Add SetDefaultsFrom to load defaults from a struct

In cases where there isn't a configuration file, none of the fields on
the struct are bound to environment variables unless BindEnv was
explicitly called. AutomaticEnv essentially doesn't work with the config
file per spf13#761.

If you need to support this scenario, call SetDefaultsFrom on the
destination struct before calling Unmarshal.

Signed-off-by: Carolyn Van Slyck <me@carolynvanslyck.com>

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/coreos/pkg v0.0.0-20180928190104-399ea9e2e55f // indirect
 	github.com/fsnotify/fsnotify v1.4.7
 	github.com/hashicorp/hcl v1.0.0
+	github.com/jeremywohl/flatten v1.0.1
 	github.com/magiconair/properties v1.8.1
 	github.com/mitchellh/mapstructure v1.3.3
 	github.com/pelletier/go-toml v1.2.0

go.sum

@@ -198,6 +198,8 @@ github.com/hpcloud/tail v1.0.0/go.mod h1:ab1qPbhIpdTxEkNHXyeSf5vhxWSCs/tWer42PpO
 github.com/hudl/fargo v1.3.0/go.mod h1:y3CKSmjA+wD2gak7sUSXTAoopbhU08POFhmITJgmKTg=
 github.com/inconshreveable/mousetrap v1.0.0/go.mod h1:PxqpIevigyE2G7u3NXJIT2ANytuPF1OarO4DADm73n8=
 github.com/influxdata/influxdb1-client v0.0.0-20191209144304-8bf82d3c094d/go.mod h1:qj24IKcXYK6Iy9ceXlo3Tc+vtHo9lIhSX5JddghvEPo=
+github.com/jeremywohl/flatten v1.0.1 h1:LrsxmB3hfwJuE+ptGOijix1PIfOoKLJ3Uee/mzbgtrs=
+github.com/jeremywohl/flatten v1.0.1/go.mod h1:4AmD/VxjWcI5SRB0n6szE2A6s2fsNHDLO0nAlMHgfLQ=
 github.com/jmespath/go-jmespath v0.0.0-20180206201540-c2b33e8439af/go.mod h1:Nht3zPeWKUH0NzdCt2Blrr5ys8VGpn0CEB0cQHVjt7k=
 github.com/jonboulle/clockwork v0.1.0 h1:VKV+ZcuP6l3yW9doeqz6ziZGgcynBVQO+obU0+0hcPo=
 github.com/jonboulle/clockwork v0.1.0/go.mod h1:Ii8DK3G1RaLaWxj9trq07+26W01tbo22gdxWY5EU2bo=

viper.go

@@ -38,6 +38,7 @@ import (
 	"github.com/fsnotify/fsnotify"
 	"github.com/hashicorp/hcl"
 	"github.com/hashicorp/hcl/hcl/printer"
+	"github.com/jeremywohl/flatten"
 	"github.com/magiconair/properties"
 	"github.com/mitchellh/mapstructure"
 	"github.com/pelletier/go-toml"
@@ -995,6 +996,35 @@ func (v *Viper) Unmarshal(rawVal interface{}, opts ...DecoderConfigOption) error
 	return decode(v.AllSettings(), defaultDecoderConfig(rawVal, opts...))
 }
 
+// ReadDefaultsFrom applies the specified data to Viper's configuration
+// using SetDefault.
+// Useful for setting defaults from the specified structs in cases
+// where a configuration file may not be present.
+func ReadDefaultsFrom(val interface{}) error {
+	return v.SetDefaultsFrom(val)
+}
+
+// ReadDefaultsFrom applies the specified data to Viper's configuration
+// using SetDefault.
+// Useful for setting defaults from the specified structs in cases
+// where a configuration file may not be present.
+func (v *Viper) SetDefaultsFrom(val interface{}) error {
+	var tmp map[string]interface{}
+	err := mapstructure.Decode(val, &tmp)
+	if err != nil {
+		return fmt.Errorf("error decoding configuration from struct: %v", err)
+	}
+
+	defaults, err := flatten.Flatten(tmp, "", flatten.DotStyle)
+	if err != nil {
+		return fmt.Errorf("error flattening default configuration from struct: %v", err)
+	}
+	for defaultKey, defaultValue := range defaults {
+		v.SetDefault(defaultKey, defaultValue)
+	}
+	return nil
+}
+
 // defaultDecoderConfig returns default mapsstructure.DecoderConfig with suppot
 // of time.Duration values & string slices
 func defaultDecoderConfig(output interface{}, opts ...DecoderConfigOption) *mapstructure.DecoderConfig {

viper_test.go

@@ -798,6 +798,52 @@ func TestUnmarshal(t *testing.T) {
 	)
 }
 
+// Check that we can unmarshal environment variables without
+// explicit bindings or a configuration file.
+func TestUnmarshalWithoutConfigFile(t *testing.T) {
+	v := New()
+	v.AutomaticEnv()
+
+	type Person struct {
+		Name string
+	}
+	type config struct {
+		Port   int
+		Person Person
+		Color  string
+	}
+
+	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
+	testutil.Setenv(t, "PERSON_NAME", "Sally")
+
+	defaults := config{
+		Person: Person{
+			Name: "default name",
+		},
+		Port:  8080,
+		Color: "blue",
+	}
+
+	err := v.SetDefaultsFrom(defaults)
+	if err != nil {
+		t.Fatalf("SetDefaultsFrom failed: %v", err)
+	}
+	v.SetDefault("port", 80)
+
+	var C config
+	err = v.Unmarshal(&C)
+	if err != nil {
+		t.Fatalf("unable to decode into struct: %v", err)
+	}
+
+	wantConfig := config{
+		Color:  "blue",                // use default
+		Port:   80,                    // override with env var
+		Person: Person{Name: "Sally"}, // override nested with env var
+	}
+	assert.Equal(t, wantConfig, C)
+}
+
 func TestUnmarshalWithDecoderOptions(t *testing.T) {
 	Set("credentials", "{\"foo\":\"bar\"}")