fix: Env variables annotated with file is not allowed to be empty (#109)

* adding support to overload env variable and load data from a file

* fixing file perm for TestFile

* refactoring in order to allow file option to act more in accoredence to prior setup

* adding more tests for file option and some extra for requierd option

* updating readme for env file tag

* refactoring to only load value from file if file annotation is given on tag

* updating readme for env file tag

* fixing formating

* fixing type-o

* Update env_test.go

Co-Authored-By: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>

* Update env.go

Co-Authored-By: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>

* lessening requierment for file annotation in order not to enforce that a file has to be given

Co-authored-by: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>

env.go

@@ -193,7 +193,7 @@ func get(field reflect.StructField) (val string, err error) {
 		return "", fmt.Errorf(`env: required environment variable %q is not set`, key)
 	}
 
-	if loadFile {
+	if loadFile && val != "" {
 		filename := val
 		val, err = getFromFile(filename)
 		if err != nil {

env_test.go

@@ -1113,8 +1113,43 @@ func TestFileNoParam(t *testing.T) {
 	cfg := config{}
 	err := Parse(&cfg)
 
+	assert.NoError(t, err)
+}
+
+func TestFileNoParamRequired(t *testing.T) {
+	type config struct {
+		SecretKey string `env:"SECRET_KEY,file,required"`
+	}
+	defer os.Clearenv()
+	cfg := config{}
+	err := Parse(&cfg)
+
+	assert.Error(t, err)
+	assert.EqualError(t, err, "env: required environment variable \"SECRET_KEY\" is not set")
+}
+
+func TestFileBadFile(t *testing.T) {
+	type config struct {
+		SecretKey string `env:"SECRET_KEY,file"`
+	}
+
+	file, err := ioutil.TempFile("", "sec_key_*")
+	assert.NoError(t, err)
+	err = ioutil.WriteFile(file.Name(), []byte("secret"), 0660)
+	assert.NoError(t, err)
+
+	filename := file.Name()
+	defer os.Clearenv()
+	os.Setenv("SECRET_KEY", filename)
+
+	err = os.Remove(filename)
+	assert.NoError(t, err)
+
+	cfg := config{}
+	err = Parse(&cfg)
+
 	assert.Error(t, err)
-	assert.EqualError(t, err, `env: could not load content of file "" from variable SECRET_KEY: open : no such file or directory`)
+	assert.EqualError(t, err, fmt.Sprintf(`env: could not load content of file "%s" from variable SECRET_KEY: open %s: no such file or directory`, filename, filename))
 }
 
 func TestFileWithDefault(t *testing.T) {