topdown: Fix for empty documents generated by partial rules

This commit fixes an issue in partial eval whereby OPA would generate
an incorrect expression referring to a non-existent/undefined virtual
document. This would occur if an expression referenced the full extent
of a virtual document generated by partial rules and those partial
rules were all undefined. In that case, the reference needs to be
unified with the default (empty) value for that virtual document.

Fixes #2492

Signed-off-by: Torin Sandall <torinsandall@gmail.com>

topdown/eval.go

@@ -1976,27 +1976,39 @@ func (e evalVirtualPartial) partialEvalSupport(iter unifyIterator) error {
 
 	path, term := e.e.savePackagePathAndTerm(e.plugged[:e.pos+1], e.ref)
 
-	if !e.e.saveSupport.Exists(path) {
+	var defined bool
+
+	if e.e.saveSupport.Exists(path) {
+		defined = true
+	} else {
 		for i := range e.ir.Rules {
-			err := e.partialEvalSupportRule(iter, e.ir.Rules[i], path)
+			ok, err := e.partialEvalSupportRule(iter, e.ir.Rules[i], path)
 			if err != nil {
 				return err
+			} else if ok {
+				defined = true
 			}
 		}
 	}
 
+	if !defined {
+		term = e.empty
+	}
+
 	return e.e.saveUnify(term, e.rterm, e.bindings, e.rbindings, iter)
 }
 
-func (e evalVirtualPartial) partialEvalSupportRule(iter unifyIterator, rule *ast.Rule, path ast.Ref) error {
+func (e evalVirtualPartial) partialEvalSupportRule(iter unifyIterator, rule *ast.Rule, path ast.Ref) (bool, error) {
 
 	child := e.e.child(rule.Body)
 	child.traceEnter(rule)
 
 	e.e.saveStack.PushQuery(nil)
+	var defined bool
 
 	err := child.eval(func(child *eval) error {
 		child.traceExit(rule)
+		defined = true
 
 		current := e.e.saveStack.PopQuery()
 		plugged := current.Plug(e.e.caller.bindings)
@@ -2031,7 +2043,7 @@ func (e evalVirtualPartial) partialEvalSupportRule(iter unifyIterator, rule *ast
 		return nil
 	})
 	e.e.saveStack.PopQuery()
-	return err
+	return defined, err
 }
 
 func (e evalVirtualPartial) evalTerm(iter unifyIterator, term *ast.Term, termbindings *bindings) error {

topdown/topdown_partial_test.go

@@ -715,6 +715,16 @@ func TestTopDownPartialEval(t *testing.T) {
 				p[x2] = y2 { x2 = input.x; y2 = input.y }
 			`},
 		},
+		{
+			note:  "automatic shallow inlining: full extent: no solutions",
+			query: "data.test.p = x",
+			modules: []string{
+				`package test
+
+				p[1] { input = 1; false }`,
+			},
+			wantQueries: []string{`x = set()`},
+		},
 		{
 			note:  "automatic shallow inlining: full extent: iteration",
 			query: "data.test[x] = y",