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, 315 insertions, 0 deletions

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)
+		}
+	})
+}