fix: sync pool race condition (#947)

Signed-off-by: Daniel Weiße <dw@edgeless.systems>

.github/workflows/cifuzz.yml

@@ -19,7 +19,7 @@ jobs:
         dry-run: false
         language: go
     - name: Upload Crash
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@v4
       if: failure() && steps.build.outcome == 'success'
       with:
         name: artifacts

.github/workflows/codeql-analysis.yml

@@ -39,7 +39,7 @@ jobs:
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL
-      uses: github/codeql-action/init@v2
+      uses: github/codeql-action/init@v3
       with:
         languages: ${{ matrix.language }}
         # If you wish to specify custom queries, you can do so here or in a config file.
@@ -50,7 +50,7 @@ jobs:
     # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
     # If this step fails, then you should remove it and run the build manually (see below)
     - name: Autobuild
-      uses: github/codeql-action/autobuild@v2
+      uses: github/codeql-action/autobuild@v3
 
     # ℹ️ Command-line programs to run using the OS shell.
     # 📚 https://git.io/JvXDl
@@ -64,4 +64,4 @@ jobs:
     #   make release
 
     - name: Perform CodeQL Analysis
-      uses: github/codeql-action/analyze@v2
+      uses: github/codeql-action/analyze@v3

.github/workflows/coverage.yml

@@ -13,8 +13,8 @@ jobs:
         with:
           fetch-depth: 0
       - name: Setup go
-        uses: actions/setup-go@v4
+        uses: actions/setup-go@v5
         with:
-          go-version: "1.21"
+          go-version: "1.22"
       - name: Run tests with coverage
         run: ./ci.sh coverage -d "${GITHUB_BASE_REF-HEAD}"

.github/workflows/release.yml

@@ -20,9 +20,9 @@ jobs:
         with:
           fetch-depth: 0
       - name: Set up Go
-        uses: actions/setup-go@v4
+        uses: actions/setup-go@v5
         with:
-          go-version: "1.21"
+          go-version: "1.22"
       - name: Login to GitHub Container Registry
         uses: docker/login-action@v3
         with:

.github/workflows/workflow.yml

@@ -11,8 +11,8 @@ jobs:
   build:
     strategy:
       matrix:
-        os: [ 'ubuntu-latest', 'windows-latest', 'macos-latest' ]
+        os: [ 'ubuntu-latest', 'windows-latest', 'macos-latest', 'macos-14' ]
-        go: [ '1.20', '1.21' ]
+        go: [ '1.21', '1.22' ]
     runs-on: ${{ matrix.os }}
     name: ${{ matrix.go }}/${{ matrix.os }}
     steps:
@@ -20,7 +20,7 @@ jobs:
         with:
           fetch-depth: 0
       - name: Setup go ${{ matrix.go }}
-        uses: actions/setup-go@v4
+        uses: actions/setup-go@v5
         with:
           go-version: ${{ matrix.go }}
       - name: Run unit tests

CONTRIBUTING.md

@@ -165,25 +165,22 @@ Checklist:
 
 ### New release
 
-1. Decide on the next version number. Use semver.
+1. Decide on the next version number. Use semver. Review commits since last
-2. Generate release notes using [`gh`][gh]. Example:
+   version to assess.
+2. Tag release. For example:
 ```
-$ gh api -X POST \
+git checkout v2
-  -F tag_name='v2.0.0-beta.5' \
+git pull
-  -F target_commitish='v2' \
+git tag v2.2.0
-  -F previous_tag_name='v2.0.0-beta.4' \
+git push --tags
-  --jq '.body' \
-  repos/pelletier/go-toml/releases/generate-notes
 ```
-3. Look for "Other changes". That would indicate a pull request not labeled
+3. CI automatically builds a draft Github release. Review it and edit as
-   properly. Tweak labels and pull request titles until changelog looks good for
+   necessary. Look for "Other changes". That would indicate a pull request not
-   users.
+   labeled properly. Tweak labels and pull request titles until changelog looks
-4. [Draft new release][new-release].
+   good for users.
-5. Fill tag and target with the same value used to generate the changelog.
+4. Check "create discussion" box, in the "Releases" category.
-6. Set title to the new tag value.
+5. If new version is an alpha or beta only, check pre-release box.
-7. Paste the generated changelog.
+
-8. Check "create discussion", in the "Releases" category.
-9. Check pre-release if new version is an alpha or beta.
 
 [issues-tracker]: https://github.com/pelletier/go-toml/issues
 [bug-report]: https://github.com/pelletier/go-toml/issues/new?template=bug_report.md

README.md

@@ -261,7 +261,7 @@ Those tools are also available as a [Docker image][docker]. For example, to use
 docker run -i ghcr.io/pelletier/go-toml:v2 tomljson < example.toml
 ```
 
-Multiple versions are availble on [ghcr.io][docker].
+Multiple versions are available on [ghcr.io][docker].
 
 [docker]: https://github.com/pelletier/go-toml/pkgs/container/go-toml
 
@@ -565,10 +565,11 @@ complete solutions exist out there.
 
 ## Versioning
 
-Go-toml follows [Semantic Versioning](https://semver.org). The supported version
+Expect for parts explicitely marked otherwise, go-toml follows [Semantic
-of [TOML](https://github.com/toml-lang/toml) is indicated at the beginning of
+Versioning](https://semver.org). The supported version of
-this document. The last two major versions of Go are supported
+[TOML](https://github.com/toml-lang/toml) is indicated at the beginning of this
-(see [Go Release Policy](https://golang.org/doc/devel/release.html#policy)).
+document. The last two major versions of Go are supported (see [Go Release
+Policy](https://golang.org/doc/devel/release.html#policy)).
 
 ## License
 

cmd/jsontoml/main.go

@@ -19,6 +19,7 @@ package main
 
 import (
 	"encoding/json"
+	"flag"
 	"io"
 
 	"github.com/pelletier/go-toml/v2"
@@ -33,7 +34,11 @@ Reading from a file:
   jsontoml file.json > file.toml
 `
 
+var useJsonNumber bool
+
 func main() {
+	flag.BoolVar(&useJsonNumber, "use-json-number", false, "unmarshal numbers into `json.Number` type instead of as `float64`")
+
 	p := cli.Program{
 		Usage: usage,
 		Fn:    convert,
@@ -45,11 +50,17 @@ func convert(r io.Reader, w io.Writer) error {
 	var v interface{}
 
 	d := json.NewDecoder(r)
+	e := toml.NewEncoder(w)
+
+	if useJsonNumber {
+		d.UseNumber()
+		e.SetMarshalJsonNumbers(true)
+	}
+
 	err := d.Decode(&v)
 	if err != nil {
 		return err
 	}
 
-	e := toml.NewEncoder(w)
 	return e.Encode(v)
 }

cmd/jsontoml/main_test.go

@@ -15,6 +15,7 @@ func TestConvert(t *testing.T) {
 		input         string
 		expected      string
 		errors        bool
+		useJsonNumber bool
 	}{
 		{
 			name: "valid json",
@@ -26,6 +27,19 @@ func TestConvert(t *testing.T) {
 }`,
 			expected: `[mytoml]
 a = 42.0
+`,
+		},
+		{
+			name:          "use json number",
+			useJsonNumber: true,
+			input: `
+{
+  "mytoml": {
+    "a": 42
+  }
+}`,
+			expected: `[mytoml]
+a = 42
 `,
 		},
 		{
@@ -37,6 +51,7 @@ a = 42.0
 
 	for _, e := range examples {
 		b := new(bytes.Buffer)
+		useJsonNumber = e.useJsonNumber
 		err := convert(strings.NewReader(e.input), b)
 		if e.errors {
 			require.Error(t, err)

fuzz_test.go

@@ -1,5 +1,5 @@
-//go:build go1.18 || go1.19 || go1.20 || go1.21
+//go:build go1.18 || go1.19 || go1.20 || go1.21 || go1.22
-// +build go1.18 go1.19 go1.20 go1.21
+// +build go1.18 go1.19 go1.20 go1.21 go1.22
 
 package toml_test
 

go.mod

@@ -2,4 +2,4 @@ module github.com/pelletier/go-toml/v2
 
 go 1.16
 
-require github.com/stretchr/testify v1.8.4
+require github.com/stretchr/testify v1.9.0

go.sum

@@ -6,10 +6,12 @@ github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZN
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
 github.com/stretchr/objx v0.5.0/go.mod h1:Yh+to48EsGEfYuaHDzXPcE3xhTkx73EhmCGUpEOglKo=
+github.com/stretchr/objx v0.5.2/go.mod h1:FRsXN1f5AsAjCGJKqEizvkpNtU+EGNCLh3NxZ/8L+MA=
 github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
-github.com/stretchr/testify v1.8.4 h1:CcVxjf3Q8PM0mHUKJCdn+eZZtm5yQwehR5yeSVQQcUk=
 github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
+github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

internal/tracker/seen.go

@@ -57,7 +57,11 @@ type SeenTracker struct {
 	currentIdx int
 }
 
-var pool sync.Pool
+var pool = sync.Pool{
+	New: func() interface{} {
+		return &SeenTracker{}
+	},
+}
 
 func (s *SeenTracker) reset() {
 	// Always contains a root element at index 0.
@@ -149,8 +153,9 @@ func (s *SeenTracker) setExplicitFlag(parentIdx int) {
 
 // CheckExpression takes a top-level node and checks that it does not contain
 // keys that have been seen in previous calls, and validates that types are
-// consistent.
+// consistent. It returns true if it is the first time this node's key is seen.
-func (s *SeenTracker) CheckExpression(node *unstable.Node) error {
+// Useful to clear array tables on first use.
+func (s *SeenTracker) CheckExpression(node *unstable.Node) (bool, error) {
 	if s.entries == nil {
 		s.reset()
 	}
@@ -166,7 +171,7 @@ func (s *SeenTracker) CheckExpression(node *unstable.Node) error {
 	}
 }
 
-func (s *SeenTracker) checkTable(node *unstable.Node) error {
+func (s *SeenTracker) checkTable(node *unstable.Node) (bool, error) {
 	if s.currentIdx >= 0 {
 		s.setExplicitFlag(s.currentIdx)
 	}
@@ -192,7 +197,7 @@ func (s *SeenTracker) checkTable(node *unstable.Node) error {
 		} else {
 			entry := s.entries[idx]
 			if entry.kind == valueKind {
-				return fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
+				return false, fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
 			}
 		}
 		parentIdx = idx
@@ -201,25 +206,27 @@ func (s *SeenTracker) checkTable(node *unstable.Node) error {
 	k := it.Node().Data
 	idx := s.find(parentIdx, k)
 
+	first := false
 	if idx >= 0 {
 		kind := s.entries[idx].kind
 		if kind != tableKind {
-			return fmt.Errorf("toml: key %s should be a table, not a %s", string(k), kind)
+			return false, fmt.Errorf("toml: key %s should be a table, not a %s", string(k), kind)
 		}
 		if s.entries[idx].explicit {
-			return fmt.Errorf("toml: table %s already exists", string(k))
+			return false, fmt.Errorf("toml: table %s already exists", string(k))
 		}
 		s.entries[idx].explicit = true
 	} else {
 		idx = s.create(parentIdx, k, tableKind, true, false)
+		first = true
 	}
 
 	s.currentIdx = idx
 
-	return nil
+	return first, nil
 }
 
-func (s *SeenTracker) checkArrayTable(node *unstable.Node) error {
+func (s *SeenTracker) checkArrayTable(node *unstable.Node) (bool, error) {
 	if s.currentIdx >= 0 {
 		s.setExplicitFlag(s.currentIdx)
 	}
@@ -242,7 +249,7 @@ func (s *SeenTracker) checkArrayTable(node *unstable.Node) error {
 		} else {
 			entry := s.entries[idx]
 			if entry.kind == valueKind {
-				return fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
+				return false, fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
 			}
 		}
 
@@ -252,22 +259,23 @@ func (s *SeenTracker) checkArrayTable(node *unstable.Node) error {
 	k := it.Node().Data
 	idx := s.find(parentIdx, k)
 
-	if idx >= 0 {
+	firstTime := idx < 0
+	if firstTime {
+		idx = s.create(parentIdx, k, arrayTableKind, true, false)
+	} else {
 		kind := s.entries[idx].kind
 		if kind != arrayTableKind {
-			return fmt.Errorf("toml: key %s already exists as a %s,  but should be an array table", kind, string(k))
+			return false, fmt.Errorf("toml: key %s already exists as a %s,  but should be an array table", kind, string(k))
 		}
 		s.clear(idx)
-	} else {
-		idx = s.create(parentIdx, k, arrayTableKind, true, false)
 	}
 
 	s.currentIdx = idx
 
-	return nil
+	return firstTime, nil
 }
 
-func (s *SeenTracker) checkKeyValue(node *unstable.Node) error {
+func (s *SeenTracker) checkKeyValue(node *unstable.Node) (bool, error) {
 	parentIdx := s.currentIdx
 	it := node.Key()
 
@@ -281,11 +289,11 @@ func (s *SeenTracker) checkKeyValue(node *unstable.Node) error {
 		} else {
 			entry := s.entries[idx]
 			if it.IsLast() {
-				return fmt.Errorf("toml: key %s is already defined", string(k))
+				return false, fmt.Errorf("toml: key %s is already defined", string(k))
 			} else if entry.kind != tableKind {
-				return fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
+				return false, fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
 			} else if entry.explicit {
-				return fmt.Errorf("toml: cannot redefine table %s that has already been explicitly defined", string(k))
+				return false, fmt.Errorf("toml: cannot redefine table %s that has already been explicitly defined", string(k))
 			}
 		}
 
@@ -303,45 +311,39 @@ func (s *SeenTracker) checkKeyValue(node *unstable.Node) error {
 		return s.checkArray(value)
 	}
 
-	return nil
+	return false, nil
 }
 
-func (s *SeenTracker) checkArray(node *unstable.Node) error {
+func (s *SeenTracker) checkArray(node *unstable.Node) (first bool, err error) {
 	it := node.Children()
 	for it.Next() {
 		n := it.Node()
 		switch n.Kind {
 		case unstable.InlineTable:
-			err := s.checkInlineTable(n)
+			first, err = s.checkInlineTable(n)
 			if err != nil {
-				return err
+				return false, err
 			}
 		case unstable.Array:
-			err := s.checkArray(n)
+			first, err = s.checkArray(n)
 			if err != nil {
-				return err
+				return false, err
 			}
 		}
 	}
-	return nil
+	return first, nil
-}
-
-func (s *SeenTracker) checkInlineTable(node *unstable.Node) error {
-	if pool.New == nil {
-		pool.New = func() interface{} {
-			return &SeenTracker{}
-		}
 }
 
+func (s *SeenTracker) checkInlineTable(node *unstable.Node) (first bool, err error) {
 	s = pool.Get().(*SeenTracker)
 	s.reset()
 
 	it := node.Children()
 	for it.Next() {
 		n := it.Node()
-		err := s.checkKeyValue(n)
+		first, err = s.checkKeyValue(n)
 		if err != nil {
-			return err
+			return false, err
 		}
 	}
 
@@ -352,5 +354,5 @@ func (s *SeenTracker) checkInlineTable(node *unstable.Node) error {
 	// redefinition of its keys: check* functions cannot walk into
 	// a value.
 	pool.Put(s)
-	return nil
+	return first, nil
 }

marshaler.go

@@ -3,6 +3,7 @@ package toml
 import (
 	"bytes"
 	"encoding"
+	"encoding/json"
 	"fmt"
 	"io"
 	"math"
@@ -41,6 +42,7 @@ type Encoder struct {
 	arraysMultiline    bool
 	indentSymbol       string
 	indentTables       bool
+	marshalJsonNumbers bool
 }
 
 // NewEncoder returns a new Encoder that writes to w.
@@ -87,6 +89,17 @@ func (enc *Encoder) SetIndentTables(indent bool) *Encoder {
 	return enc
 }
 
+// SetMarshalJsonNumbers forces the encoder to serialize `json.Number` as a
+// float or integer instead of relying on TextMarshaler to emit a string.
+//
+// *Unstable:* This method does not follow the compatibility guarantees of
+// semver. It can be changed or removed without a new major version being
+// issued.
+func (enc *Encoder) SetMarshalJsonNumbers(indent bool) *Encoder {
+	enc.marshalJsonNumbers = indent
+	return enc
+}
+
 // Encode writes a TOML representation of v to the stream.
 //
 // If v cannot be represented to TOML it returns an error.
@@ -252,6 +265,18 @@ func (enc *Encoder) encode(b []byte, ctx encoderCtx, v reflect.Value) ([]byte, e
 		return append(b, x.String()...), nil
 	case LocalDateTime:
 		return append(b, x.String()...), nil
+	case json.Number:
+		if enc.marshalJsonNumbers {
+			if x == "" { /// Useful zero value.
+				return append(b, "0"...), nil
+			} else if v, err := x.Int64(); err == nil {
+				return enc.encode(b, ctx, reflect.ValueOf(v))
+			} else if f, err := x.Float64(); err == nil {
+				return enc.encode(b, ctx, reflect.ValueOf(f))
+			} else {
+				return nil, fmt.Errorf("toml: unable to convert %q to int64 or float64", x)
+			}
+		}
 	}
 
 	hasTextMarshaler := v.Type().Implements(textMarshalerType)
@@ -707,6 +732,8 @@ func walkStruct(ctx encoderCtx, t *table, v reflect.Value) {
 			if fieldType.Anonymous {
 				if fieldType.Type.Kind() == reflect.Struct {
 					walkStruct(ctx, t, f)
+				} else if fieldType.Type.Kind() == reflect.Pointer && !f.IsNil() && f.Elem().Kind() == reflect.Struct {
+					walkStruct(ctx, t, f.Elem())
 				}
 				continue
 			} else {
@@ -998,6 +1025,10 @@ func (enc *Encoder) encodeSliceAsArrayTable(b []byte, ctx encoderCtx, v reflect.
 
 	scratch = enc.commented(ctx.commented, scratch)
 
+	if enc.indentTables {
+		scratch = enc.indent(ctx.indent, scratch)
+	}
+
 	scratch = append(scratch, "[["...)
 
 	for i, k := range ctx.parentKey {

marshaler_test.go

@@ -948,6 +948,29 @@ func TestEncoderSetIndentSymbol(t *testing.T) {
 	assert.Equal(t, expected, w.String())
 }
 
+func TestEncoderSetMarshalJsonNumbers(t *testing.T) {
+	var w strings.Builder
+	enc := toml.NewEncoder(&w)
+	enc.SetMarshalJsonNumbers(true)
+	err := enc.Encode(map[string]interface{}{
+		"A": json.Number("1.1"),
+		"B": json.Number("42e-3"),
+		"C": json.Number("42"),
+		"D": json.Number("0"),
+		"E": json.Number("0.0"),
+		"F": json.Number(""),
+	})
+	require.NoError(t, err)
+	expected := `A = 1.1
+B = 0.042
+C = 42
+D = 0
+E = 0.0
+F = 0
+`
+	assert.Equal(t, expected, w.String())
+}
+
 func TestEncoderOmitempty(t *testing.T) {
 	type doc struct {
 		String  string            `toml:",omitempty,multiline"`
@@ -1304,6 +1327,38 @@ value = ''
 	require.Equal(t, expected, string(result))
 }
 
+func TestMarshalNestedAnonymousStructs_PointerEmbedded(t *testing.T) {
+	type Embedded struct {
+		Value   string  `toml:"value" json:"value"`
+		Omitted string  `toml:"omitted,omitempty"`
+		Ptr     *string `toml:"ptr"`
+	}
+
+	type Named struct {
+		Value string `toml:"value" json:"value"`
+	}
+
+	type Doc struct {
+		*Embedded
+		*Named    `toml:"named" json:"named"`
+		Anonymous struct {
+			*Embedded
+			Value *string `toml:"value" json:"value"`
+		} `toml:"anonymous,omitempty" json:"anonymous,omitempty"`
+	}
+
+	doc := &Doc{
+		Embedded: &Embedded{Value: "foo"},
+	}
+
+	expected := `value = 'foo'
+`
+
+	result, err := toml.Marshal(doc)
+	require.NoError(t, err)
+	require.Equal(t, expected, string(result))
+}
+
 func TestLocalTime(t *testing.T) {
 	v := map[string]toml.LocalTime{
 		"a": {
@@ -1444,6 +1499,43 @@ func TestMarshalCommented(t *testing.T) {
 	require.Equal(t, expected, string(out))
 }
 
+func TestMarshalIndentedCustomTypeArray(t *testing.T) {
+	c := struct {
+		Nested struct {
+			NestedArray []struct {
+				Value int
+			}
+		}
+	}{
+		Nested: struct {
+			NestedArray []struct {
+				Value int
+			}
+		}{
+			NestedArray: []struct {
+				Value int
+			}{
+				{Value: 1},
+				{Value: 2},
+			},
+		},
+	}
+
+	expected := `[Nested]
+  [[Nested.NestedArray]]
+    Value = 1
+
+  [[Nested.NestedArray]]
+    Value = 2
+`
+
+	var buf bytes.Buffer
+	enc := toml.NewEncoder(&buf)
+	enc.SetIndentTables(true)
+	require.NoError(t, enc.Encode(c))
+	require.Equal(t, expected, buf.String())
+}
+
 func ExampleMarshal() {
 	type MyConfig struct {
 		Version int

ossfuzz/fuzz.go

@@ -1,5 +1,5 @@
-//go:build go1.18 || go1.19 || go1.20 || go1.21
+//go:build go1.18 || go1.19 || go1.20 || go1.21 || go1.22
-// +build go1.18 go1.19 go1.20 go1.21
+// +build go1.18 go1.19 go1.20 go1.21 go1.22
 
 package ossfuzz
 

unmarshaler.go

@@ -35,6 +35,9 @@ type Decoder struct {
 
 	// global settings
 	strict bool
+
+	// toggles unmarshaler interface
+	unmarshalerInterface bool
 }
 
 // NewDecoder creates a new Decoder that will read from r.
@@ -54,6 +57,24 @@ func (d *Decoder) DisallowUnknownFields() *Decoder {
 	return d
 }
 
+// EnableUnmarshalerInterface allows to enable unmarshaler interface.
+//
+// With this feature enabled, types implementing the unstable/Unmarshaler
+// interface can be decoded from any structure of the document. It allows types
+// that don't have a straightfoward TOML representation to provide their own
+// decoding logic.
+//
+// Currently, types can only decode from a single value. Tables and array tables
+// are not supported.
+//
+// *Unstable:* This method does not follow the compatibility guarantees of
+// semver. It can be changed or removed without a new major version being
+// issued.
+func (d *Decoder) EnableUnmarshalerInterface() *Decoder {
+	d.unmarshalerInterface = true
+	return d
+}
+
 // Decode the whole content of r into v.
 //
 // By default, values in the document that don't exist in the target Go value
@@ -108,6 +129,7 @@ func (d *Decoder) Decode(v interface{}) error {
 		strict: strict{
 			Enabled: d.strict,
 		},
+		unmarshalerInterface: d.unmarshalerInterface,
 	}
 
 	return dec.FromParser(v)
@@ -127,6 +149,10 @@ type decoder struct {
 	// need to be skipped.
 	skipUntilTable bool
 
+	// Flag indicating that the current array/slice table should be cleared because
+	// it is the first encounter of an array table.
+	clearArrayTable bool
+
 	// Tracks position in Go arrays.
 	// This is used when decoding [[array tables]] into Go arrays. Given array
 	// tables are separate TOML expression, we need to keep track of where we
@@ -139,6 +165,9 @@ type decoder struct {
 	// Strict mode
 	strict strict
 
+	// Flag that enables/disables unmarshaler interface.
+	unmarshalerInterface bool
+
 	// Current context for the error.
 	errorContext *errorContext
 }
@@ -246,9 +275,10 @@ Rules for the unmarshal code:
 func (d *decoder) handleRootExpression(expr *unstable.Node, v reflect.Value) error {
 	var x reflect.Value
 	var err error
+	var first bool // used for to clear array tables on first use
 
 	if !(d.skipUntilTable && expr.Kind == unstable.KeyValue) {
-		err = d.seen.CheckExpression(expr)
+		first, err = d.seen.CheckExpression(expr)
 		if err != nil {
 			return err
 		}
@@ -267,6 +297,7 @@ func (d *decoder) handleRootExpression(expr *unstable.Node, v reflect.Value) err
 	case unstable.ArrayTable:
 		d.skipUntilTable = false
 		d.strict.EnterArrayTable(expr)
+		d.clearArrayTable = first
 		x, err = d.handleArrayTable(expr.Key(), v)
 	default:
 		panic(fmt.Errorf("parser should not permit expression of kind %s at document root", expr.Kind))
@@ -307,6 +338,10 @@ func (d *decoder) handleArrayTableCollectionLast(key unstable.Iterator, v reflec
 				reflect.Copy(nelem, elem)
 				elem = nelem
 			}
+			if d.clearArrayTable && elem.Len() > 0 {
+				elem.SetLen(0)
+				d.clearArrayTable = false
+			}
 		}
 		return d.handleArrayTableCollectionLast(key, elem)
 	case reflect.Ptr:
@@ -325,6 +360,10 @@ func (d *decoder) handleArrayTableCollectionLast(key unstable.Iterator, v reflec
 
 		return v, nil
 	case reflect.Slice:
+		if d.clearArrayTable && v.Len() > 0 {
+			v.SetLen(0)
+			d.clearArrayTable = false
+		}
 		elemType := v.Type().Elem()
 		var elem reflect.Value
 		if elemType.Kind() == reflect.Interface {
@@ -576,7 +615,7 @@ func (d *decoder) handleKeyValues(v reflect.Value) (reflect.Value, error) {
 			break
 		}
 
-		err := d.seen.CheckExpression(expr)
+		_, err := d.seen.CheckExpression(expr)
 		if err != nil {
 			return reflect.Value{}, err
 		}
@@ -634,6 +673,14 @@ func (d *decoder) handleValue(value *unstable.Node, v reflect.Value) error {
 		v = initAndDereferencePointer(v)
 	}
 
+	if d.unmarshalerInterface {
+		if v.CanAddr() && v.Addr().CanInterface() {
+			if outi, ok := v.Addr().Interface().(unstable.Unmarshaler); ok {
+				return outi.UnmarshalTOML(value)
+			}
+		}
+	}
+
 	ok, err := d.tryTextUnmarshaler(value, v)
 	if ok || err != nil {
 		return err

unmarshaler_test.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/pelletier/go-toml/v2"
+	"github.com/pelletier/go-toml/v2/unstable"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -2823,6 +2824,76 @@ blah.a = "def"`)
 	require.Equal(t, "def", cfg.A)
 }
 
+func TestIssue931(t *testing.T) {
+	type item struct {
+		Name string
+	}
+
+	type items struct {
+		Slice []item
+	}
+
+	its := items{[]item{{"a"}, {"b"}}}
+
+	b := []byte(`
+	[[Slice]]
+  Name = 'c'
+
+[[Slice]]
+  Name = 'd'
+	`)
+
+	toml.Unmarshal(b, &its)
+	require.Equal(t, items{[]item{{"c"}, {"d"}}}, its)
+}
+
+func TestIssue931Interface(t *testing.T) {
+	type items struct {
+		Slice interface{}
+	}
+
+	type item = map[string]interface{}
+
+	its := items{[]interface{}{item{"Name": "a"}, item{"Name": "b"}}}
+
+	b := []byte(`
+	[[Slice]]
+  Name = 'c'
+
+[[Slice]]
+  Name = 'd'
+	`)
+
+	toml.Unmarshal(b, &its)
+	require.Equal(t, items{[]interface{}{item{"Name": "c"}, item{"Name": "d"}}}, its)
+}
+
+func TestIssue931SliceInterface(t *testing.T) {
+	type items struct {
+		Slice []interface{}
+	}
+
+	type item = map[string]interface{}
+
+	its := items{
+		[]interface{}{
+			item{"Name": "a"},
+			item{"Name": "b"},
+		},
+	}
+
+	b := []byte(`
+	[[Slice]]
+  Name = 'c'
+
+[[Slice]]
+  Name = 'd'
+	`)
+
+	toml.Unmarshal(b, &its)
+	require.Equal(t, items{[]interface{}{item{"Name": "c"}, item{"Name": "d"}}}, its)
+}
+
 func TestUnmarshalDecodeErrors(t *testing.T) {
 	examples := []struct {
 		desc string
@@ -3702,3 +3773,95 @@ func TestUnmarshal_Nil(t *testing.T) {
 		})
 	}
 }
+
+type CustomUnmarshalerKey struct {
+	A int64
+}
+
+func (k *CustomUnmarshalerKey) UnmarshalTOML(value *unstable.Node) error {
+	item, err := strconv.ParseInt(string(value.Data), 10, 64)
+	if err != nil {
+		return fmt.Errorf("error converting to int64, %v", err)
+	}
+	k.A = item
+	return nil
+
+}
+
+func TestUnmarshal_CustomUnmarshaler(t *testing.T) {
+	type MyConfig struct {
+		Unmarshalers []CustomUnmarshalerKey `toml:"unmarshalers"`
+		Foo          *string                `toml:"foo,omitempty"`
+	}
+
+	examples := []struct {
+		desc                        string
+		disableUnmarshalerInterface bool
+		input                       string
+		expected                    MyConfig
+		err                         bool
+	}{
+		{
+			desc:     "empty",
+			input:    ``,
+			expected: MyConfig{Unmarshalers: []CustomUnmarshalerKey{}, Foo: nil},
+		},
+		{
+			desc:  "simple",
+			input: `unmarshalers = [1,2,3]`,
+			expected: MyConfig{
+				Unmarshalers: []CustomUnmarshalerKey{
+					{A: 1},
+					{A: 2},
+					{A: 3},
+				},
+				Foo: nil,
+			},
+		},
+		{
+			desc: "unmarshal string and custom unmarshaler",
+			input: `unmarshalers = [1,2,3]
+foo = "bar"`,
+			expected: MyConfig{
+				Unmarshalers: []CustomUnmarshalerKey{
+					{A: 1},
+					{A: 2},
+					{A: 3},
+				},
+				Foo: func(v string) *string {
+					return &v
+				}("bar"),
+			},
+		},
+		{
+			desc:                        "simple example, but unmarshaler interface disabled",
+			disableUnmarshalerInterface: true,
+			input:                       `unmarshalers = [1,2,3]`,
+			err:                         true,
+		},
+	}
+
+	for _, ex := range examples {
+		e := ex
+		t.Run(e.desc, func(t *testing.T) {
+			foo := MyConfig{}
+
+			decoder := toml.NewDecoder(bytes.NewReader([]byte(e.input)))
+			if !ex.disableUnmarshalerInterface {
+				decoder.EnableUnmarshalerInterface()
+			}
+			err := decoder.Decode(&foo)
+
+			if e.err {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+				require.Equal(t, len(foo.Unmarshalers), len(e.expected.Unmarshalers))
+				for i := 0; i < len(foo.Unmarshalers); i++ {
+					require.Equal(t, foo.Unmarshalers[i], e.expected.Unmarshalers[i])
+				}
+				require.Equal(t, foo.Foo, e.expected.Foo)
+			}
+		})
+	}
+}

unstable/unmarshaler.go

@@ -0,0 +1,7 @@
+package unstable
+
+// The Unmarshaler interface may be implemented by types to customize their
+// behavior when being unmarshaled from a TOML document.
+type Unmarshaler interface {
+	UnmarshalTOML(value *Node) error
+}