jsondb: Allow passing nil as document to Read()

This makes Read() usable in situations where only the existence of a
value has to be checked.

internal/jsondb/db.go

@@ -36,7 +36,8 @@ func New(dir string, perm os.FileMode) *JSONDatabase {
 	return &JSONDatabase{dir, perm}
 }
 
-// Reads the value at `name`. `document` must deserializable from JSON. Returns
+// Reads the value at `name`. `document` must be a type that is deserializable
+// from the JSON document `name`, or nil to not deserialize at all. Returns
 // false if a document with `name` does not exist.
 func (db *JSONDatabase) Read(name string, document interface{}) (bool, error) {
 	f, err := os.Open(path.Join(db.dir, name+".json"))
@@ -48,9 +49,11 @@ func (db *JSONDatabase) Read(name string, document interface{}) (bool, error) {
 	}
 	defer f.Close()
 
-	err = json.NewDecoder(f).Decode(&document)
+	if document != nil {
-	if err != nil {
+		err = json.NewDecoder(f).Decode(&document)
-		return false, fmt.Errorf("error reading db file %s: %v", name, err)
+		if err != nil {
+			return false, fmt.Errorf("error reading db file %s: %v", name, err)
+		}
 	}
 
 	return true, nil

internal/jsondb/db_test.go

@@ -72,6 +72,39 @@ func TestCorrupt(t *testing.T) {
 	require.Error(t, err)
 }
 
+func TestRead(t *testing.T) {
+	dir, err := ioutil.TempDir("", "jsondb-test-")
+	require.NoError(t, err)
+	defer cleanupTempDir(t, dir)
+
+	err = ioutil.WriteFile(path.Join(dir, "one.json"), []byte("true"), 0755)
+	require.NoError(t, err)
+
+	db := jsondb.New(dir, 0755)
+
+	var b bool
+	exists, err := db.Read("one", &b)
+	require.NoError(t, err)
+	require.True(t, exists)
+	require.True(t, b)
+
+	// nil means don't deserialize
+	exists, err = db.Read("one", nil)
+	require.NoError(t, err)
+	require.True(t, exists)
+
+	b = false
+	exists, err = db.Read("two", &b)
+	require.NoError(t, err)
+	require.False(t, exists)
+	require.False(t, b)
+
+	// nil means don't deserialize
+	exists, err = db.Read("two", nil)
+	require.NoError(t, err)
+	require.False(t, exists)
+}
+
 func TestMultiple(t *testing.T) {
 	dir, err := ioutil.TempDir("", "jsondb-test-")
 	require.NoError(t, err)