Adds MergeConfig functionality

This patch adds the `MergeConfig` and `MergeInConfig` functions to
enable reading new configuration files via a merge strategy rather
than replace. For example, take the following as the base YAML for a
configuration:

    hello:
        pop: 37890
        world:
        - us
        - uk
        - fr
        - de

Now imagine we want to read the following, new configuration data:

    hello:
        pop: 45000
        universe:
        - mw
        - ad
    fu: bar

Using the standard `ReadConfig` function the value returned by the
nested key `hello.world` would no longer be present after the second
configuration is read. This is because the `ReadConfig` function and
its relatives replace nested structures entirely.

The new `MergeConfig` function would produce the following config
after the second YAML snippet was merged with the first:

    hello:
        pop: 45000
        world:
        - us
        - uk
        - fr
        - de
        universe:
        - mw
        - ad
    fu: bar

Examples showing how this works can be found in the two unit tests
named `TestMergeConfig` and `TestMergeConfigNoMerge`.

viper.go

@@ -901,12 +901,124 @@ func (v *Viper) ReadInConfig() error {
 	return v.unmarshalReader(bytes.NewReader(file), v.config)
 }
 
+// MergeInConfig merges a new configuration with an existing config.
+func MergeInConfig() error { return v.MergeInConfig() }
+func (v *Viper) MergeInConfig() error {
+	jww.INFO.Println("Attempting to merge in config file")
+	if !stringInSlice(v.getConfigType(), SupportedExts) {
+		return UnsupportedConfigError(v.getConfigType())
+	}
+
+	file, err := ioutil.ReadFile(v.getConfigFile())
+	if err != nil {
+		return err
+	}
+
+	return v.MergeConfig(bytes.NewReader(file))
+}
+
+// Viper will read a configuration file, setting existing keys to nil if the
+// key does not exist in the file.
 func ReadConfig(in io.Reader) error { return v.ReadConfig(in) }
 func (v *Viper) ReadConfig(in io.Reader) error {
 	v.config = make(map[string]interface{})
 	return v.unmarshalReader(in, v.config)
 }
 
+// MergeConfig merges a new configuration with an existing config.
+func MergeConfig(in io.Reader) error { return v.MergeConfig(in) }
+func (v *Viper) MergeConfig(in io.Reader) error {
+	if v.config == nil {
+		v.config = make(map[string]interface{})
+	}
+	cfg := make(map[string]interface{})
+	if err := v.unmarshalReader(in, cfg); err != nil {
+		return err
+	}
+	mergeMaps(cfg, v.config, nil)
+	return nil
+}
+
+func keyExists(k string, m map[string]interface{}) string {
+	lk := strings.ToLower(k)
+	for mk := range m {
+		lmk := strings.ToLower(mk)
+		if lmk == lk {
+			return mk
+		}
+	}
+	return ""
+}
+
+func castToMapStringInterface(
+	src map[interface{}]interface{}) map[string]interface{} {
+	tgt := map[string]interface{}{}
+	for k, v := range src {
+		tgt[fmt.Sprintf("%v", k)] = v
+	}
+	return tgt
+}
+
+// mergeMaps merges two maps. The `itgt` parameter is for handling go-yaml's
+// insistence on parsing nested structures as `map[interface{}]interface{}`
+// instead of using a `string` as the key for nest structures beyond one level
+// deep. Both map types are supported as there is a go-yaml fork that uses
+// `map[string]interface{}` instead.
+func mergeMaps(
+	src, tgt map[string]interface{}, itgt map[interface{}]interface{}) {
+	for sk, sv := range src {
+		tk := keyExists(sk, tgt)
+		if tk == "" {
+			jww.TRACE.Printf("tk=\"\", tgt[%s]=%v", sk, sv)
+			tgt[sk] = sv
+			if itgt != nil {
+				itgt[sk] = sv
+			}
+			continue
+		}
+
+		tv, ok := tgt[tk]
+		if !ok {
+			jww.TRACE.Printf("tgt[%s] != ok, tgt[%s]=%v", tk, sk, sv)
+			tgt[sk] = sv
+			if itgt != nil {
+				itgt[sk] = sv
+			}
+			continue
+		}
+
+		svType := reflect.TypeOf(sv)
+		tvType := reflect.TypeOf(tv)
+		if svType != tvType {
+			jww.ERROR.Printf(
+				"svType != tvType; key=%s, st=%v, tt=%v, sv=%v, tv=%v",
+				sk, svType, tvType, sv, tv)
+			continue
+		}
+
+		jww.TRACE.Printf("processing key=%s, st=%v, tt=%v, sv=%v, tv=%v",
+			sk, svType, tvType, sv, tv)
+
+		switch ttv := tv.(type) {
+		case map[interface{}]interface{}:
+			jww.TRACE.Printf("merging maps (must convert)")
+			tsv := sv.(map[interface{}]interface{})
+			ssv := castToMapStringInterface(tsv)
+			stv := castToMapStringInterface(ttv)
+			mergeMaps(ssv, stv, ttv)
+		case map[string]interface{}:
+			jww.TRACE.Printf("merging maps")
+			mergeMaps(sv.(map[string]interface{}), ttv, nil)
+		default:
+			jww.TRACE.Printf("setting value")
+			tgt[tk] = sv
+			if itgt != nil {
+				itgt[tk] = sv
+			}
+		}
+	}
+}
+
 // func ReadBufConfig(buf *bytes.Buffer) error { return v.ReadBufConfig(buf) }
 // func (v *Viper) ReadBufConfig(buf *bytes.Buffer) error {
 // 	v.config = make(map[string]interface{})

viper_test.go

@@ -737,3 +737,101 @@ func TestSub(t *testing.T) {
 	assert.Equal(t, subv, (*Viper)(nil))
 }
 
+var yamlMergeExampleTgt = []byte(`
+hello:
+    pop: 37890
+    world:
+    - us
+    - uk
+    - fr
+    - de
+`)
+
+var yamlMergeExampleSrc = []byte(`
+hello:
+    pop: 45000
+    universe:
+    - mw
+    - ad
+fu: bar
+`)
+
+func TestMergeConfig(t *testing.T) {
+	v := New()
+	v.SetConfigType("yml")
+	if err := v.ReadConfig(bytes.NewBuffer(yamlMergeExampleTgt)); err != nil {
+		t.Fatal(err)
+	}
+
+	if pop := v.GetInt("hello.pop"); pop != 37890 {
+		t.Fatalf("pop != 37890, = %d", pop)
+	}
+
+	if world := v.GetStringSlice("hello.world"); len(world) != 4 {
+		t.Fatalf("len(world) != 4, = %d", len(world))
+	}
+
+	if fu := v.GetString("fu"); fu != "" {
+		t.Fatalf("fu != \"\", = %s", fu)
+	}
+
+	if err := v.MergeConfig(bytes.NewBuffer(yamlMergeExampleSrc)); err != nil {
+		t.Fatal(err)
+	}
+
+	if pop := v.GetInt("hello.pop"); pop != 45000 {
+		t.Fatalf("pop != 45000, = %d", pop)
+	}
+
+	if world := v.GetStringSlice("hello.world"); len(world) != 4 {
+		t.Fatalf("len(world) != 4, = %d", len(world))
+	}
+
+	if universe := v.GetStringSlice("hello.universe"); len(universe) != 2 {
+		t.Fatalf("len(universe) != 2, = %d", len(universe))
+	}
+
+	if fu := v.GetString("fu"); fu != "bar" {
+		t.Fatalf("fu != \"bar\", = %s", fu)
+	}
+}
+
+func TestMergeConfigNoMerge(t *testing.T) {
+	v := New()
+	v.SetConfigType("yml")
+	if err := v.ReadConfig(bytes.NewBuffer(yamlMergeExampleTgt)); err != nil {
+		t.Fatal(err)
+	}
+
+	if pop := v.GetInt("hello.pop"); pop != 37890 {
+		t.Fatalf("pop != 37890, = %d", pop)
+	}
+
+	if world := v.GetStringSlice("hello.world"); len(world) != 4 {
+		t.Fatalf("len(world) != 4, = %d", len(world))
+	}
+
+	if fu := v.GetString("fu"); fu != "" {
+		t.Fatalf("fu != \"\", = %s", fu)
+	}
+
+	if err := v.ReadConfig(bytes.NewBuffer(yamlMergeExampleSrc)); err != nil {
+		t.Fatal(err)
+	}
+
+	if pop := v.GetInt("hello.pop"); pop != 45000 {
+		t.Fatalf("pop != 45000, = %d", pop)
+	}
+
+	if world := v.GetStringSlice("hello.world"); len(world) != 0 {
+		t.Fatalf("len(world) != 0, = %d", len(world))
+	}
+
+	if universe := v.GetStringSlice("hello.universe"); len(universe) != 2 {
+		t.Fatalf("len(universe) != 2, = %d", len(universe))
+	}
+
+	if fu := v.GetString("fu"); fu != "bar" {
+		t.Fatalf("fu != \"bar\", = %s", fu)
+	}
+}