server: Update server to support notes explanation

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

server/server.go

@@ -38,6 +38,7 @@ import (
 	"github.com/open-policy-agent/opa/server/writer"
 	"github.com/open-policy-agent/opa/storage"
 	"github.com/open-policy-agent/opa/topdown"
+	"github.com/open-policy-agent/opa/topdown/notes"
 	"github.com/open-policy-agent/opa/util"
 	"github.com/open-policy-agent/opa/version"
 	"github.com/open-policy-agent/opa/watch"
@@ -1857,6 +1858,12 @@ func (s *Server) evalDiagnosticPolicy(r *http.Request) (logger diagnosticsLogger
 
 func (s *Server) getExplainResponse(explainMode types.ExplainModeV1, trace []*topdown.Event, pretty bool) (explanation types.TraceV1) {
 	switch explainMode {
+	case types.ExplainNotesV1:
+		var err error
+		explanation, err = types.NewTraceV1(notes.Filter(trace), pretty)
+		if err != nil {
+			break
+		}
 	case types.ExplainFullV1:
 		var err error
 		explanation, err = types.NewTraceV1(trace, pretty)
@@ -2105,6 +2112,8 @@ func getWatch(p []string) (watch bool) {
 func getExplain(p []string, zero types.ExplainModeV1) types.ExplainModeV1 {
 	for _, x := range p {
 		switch x {
+		case string(types.ExplainNotesV1):
+			return types.ExplainNotesV1
 		case string(types.ExplainFullV1):
 			return types.ExplainFullV1
 		}

server/server_test.go

@@ -1433,6 +1433,43 @@ p = [1, 2, 3, 4] { true }`, 200, "")
 
 }
 
+func TestDataPostExplainNotes(t *testing.T) {
+	f := newFixture(t)
+
+	f.v1(http.MethodPut, "/policies/test", `
+		package test
+		p {
+			data.a[i] = x; x > 1
+			trace(sprintf("found x = %d", [x]))
+		}`, 200, "")
+
+	f.v1(http.MethodPut, "/data/a", `[1,2,3]`, 200, "")
+	f.reset()
+
+	req := newReqV1(http.MethodPost, "/data/test/p?explain=notes", "")
+	f.server.Handler.ServeHTTP(f.recorder, req)
+
+	var result types.DataResponseV1
+
+	if err := util.NewJSONDecoder(f.recorder.Body).Decode(&result); err != nil {
+		t.Fatalf("Unexpected JSON decode err: %v", err)
+	}
+
+	var trace types.TraceV1Raw
+
+	if err := trace.UnmarshalJSON(result.Explanation); err != nil {
+		t.Fatal(err)
+	}
+
+	if len(trace) != 6 || trace[2].Op != "note" || trace[5].Op != "note" {
+		t.Logf("Found %d events in trace", len(trace))
+		for i := range trace {
+			t.Logf("Event #%d: %v\n", i, trace[i])
+		}
+		t.Fatal("Unexpected trace")
+	}
+}
+
 func TestDataProvenance(t *testing.T) {
 
 	f := newFixture(t)

server/types/types.go

@@ -191,8 +191,9 @@ type ExplainModeV1 string
 
 // Explanation mode enumeration.
 const (
-	ExplainOffV1  ExplainModeV1 = "off"
-	ExplainFullV1 ExplainModeV1 = "full"
+	ExplainOffV1   ExplainModeV1 = "off"
+	ExplainFullV1  ExplainModeV1 = "full"
+	ExplainNotesV1 ExplainModeV1 = "notes"
 )
 
 // TraceV1 models the trace result returned for queries that include the