rego: Fix panic when partial evaluating w/ tracers

There was a typo from a while back that caused problems with this. It
would only occur in cases where the Rego object was created without
tracers, prepared, and then partially evaluated with tracers supplied
as part of the evaluation context. Any of the other code paths would
actually work as the original Rego object would still have them and
the (wrong) reference wouldn't cause a panic.

At some point more recently we changed `opa eval` to split up its
options for the Rego object and evaluation. Doing this caused the
problem to surface when doing anything with `opa eval -p --explain ..`

Fixes: #2007
Signed-off-by: Patrick East <east.patrick@gmail.com>

rego/rego.go

@@ -1713,7 +1713,7 @@ func (r *Rego) partial(ctx context.Context, ectx *EvalContext) (*PartialQueries,
 		WithIndexing(ectx.indexing)
 
 	for i := range ectx.tracers {
-		q = q.WithTracer(r.tracers[i])
+		q = q.WithTracer(ectx.tracers[i])
 	}
 
 	if ectx.parsedInput != nil {

rego/rego_test.go

@@ -953,6 +953,46 @@ func TestPartialResultWithInput(t *testing.T) {
 	assertEval(t, r2, "[[true]]")
 }
 
+func TestPreparedPartialResultWithTracer(t *testing.T) {
+	mod := `
+	package test
+	default p = false
+	p {
+		input.x = 1
+	}
+	`
+	r := New(
+		Query("data.test.p == true"),
+		Module("test.rego", mod),
+	)
+
+	tracer := topdown.NewBufferTracer()
+
+	ctx := context.Background()
+	pq, err := r.PrepareForPartial(ctx)
+	if err != nil {
+		t.Fatalf("unexpected error from Rego.PrepareForPartial(): %s", err.Error())
+	}
+
+	pqs, err := pq.Partial(ctx, EvalTracer(tracer))
+	if err != nil {
+		t.Fatalf("unexpected error from PreparedEvalQuery.Partial(): %s", err.Error())
+	}
+
+	expectedQuery := "input.x = 1"
+	if len(pqs.Queries) != 1 {
+		t.Errorf("expected 1 query but found %d: %+v", len(pqs.Queries), pqs)
+	}
+	if pqs.Queries[0].String() != expectedQuery {
+		t.Errorf("unexpected query in result, expected='%s' found='%s'",
+			expectedQuery, pqs.Queries[0].String())
+	}
+
+	if len(*tracer) == 0 {
+		t.Errorf("Expected buffer tracer to contain > 0 traces")
+	}
+}
+
 func TestMissingLocation(t *testing.T) {
 
 	// Create a query programmatically and evaluate it. The Location information