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

1 files changed, 372 insertions, 0 deletions

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
+}