PMM-685: let explain operate on []byte

src/go/mongolib/explain/explain.go

@@ -16,9 +16,21 @@ func New(session pmgo.SessionManager) *explain {
 	}
 }
 
-func (e *explain) Explain(eq proto.ExampleQuery) ([]byte, error) {
+func (e *explain) Explain(db string, query []byte) ([]byte, error) {
+	var err error
+	var eq proto.ExampleQuery
+
+	err = bson.UnmarshalJSON(query, &eq)
+	if err != nil {
+		return nil, err
+	}
+
+	if db == "" {
+		db = eq.Db()
+	}
+
 	var result proto.BsonD
-	err := e.session.DB(eq.Db()).Run(eq.ExplainCmd(), &result)
+	err = e.session.DB(db).Run(eq.ExplainCmd(), &result)
 	if err != nil {
 		return nil, err
 	}

src/go/mongolib/explain/explain_test.go

@@ -138,22 +138,24 @@ func TestExplain(t *testing.T) {
 			if err != nil {
 				t.Fatalf("cannot load sample %s: %s", dir+file.Name(), err)
 			}
-			got, err := ex.Explain(eq)
+			query, err := bson.MarshalJSON(eq)
+			if err != nil {
+				t.Fatalf("cannot marshal json %s: %s", dir+file.Name(), err)
+			}
+			got, err := ex.Explain("", query)
 			expectErrMsg := expectError[file.Name()]
 			gotErrMsg := fmt.Sprintf("%v", err)
 			if gotErrMsg != expectErrMsg {
 				t.Fatalf("explain error should be '%s' but was '%s'", expectErrMsg, gotErrMsg)
 			}
 
-			if err != nil {
+			if err == nil {
-				return
-			}
-
 				result := proto.BsonD{}
 				err = bson.UnmarshalJSON(got, &result)
 				if err != nil {
 					t.Fatalf("cannot unmarshal json explain result: %s", err)
 				}
+			}
 		})
 	}
 }