with the wildcard. If i edit the build flow to use __result__ rather than $result The plan looks more correct.

Update: So yea, we explicitly exclude wildcard vars that were in the query:

opa/internal/planner/planner.go

Line 403 in b00f6aa

if !qv.IsGenerated() && !qv.IsWildcard() {

and

opa/internal/planner/planner.go

Line 437 in b00f6aa

if !rw.IsGenerated() && !rw.IsWildcard() {