topdown: cache undefined rule evaluations (#5523)

With this change, `undefined` outcomes of complete rule evaluations
are now also cached. Previously, only defined results had been cached,
and empty partial sets/objects.

In the case of partial rules with string keys, the introduction of ref heads
changed how they had been evaluated: Before, they had been evaluated
as partial sets, and thus got cached when empty. After, they had been
evaluated as complete rules (with ref heads), and if they were undefined,
they had _not_ been cached. This caused a performance regression.

Fixes #593.

Signed-off-by: Edward Paget <edward.paget@chime.com>

util/test/benchmark.go

@@ -11,6 +11,73 @@ import (
 	"text/template"
 )
 
+// PartialObjectBenchmarkCrossModule returns a module with n "bench_test_" prefixed rules
+// that each refer to another "cond_bench_" prefixed rule
+func PartialObjectBenchmarkCrossModule(n int) []string {
+	fooMod := `package test.foo
+    import data.test.bar
+	import data.test.baz
+
+	output[key] := value {
+		value := bar[key]
+		startswith("bench_test_", key)
+	}`
+	barMod := "package test.bar\n"
+	barMod += `
+	cond_bench_0 {
+		contains(lower(input.test_input_0), lower("input_01"))
+	}
+	cond_bench_1 {
+		contains(lower(input.test_input_1), lower("input"))
+	}
+	cond_bench_2 {
+		contains(lower(input.test_input_2), lower("input_10"))
+	}
+    bench_test_out_result := load_tests(test_collector)
+
+    load_tests(in) := out {
+		out := in
+	}
+    `
+
+	bazMod := "package test.baz\nimport data.test.bar\n"
+	ruleBuilder := ""
+
+	for idx := 1; idx <= n; idx++ {
+		barMod += fmt.Sprintf(`
+		bench_test_%[1]d := result {
+            input.bench_test_collector_mambo_number_%[3]d
+			result := input.bench_test_collector_mambo_number_%[3]d
+        } else := result {
+			is_null(bench_test_out_result.mambo_number_%[3]d.error)
+			result := bench_test_out_result.mambo_number_%[3]d.result
+		}
+
+        test_collector["mambo_number_%[3]d"] := result {
+			cond_bench_%[2]d
+			not %[3]d == 2
+			not %[3]d == 3
+			not input.bench_test_collector_mambo_number_%[3]d
+			result := { "result": %[3]d, "error": null }
+		}
+		`, idx, idx%3, idx%5)
+		ruleBuilder += fmt.Sprintf("    bar.bench_test_%[1]d == %[1]d\n", idx)
+		if idx%10 == 0 {
+			bazMod += fmt.Sprintf(`rule_%d {
+				%s
+			}`, idx, ruleBuilder)
+			fooMod += fmt.Sprintf(`
+			final_decision = "allow" {
+				baz.rule_%d
+			}
+			`, idx)
+			ruleBuilder = ""
+		}
+	}
+
+	return []string{fooMod, barMod, bazMod}
+}
+
 // ArrayIterationBenchmarkModule returns a module that iterates an array
 // with `n` elements
 func ArrayIterationBenchmarkModule(n int) string {