http.send caching doesn't work in system.authz policies

[anderseknert]

Expected Behavior

http.send with force_cache: true to cache responses when used in a system.authz policy.

Actual Behavior

No caching seems to be done.

Steps to Reproduce the Problem

1. Set up a server that responds with a static JSON response, like {"status": "OK"} and logs each request.
2. Create a system.authz policy.

package system.authz

default allow = false

allow {
    http.send({
        "url": "http://localhost:8000",
        "method": "GET",
        "force_cache": true,
        "force_cache_duration_seconds": 3600
    }).body.status == "OK"
}

3. Start OPA with authz policy configured opa run --server --authorization=basic --authentication=token
4. Query OPA a couple of times and see how the HTTP server set up in step 1 logs one request per policy invocation (i,.e. no cache).

If the policy is renamed and used as a normal policy, http.send caching works as expected.

Originally reported on Slack by @amrap030

[anderseknert]

Taking a look at the code, comparing how the authz query is executed:

opa/server/authorizer/authorizer.go

Lines 73 to 79 in c23bf8d

	rego := rego.New(
	rego.Query(h.decision().String()),
	rego.Compiler(h.compiler()),
	rego.Store(h.store),
	rego.Input(input),
	rego.Runtime(h.runtime),
	)

To that in the server:

opa/server/server.go

Lines 718 to 732 in c23bf8d

	opts := []func(*rego.Rego){
	rego.Store(s.store),
	rego.Transaction(txn),
	rego.Compiler(s.getCompiler()),
	rego.ParsedQuery(parsedQuery),
	rego.ParsedInput(input),
	rego.Metrics(m),
	rego.Instrument(includeInstrumentation),
	rego.QueryTracer(buf),
	rego.Runtime(s.runtime),
	rego.UnsafeBuiltins(unsafeBuiltinsMap),
	rego.InterQueryBuiltinCache(s.interQueryBuiltinCache),
	rego.PrintHook(s.manager.PrintHook()),
	rego.EnablePrintStatements(s.manager.EnablePrintStatements()),
	}

We might want to consider adding also adding some of the other stuff, like handling print consistently. Not sure about the other options, like unsafe built-ins, but I'm sure others have ideas.