Misc config loading fixes

The main motivation behind this is simplicity and correctnes, but the new small config library is also faster:

```
BenchmarkDefaultConfigProvider/Viper-16         	  252418	      4546 ns/op	    2720 B/op	      30 allocs/op
BenchmarkDefaultConfigProvider/Custom-16        	  450756	      2651 ns/op	    1008 B/op	       6 allocs/op
```

Fixes #8633
Fixes #8618
Fixes #8630
Updates #8591
Closes #6680
Closes #5192

11 files changed, 897 insertions, 30 deletions

diff --git a/config/commonConfig_test.go b/config/commonConfig_test.go
index d4273277a..55767913f 100644
--- a/config/commonConfig_test.go
+++ b/config/commonConfig_test.go
@@ -21,14 +21,12 @@ import (
 	"github.com/gohugoio/hugo/common/types"
 
 	qt "github.com/frankban/quicktest"
-
-	"github.com/spf13/viper"
 )
 
 func TestBuild(t *testing.T) {
 	c := qt.New(t)
 
-	v := viper.New()
+	v := New()
 	v.Set("build", map[string]interface{}{
 		"useResourceCacheWhen": "always",
 	})
diff --git a/config/compositeConfig.go b/config/compositeConfig.go
new file mode 100644
index 000000000..c68419533
--- /dev/null
+++ b/config/compositeConfig.go
@@ -0,0 +1,113 @@
+// Copyright 2021 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package config
+
+import (
+	"github.com/gohugoio/hugo/common/maps"
+)
+
+// NewCompositeConfig creates a new composite Provider with a read-only base
+// and a writeable layer.
+func NewCompositeConfig(base, layer Provider) Provider {
+	return &compositeConfig{
+		base:  base,
+		layer: layer,
+	}
+}
+
+// compositeConfig contains a read only config base with
+// a possibly writeable config layer on top.
+type compositeConfig struct {
+	base  Provider
+	layer Provider
+}
+
+func (c *compositeConfig) GetBool(key string) bool {
+	if c.layer.IsSet(key) {
+		return c.layer.GetBool(key)
+	}
+	return c.base.GetBool(key)
+}
+
+func (c *compositeConfig) GetInt(key string) int {
+	if c.layer.IsSet(key) {
+		return c.layer.GetInt(key)
+	}
+	return c.base.GetInt(key)
+}
+
+func (c *compositeConfig) Merge(key string, value interface{}) {
+	c.layer.Merge(key, value)
+}
+
+func (c *compositeConfig) GetParams(key string) maps.Params {
+	if c.layer.IsSet(key) {
+		return c.layer.GetParams(key)
+	}
+	return c.base.GetParams(key)
+}
+
+func (c *compositeConfig) GetStringMap(key string) map[string]interface{} {
+	if c.layer.IsSet(key) {
+		return c.layer.GetStringMap(key)
+	}
+	return c.base.GetStringMap(key)
+}
+
+func (c *compositeConfig) GetStringMapString(key string) map[string]string {
+	if c.layer.IsSet(key) {
+		return c.layer.GetStringMapString(key)
+	}
+	return c.base.GetStringMapString(key)
+}
+
+func (c *compositeConfig) GetStringSlice(key string) []string {
+	if c.layer.IsSet(key) {
+		return c.layer.GetStringSlice(key)
+	}
+	return c.base.GetStringSlice(key)
+}
+
+func (c *compositeConfig) Get(key string) interface{} {
+	if c.layer.IsSet(key) {
+		return c.layer.Get(key)
+	}
+	return c.base.Get(key)
+}
+
+func (c *compositeConfig) IsSet(key string) bool {
+	if c.layer.IsSet(key) {
+		return true
+	}
+	return c.base.IsSet(key)
+}
+
+func (c *compositeConfig) GetString(key string) string {
+	if c.layer.IsSet(key) {
+		return c.layer.GetString(key)
+	}
+	return c.base.GetString(key)
+}
+
+func (c *compositeConfig) Set(key string, value interface{}) {
+	c.layer.Set(key, value)
+}
+
+func (c *compositeConfig) WalkParams(walkFn func(params ...KeyParams) bool) {
+	panic("not supported")
+}
+
+func (c *compositeConfig) SetDefaultMergeStrategy() {
+	panic("not supported")
+}
diff --git a/config/compositeConfig_test.go b/config/compositeConfig_test.go
new file mode 100644
index 000000000..60644102f
--- /dev/null
+++ b/config/compositeConfig_test.go
@@ -0,0 +1,40 @@
+// Copyright 2021 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package config
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestCompositeConfig(t *testing.T) {
+	c := qt.New(t)
+
+	c.Run("Set and get", func(c *qt.C) {
+		base, layer := New(), New()
+		cfg := NewCompositeConfig(base, layer)
+
+		layer.Set("a1", "av")
+		base.Set("b1", "bv")
+		cfg.Set("c1", "cv")
+
+		c.Assert(cfg.Get("a1"), qt.Equals, "av")
+		c.Assert(cfg.Get("b1"), qt.Equals, "bv")
+		c.Assert(cfg.Get("c1"), qt.Equals, "cv")
+		c.Assert(cfg.IsSet("c1"), qt.IsTrue)
+		c.Assert(layer.IsSet("c1"), qt.IsTrue)
+		c.Assert(base.IsSet("c1"), qt.IsFalse)
+	})
+}
diff --git a/config/configLoader.go b/config/configLoader.go
index 6d94f0b79..0998b1bef 100644
--- a/config/configLoader.go
+++ b/config/configLoader.go
@@ -20,7 +20,6 @@ import (
 	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/parser/metadecoders"
 	"github.com/spf13/afero"
-	"github.com/spf13/viper"
 )
 
 var (
@@ -43,15 +42,11 @@ func IsValidConfigFilename(filename string) bool {
 
 // FromConfigString creates a config from the given YAML, JSON or TOML config. This is useful in tests.
 func FromConfigString(config, configType string) (Provider, error) {
-	v := newViper()
 	m, err := readConfig(metadecoders.FormatFromString(configType), []byte(config))
 	if err != nil {
 		return nil, err
 	}
-
-	v.MergeConfigMap(m)
-
-	return v, nil
+	return NewFrom(m), nil
 }
 
 // FromFile loads the configuration from the given filename.
@@ -60,15 +55,7 @@ func FromFile(fs afero.Fs, filename string) (Provider, error) {
 	if err != nil {
 		return nil, err
 	}
-
-	v := newViper()
-
-	err = v.MergeConfigMap(m)
-	if err != nil {
-		return nil, err
-	}
-
-	return v, nil
+	return NewFrom(m), nil
 }
 
 // FromFileToMap is the same as FromFile, but it returns the config values
@@ -116,9 +103,3 @@ func init() {
 func RenameKeys(m map[string]interface{}) {
 	keyAliases.Rename(m)
 }
-
-func newViper() *viper.Viper {
-	v := viper.New()
-
-	return v
-}
diff --git a/config/configProvider.go b/config/configProvider.go
index 928bf948a..92206ca9e 100644
--- a/config/configProvider.go
+++ b/config/configProvider.go
@@ -14,6 +14,7 @@
 package config
 
 import (
+	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/common/types"
 )
 
@@ -22,11 +23,15 @@ type Provider interface {
 	GetString(key string) string
 	GetInt(key string) int
 	GetBool(key string) bool
+	GetParams(key string) maps.Params
 	GetStringMap(key string) map[string]interface{}
 	GetStringMapString(key string) map[string]string
 	GetStringSlice(key string) []string
 	Get(key string) interface{}
 	Set(key string, value interface{})
+	Merge(key string, value interface{})
+	SetDefaultMergeStrategy()
+	WalkParams(walkFn func(params ...KeyParams) bool)
 	IsSet(key string) bool
 }
 
diff --git a/config/configProvider_test.go b/config/configProvider_test.go
index d9fff56b6..0afba1e58 100644
--- a/config/configProvider_test.go
+++ b/config/configProvider_test.go
@@ -17,12 +17,11 @@ import (
 	"testing"
 
 	qt "github.com/frankban/quicktest"
-	"github.com/spf13/viper"
 )
 
 func TestGetStringSlicePreserveString(t *testing.T) {
 	c := qt.New(t)
-	cfg := viper.New()
+	cfg := New()
 
 	s := "This is a string"
 	sSlice := []string{"This", "is", "a", "slice"}
diff --git a/config/defaultConfigProvider.go b/config/defaultConfigProvider.go
new file mode 100644
index 000000000..d9c9db7f1
--- /dev/null
+++ b/config/defaultConfigProvider.go
@@ -0,0 +1,372 @@
+// Copyright 2021 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package config
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+	"sync"
+
+	"github.com/spf13/cast"
+
+	"github.com/gohugoio/hugo/common/maps"
+)
+
+var (
+
+	// ConfigRootKeysSet contains all of the config map root keys.
+	// TODO(bep) use this for something (docs etc.)
+	ConfigRootKeysSet = map[string]bool{
+		"build":         true,
+		"caches":        true,
+		"frontmatter":   true,
+		"languages":     true,
+		"imaging":       true,
+		"markup":        true,
+		"mediatypes":    true,
+		"menus":         true,
+		"minify":        true,
+		"module":        true,
+		"outputformats": true,
+		"params":        true,
+		"permalinks":    true,
+		"related":       true,
+		"sitemap":       true,
+		"taxonomies":    true,
+	}
+
+	// ConfigRootKeys is a sorted version of ConfigRootKeysSet.
+	ConfigRootKeys []string
+)
+
+func init() {
+	for k := range ConfigRootKeysSet {
+		ConfigRootKeys = append(ConfigRootKeys, k)
+	}
+	sort.Strings(ConfigRootKeys)
+}
+
+// New creates a Provider backed by an empty maps.Params.
+func New() Provider {
+	return &defaultConfigProvider{
+		root: make(maps.Params),
+	}
+}
+
+// NewFrom creates a Provider backed by params.
+func NewFrom(params maps.Params) Provider {
+	maps.PrepareParams(params)
+	return &defaultConfigProvider{
+		root: params,
+	}
+}
+
+// defaultConfigProvider is a Provider backed by a map where all keys are lower case.
+// All methods are thread safe.
+type defaultConfigProvider struct {
+	mu   sync.RWMutex
+	root maps.Params
+
+	keyCache sync.Map
+}
+
+func (c *defaultConfigProvider) Get(k string) interface{} {
+	if k == "" {
+		return c.root
+	}
+	c.mu.RLock()
+	key, m := c.getNestedKeyAndMap(strings.ToLower(k), false)
+	if m == nil {
+		return nil
+	}
+	v := m[key]
+	c.mu.RUnlock()
+	return v
+}
+
+func (c *defaultConfigProvider) GetBool(k string) bool {
+	v := c.Get(k)
+	return cast.ToBool(v)
+}
+
+func (c *defaultConfigProvider) GetInt(k string) int {
+	v := c.Get(k)
+	return cast.ToInt(v)
+}
+
+func (c *defaultConfigProvider) IsSet(k string) bool {
+	var found bool
+	c.mu.RLock()
+	key, m := c.getNestedKeyAndMap(strings.ToLower(k), false)
+	if m != nil {
+		_, found = m[key]
+	}
+	c.mu.RUnlock()
+	return found
+}
+
+func (c *defaultConfigProvider) GetString(k string) string {
+	v := c.Get(k)
+	return cast.ToString(v)
+}
+
+func (c *defaultConfigProvider) GetParams(k string) maps.Params {
+	v := c.Get(k)
+	if v == nil {
+		return nil
+	}
+	return v.(maps.Params)
+}
+
+func (c *defaultConfigProvider) GetStringMap(k string) map[string]interface{} {
+	v := c.Get(k)
+	return maps.ToStringMap(v)
+}
+
+func (c *defaultConfigProvider) GetStringMapString(k string) map[string]string {
+	v := c.Get(k)
+	return maps.ToStringMapString(v)
+}
+
+func (c *defaultConfigProvider) GetStringSlice(k string) []string {
+	v := c.Get(k)
+	return cast.ToStringSlice(v)
+}
+
+func (c *defaultConfigProvider) Set(k string, v interface{}) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	k = strings.ToLower(k)
+
+	if k == "" {
+		if p, ok := maps.ToParamsAndPrepare(v); ok {
+			// Set the values directly in root.
+			c.root.Set(p)
+		} else {
+			c.root[k] = v
+		}
+
+		return
+	}
+
+	switch vv := v.(type) {
+	case map[string]interface{}:
+		var p maps.Params = vv
+		v = p
+		maps.PrepareParams(p)
+	}
+
+	key, m := c.getNestedKeyAndMap(k, true)
+
+	if existing, found := m[key]; found {
+		if p1, ok := existing.(maps.Params); ok {
+			if p2, ok := v.(maps.Params); ok {
+				p1.Set(p2)
+				return
+			}
+		}
+	}
+
+	m[key] = v
+}
+
+func (c *defaultConfigProvider) Merge(k string, v interface{}) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	k = strings.ToLower(k)
+
+	if k == "" {
+		rs, f := c.root.GetMergeStrategy()
+		if f && rs == maps.ParamsMergeStrategyNone {
+			// The user has set a "no merge" strategy on this,
+			// nothing more to do.
+			return
+		}
+
+		if p, ok := maps.ToParamsAndPrepare(v); ok {
+			// As there may be keys in p not in root, we need to handle
+			// those as a special case.
+			for kk, vv := range p {
+				if pp, ok := vv.(maps.Params); ok {
+					if ppp, ok := c.root[kk]; ok {
+						ppp.(maps.Params).Merge(pp)
+					} else {
+						// We need to use the default merge strategy for
+						// this key.
+						np := make(maps.Params)
+						strategy := c.determineMergeStrategy(KeyParams{Key: "", Params: c.root}, KeyParams{Key: kk, Params: np})
+						np.SetDefaultMergeStrategy(strategy)
+						np.Merge(pp)
+						if len(np) > 0 {
+							c.root[kk] = np
+						}
+					}
+				}
+			}
+			// Merge the rest.
+			c.root.Merge(p)
+		} else {
+			panic(fmt.Sprintf("unsupported type %T received in Merge", v))
+		}
+
+		return
+	}
+
+	switch vv := v.(type) {
+	case map[string]interface{}:
+		var p maps.Params = vv
+		v = p
+		maps.PrepareParams(p)
+	}
+
+	key, m := c.getNestedKeyAndMap(k, true)
+
+	if existing, found := m[key]; found {
+		if p1, ok := existing.(maps.Params); ok {
+			if p2, ok := v.(maps.Params); ok {
+				p1.Merge(p2)
+			}
+		}
+	} else {
+		m[key] = v
+	}
+}
+
+func (c *defaultConfigProvider) WalkParams(walkFn func(params ...KeyParams) bool) {
+	var walk func(params ...KeyParams)
+	walk = func(params ...KeyParams) {
+		if walkFn(params...) {
+			return
+		}
+		p1 := params[len(params)-1]
+		i := len(params)
+		for k, v := range p1.Params {
+			if p2, ok := v.(maps.Params); ok {
+				paramsplus1 := make([]KeyParams, i+1)
+				copy(paramsplus1, params)
+				paramsplus1[i] = KeyParams{Key: k, Params: p2}
+				walk(paramsplus1...)
+			}
+		}
+	}
+	walk(KeyParams{Key: "", Params: c.root})
+}
+
+func (c *defaultConfigProvider) determineMergeStrategy(params ...KeyParams) maps.ParamsMergeStrategy {
+	if len(params) == 0 {
+		return maps.ParamsMergeStrategyNone
+	}
+
+	var (
+		strategy   maps.ParamsMergeStrategy
+		prevIsRoot bool
+		curr       = params[len(params)-1]
+	)
+
+	if len(params) > 1 {
+		prev := params[len(params)-2]
+		prevIsRoot = prev.Key == ""
+
+		// Inherit from parent (but not from the root unless it's set by user).
+		s, found := prev.Params.GetMergeStrategy()
+		if !prevIsRoot && !found {
+			panic("invalid state, merge strategy not set on parent")
+		}
+		if found || !prevIsRoot {
+			strategy = s
+		}
+	}
+
+	switch curr.Key {
+	case "":
+	// Don't set a merge strategy on the root unless set by user.
+	// This will be handled as a special case.
+	case "params":
+		strategy = maps.ParamsMergeStrategyDeep
+	case "outputformats", "mediatypes":
+		if prevIsRoot {
+			strategy = maps.ParamsMergeStrategyShallow
+		}
+	case "menus":
+		isMenuKey := prevIsRoot
+		if !isMenuKey {
+			// Can also be set below languages.
+			// root > languages > en > menus
+			if len(params) == 4 && params[1].Key == "languages" {
+				isMenuKey = true
+			}
+		}
+		if isMenuKey {
+			strategy = maps.ParamsMergeStrategyShallow
+		}
+	default:
+		if strategy == "" {
+			strategy = maps.ParamsMergeStrategyNone
+		}
+	}
+
+	return strategy
+}
+
+type KeyParams struct {
+	Key    string
+	Params maps.Params
+}
+
+func (c *defaultConfigProvider) SetDefaultMergeStrategy() {
+	c.WalkParams(func(params ...KeyParams) bool {
+		if len(params) == 0 {
+			return false
+		}
+		p := params[len(params)-1].Params
+		var found bool
+		if _, found = p.GetMergeStrategy(); found {
+			// Set by user.
+			return false
+		}
+		strategy := c.determineMergeStrategy(params...)
+		if strategy != "" {
+			p.SetDefaultMergeStrategy(strategy)
+		}
+		return false
+	})
+
+}
+
+func (c *defaultConfigProvider) getNestedKeyAndMap(key string, create bool) (string, maps.Params) {
+	var parts []string
+	v, ok := c.keyCache.Load(key)
+	if ok {
+		parts = v.([]string)
+	} else {
+		parts = strings.Split(key, ".")
+		c.keyCache.Store(key, parts)
+	}
+	current := c.root
+	for i := 0; i < len(parts)-1; i++ {
+		next, found := current[parts[i]]
+		if !found {
+			if create {
+				next = make(maps.Params)
+				current[parts[i]] = next
+			} else {
+				return "", nil
+			}
+		}
+		current = next.(maps.Params)
+	}
+	return parts[len(parts)-1], current
+}
diff --git a/config/defaultConfigProvider_test.go b/config/defaultConfigProvider_test.go
new file mode 100644
index 000000000..834165d96
--- /dev/null
+++ b/config/defaultConfigProvider_test.go
@@ -0,0 +1,315 @@
+// Copyright 2021 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package config
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+	"testing"
+
+	"github.com/spf13/viper"
+
+	"github.com/gohugoio/hugo/common/para"
+
+	"github.com/gohugoio/hugo/common/maps"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestDefaultConfigProvider(t *testing.T) {
+	c := qt.New(t)
+
+	c.Run("Set and get", func(c *qt.C) {
+		cfg := New()
+		var k string
+		var v interface{}
+
+		k, v = "foo", "bar"
+		cfg.Set(k, v)
+		c.Assert(cfg.Get(k), qt.Equals, v)
+		c.Assert(cfg.Get(strings.ToUpper(k)), qt.Equals, v)
+		c.Assert(cfg.GetString(k), qt.Equals, v)
+
+		k, v = "foo", 42
+		cfg.Set(k, v)
+		c.Assert(cfg.Get(k), qt.Equals, v)
+		c.Assert(cfg.GetInt(k), qt.Equals, v)
+
+		c.Assert(cfg.Get(""), qt.DeepEquals, maps.Params{
+			"foo": 42,
+		})
+	})
+
+	c.Run("Set and get map", func(c *qt.C) {
+		cfg := New()
+
+		cfg.Set("foo", map[string]interface{}{
+			"bar": "baz",
+		})
+
+		c.Assert(cfg.Get("foo"), qt.DeepEquals, maps.Params{
+			"bar": "baz",
+		})
+
+		c.Assert(cfg.GetStringMap("foo"), qt.DeepEquals, map[string]interface{}{"bar": string("baz")})
+		c.Assert(cfg.GetStringMapString("foo"), qt.DeepEquals, map[string]string{"bar": string("baz")})
+	})
+
+	c.Run("Set and get nested", func(c *qt.C) {
+		cfg := New()
+
+		cfg.Set("a", map[string]interface{}{
+			"B": "bv",
+		})
+		cfg.Set("a.c", "cv")
+
+		c.Assert(cfg.Get("a"), qt.DeepEquals, maps.Params{
+			"b": "bv",
+			"c": "cv",
+		})
+		c.Assert(cfg.Get("a.c"), qt.Equals, "cv")
+
+		cfg.Set("b.a", "av")
+		c.Assert(cfg.Get("b"), qt.DeepEquals, maps.Params{
+			"a": "av",
+		})
+
+		cfg.Set("b", map[string]interface{}{
+			"b": "bv",
+		})
+
+		c.Assert(cfg.Get("b"), qt.DeepEquals, maps.Params{
+			"a": "av",
+			"b": "bv",
+		})
+
+		cfg = New()
+
+		cfg.Set("a", "av")
+
+		cfg.Set("", map[string]interface{}{
+			"a": "av2",
+			"b": "bv2",
+		})
+
+		c.Assert(cfg.Get(""), qt.DeepEquals, maps.Params{
+			"a": "av2",
+			"b": "bv2",
+		})
+
+		cfg = New()
+
+		cfg.Set("a", "av")
+
+		cfg.Set("", map[string]interface{}{
+			"b": "bv2",
+		})
+
+		c.Assert(cfg.Get(""), qt.DeepEquals, maps.Params{
+			"a": "av",
+			"b": "bv2",
+		})
+
+		cfg = New()
+
+		cfg.Set("", map[string]interface{}{
+			"foo": map[string]interface{}{
+				"a": "av",
+			},
+		})
+
+		cfg.Set("", map[string]interface{}{
+			"foo": map[string]interface{}{
+				"b": "bv2",
+			},
+		})
+
+		c.Assert(cfg.Get("foo"), qt.DeepEquals, maps.Params{
+			"a": "av",
+			"b": "bv2",
+		})
+	})
+
+	c.Run("Merge default strategy", func(c *qt.C) {
+		cfg := New()
+
+		cfg.Set("a", map[string]interface{}{
+			"B": "bv",
+		})
+
+		cfg.Merge("a", map[string]interface{}{
+			"B": "bv2",
+			"c": "cv2",
+		})
+
+		c.Assert(cfg.Get("a"), qt.DeepEquals, maps.Params{
+			"b": "bv",
+			"c": "cv2",
+		})
+
+		cfg = New()
+
+		cfg.Set("a", "av")
+
+		cfg.Merge("", map[string]interface{}{
+			"a": "av2",
+			"b": "bv2",
+		})
+
+		c.Assert(cfg.Get(""), qt.DeepEquals, maps.Params{
+			"a": "av",
+			"b": "bv2",
+		})
+	})
+
+	c.Run("Merge shallow", func(c *qt.C) {
+		cfg := New()
+
+		cfg.Set("a", map[string]interface{}{
+			"_merge": "shallow",
+			"B":      "bv",
+			"c": map[string]interface{}{
+				"b": "bv",
+			},
+		})
+
+		cfg.Merge("a", map[string]interface{}{
+			"c": map[string]interface{}{
+				"d": "dv2",
+			},
+			"e": "ev2",
+		})
+
+		c.Assert(cfg.Get("a"), qt.DeepEquals, maps.Params{
+			"e":      "ev2",
+			"_merge": maps.ParamsMergeStrategyShallow,
+			"b":      "bv",
+			"c": maps.Params{
+				"b": "bv",
+			},
+		})
+	})
+
+	c.Run("IsSet", func(c *qt.C) {
+		cfg := New()
+
+		cfg.Set("a", map[string]interface{}{
+			"B": "bv",
+		})
+
+		c.Assert(cfg.IsSet("A"), qt.IsTrue)
+		c.Assert(cfg.IsSet("a.b"), qt.IsTrue)
+		c.Assert(cfg.IsSet("z"), qt.IsFalse)
+	})
+
+	c.Run("Para", func(c *qt.C) {
+		cfg := New()
+		p := para.New(4)
+		r, _ := p.Start(context.Background())
+
+		setAndGet := func(k string, v int) error {
+			vs := strconv.Itoa(v)
+			cfg.Set(k, v)
+			err := errors.New("get failed")
+			if cfg.Get(k) != v {
+				return err
+			}
+			if cfg.GetInt(k) != v {
+				return err
+			}
+			if cfg.GetString(k) != vs {
+				return err
+			}
+			if !cfg.IsSet(k) {
+				return err
+			}
+			return nil
+		}
+
+		for i := 0; i < 20; i++ {
+			i := i
+			r.Run(func() error {
+				const v = 42
+				k := fmt.Sprintf("k%d", i)
+				if err := setAndGet(k, v); err != nil {
+					return err
+				}
+
+				m := maps.Params{
+					"new": 42,
+				}
+
+				cfg.Merge("", m)
+
+				return nil
+			})
+		}
+
+		c.Assert(r.Wait(), qt.IsNil)
+	})
+}
+
+func BenchmarkDefaultConfigProvider(b *testing.B) {
+	type cfger interface {
+		Get(key string) interface{}
+		Set(key string, value interface{})
+		IsSet(key string) bool
+	}
+
+	newMap := func() map[string]interface{} {
+		return map[string]interface{}{
+			"a": map[string]interface{}{
+				"b": map[string]interface{}{
+					"c": 32,
+					"d": 43,
+				},
+			},
+			"b": 62,
+		}
+	}
+
+	runMethods := func(b *testing.B, cfg cfger) {
+		m := newMap()
+		cfg.Set("mymap", m)
+		cfg.Set("num", 32)
+		if !(cfg.IsSet("mymap") && cfg.IsSet("mymap.a") && cfg.IsSet("mymap.a.b") && cfg.IsSet("mymap.a.b.c")) {
+			b.Fatal("IsSet failed")
+		}
+
+		if cfg.Get("num") != 32 {
+			b.Fatal("Get failed")
+		}
+
+		if cfg.Get("mymap.a.b.c") != 32 {
+			b.Fatal("Get failed")
+		}
+	}
+
+	b.Run("Viper", func(b *testing.B) {
+		v := viper.New()
+		for i := 0; i < b.N; i++ {
+			runMethods(b, v)
+		}
+	})
+
+	b.Run("Custom", func(b *testing.B) {
+		cfg := New()
+		for i := 0; i < b.N; i++ {
+			runMethods(b, cfg)
+		}
+	})
+}
diff --git a/config/docshelper.go b/config/docshelper.go
new file mode 100644
index 000000000..336a0dc16
--- /dev/null
+++ b/config/docshelper.go
@@ -0,0 +1,45 @@
+// Copyright 2021 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package config
+
+import (
+	"github.com/gohugoio/hugo/common/maps"
+	"github.com/gohugoio/hugo/docshelper"
+)
+
+// This is is just some helpers used to create some JSON used in the Hugo docs.
+func init() {
+	docsProvider := func() docshelper.DocProvider {
+
+		cfg := New()
+		for _, configRoot := range ConfigRootKeys {
+			cfg.Set(configRoot, make(maps.Params))
+		}
+		lang := maps.Params{
+			"en": maps.Params{
+				"menus":  maps.Params{},
+				"params": maps.Params{},
+			},
+		}
+		cfg.Set("languages", lang)
+		cfg.SetDefaultMergeStrategy()
+
+		configHelpers := map[string]interface{}{
+			"mergeStrategy": cfg.Get(""),
+		}
+		return docshelper.DocProvider{"config": configHelpers}
+	}
+
+	docshelper.AddDocProviderFunc(docsProvider)
+}
diff --git a/config/privacy/privacyConfig_test.go b/config/privacy/privacyConfig_test.go
index 0fb599c0a..c17ce713d 100644
--- a/config/privacy/privacyConfig_test.go
+++ b/config/privacy/privacyConfig_test.go
@@ -18,7 +18,6 @@ import (
 
 	qt "github.com/frankban/quicktest"
 	"github.com/gohugoio/hugo/config"
-	"github.com/spf13/viper"
 )
 
 func TestDecodeConfigFromTOML(t *testing.T) {
@@ -94,7 +93,7 @@ PrivacyENhanced = true
 func TestDecodeConfigDefault(t *testing.T) {
 	c := qt.New(t)
 
-	pc, err := DecodeConfig(viper.New())
+	pc, err := DecodeConfig(config.New())
 	c.Assert(err, qt.IsNil)
 	c.Assert(pc, qt.Not(qt.IsNil))
 	c.Assert(pc.YouTube.PrivacyEnhanced, qt.Equals, false)
diff --git a/config/services/servicesConfig_test.go b/config/services/servicesConfig_test.go
index 6e979b999..d7a52ba4f 100644
--- a/config/services/servicesConfig_test.go
+++ b/config/services/servicesConfig_test.go
@@ -18,7 +18,7 @@ import (
 
 	qt "github.com/frankban/quicktest"
 	"github.com/gohugoio/hugo/config"
-	"github.com/spf13/viper"
+	
 )
 
 func TestDecodeConfigFromTOML(t *testing.T) {
@@ -55,7 +55,7 @@ disableInlineCSS = true
 func TestUseSettingsFromRootIfSet(t *testing.T) {
 	c := qt.New(t)
 
-	cfg := viper.New()
+	cfg := config.New()
 	cfg.Set("disqusShortname", "root_short")
 	cfg.Set("googleAnalytics", "ga_root")