feat: add the required tag option and UnmarshalerContext

Assisted-by: GLM 5.3 Flash

CHANGELOG.md

@@ -46,6 +46,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   (10000 levels, which no hand-written document approaches): a document that
   nests arrays or inline tables deeper used to run the stack out and is now
   rejected with a `SyntaxError` naming the limit.
+- The `toml` tag gained the `required` option: a field tagged
+  `toml:"host,required"` makes the decode fail with
+  `missing required key "host"` when the document carries no key that
+  resolves to it. The option shapes decoding only, and the encoder ignores
+  it.
+- `UnmarshalerContext`, the custom-decode interface that hands the decode's
+  context to the method, `UnmarshalTOMLContext(ctx, data)`. It wins over
+  `UnmarshalTOML` when a type implements both, so a long custom decode can
+  abort on cancellation; a non-cancellable entry point hands in
+  `context.Background`, never nil.
 - A TOML array decodes into a Go fixed-size array, `[N]T`, where only a slice
   was accepted before; the encoder could already encode one. A length mismatch
   is an error wrapped with the key path.

decode.go

@@ -4,6 +4,7 @@
 package interpres
 
 import (
+	"context"
 	"encoding"
 	"fmt"
 	"reflect"
@@ -14,17 +15,30 @@ import (
 	"time"
 )
 
-// decoder maps a parsed TOML tree onto Go values via reflection.
+// decoder maps a parsed TOML tree onto Go values via reflection. ctx is the
+// context a cancellable entry point handed in, and reaches an
+// UnmarshalerContext destination; entry points without one leave it nil.
 type decoder struct {
 	disallowUnknown bool
+	ctx             context.Context
 }
 
 func newDecoder() *decoder { return &decoder{} }
 
+// ctxOrBackground returns the context the decode carries, and Background when
+// none was given, so a custom decoder never receives a nil context.
+func (d *decoder) ctxOrBackground() context.Context {
+	if d.ctx == nil {
+		return context.Background()
+	}
+	return d.ctx
+}
+
 var timeType = reflect.TypeFor[time.Time]()
 
 var (
 	unmarshalerType     = reflect.TypeFor[Unmarshaler]()
+	ctxUnmarshalerType  = reflect.TypeFor[UnmarshalerContext]()
 	textUnmarshalerType = reflect.TypeFor[encoding.TextUnmarshaler]()
 	numberType          = reflect.TypeFor[Number]()
 )
@@ -36,6 +50,8 @@ var (
 const (
 	flagUnmarshaler uint8 = 1 << iota
 	flagAddrUnmarshaler
+	flagCtxUnmarshaler
+	flagAddrCtxUnmarshaler
 	flagTextUnmarshaler
 	flagAddrTextUnmarshaler
 )
@@ -72,10 +88,16 @@ func typeFlags(t reflect.Type) uint8 {
 	if t.Implements(unmarshalerType) {
 		f |= flagUnmarshaler
 	}
+	if t.Implements(ctxUnmarshalerType) {
+		f |= flagCtxUnmarshaler
+	}
 	pt := reflect.PointerTo(t)
 	if pt.Implements(unmarshalerType) {
 		f |= flagAddrUnmarshaler
 	}
+	if pt.Implements(ctxUnmarshalerType) {
+		f |= flagAddrCtxUnmarshaler
+	}
 	// The date-time types are excluded from the text path: they carry
 	// time.Time's UnmarshalText through an embedded field while their only
 	// accepted form is a bare timestamp.
@@ -115,6 +137,24 @@ func unmarshalerOf(dst reflect.Value) (Unmarshaler, bool) {
 	return nil, false
 }
 
+// ctxUnmarshalerOf is the same resolution for UnmarshalerContext.
+func ctxUnmarshalerOf(dst reflect.Value) (UnmarshalerContext, bool) {
+	if dst.Kind() == reflect.Interface {
+		u, ok := dst.Interface().(UnmarshalerContext)
+		return u, ok
+	}
+	f := typeFlags(dst.Type())
+	if f&flagCtxUnmarshaler != 0 {
+		u, ok := dst.Interface().(UnmarshalerContext)
+		return u, ok
+	}
+	if f&flagAddrCtxUnmarshaler != 0 && dst.CanAddr() {
+		u, ok := dst.Addr().Interface().(UnmarshalerContext)
+		return u, ok
+	}
+	return nil, false
+}
+
 func (d *decoder) decode(tree map[string]any, v any) error {
 	rv := reflect.ValueOf(v)
 	if rv.Kind() != reflect.Pointer || rv.IsNil() {
@@ -139,12 +179,18 @@ func (d *decoder) assign(data any, dst reflect.Value) error {
 		return nil
 	}
 
-	// Types implementing Unmarshaler get the parsed data wholesale and are
-	// responsible for setting their own state. The decoder does not consult
-	// any return value; whatever the receiver stores is kept. The lookup
-	// covers both T and *T so a pointer-receiver UnmarshalTOML method is
-	// invoked on an addressable struct field.
+	// Types implementing UnmarshalerContext get the context beside the parsed
+	// data, and are responsible for setting their own state. They win over
+	// Unmarshaler, which wins over the text path. The lookups cover both T and
+	// *T so a pointer-receiver method is invoked on an addressable struct
+	// field.
 	if dst.CanInterface() {
+		if u, ok := ctxUnmarshalerOf(dst); ok {
+			if err := u.UnmarshalTOMLContext(d.ctxOrBackground(), data); err != nil {
+				return fmt.Errorf("unmarshal: %w", err)
+			}
+			return nil
+		}
 		u, ok := dst.Interface().(Unmarshaler)
 		if !ok && dst.CanAddr() {
 			u, ok = dst.Addr().Interface().(Unmarshaler)
@@ -258,12 +304,20 @@ func (d *decoder) assignStruct(tbl map[string]any, dst reflect.Value) error {
 			return fmt.Errorf("interpres: unknown field %q for %s", unknown, dst.Type())
 		}
 	}
+	// The keys that resolved to a field are remembered while the table walks,
+	// but only a struct that demands one pays for the set.
+	var seen map[string]bool
+	if len(schema.required) > 0 {
+		seen = make(map[string]bool, len(tbl))
+	}
 	for key, val := range tbl {
 		// A key that is already lowercase, which document keys usually are,
 		// hits the map directly; only a miss pays for the case fold.
+		resolved := key
 		field, ok := schema.byName[key]
 		if !ok {
-			field, ok = schema.byName[strings.ToLower(key)]
+			resolved = strings.ToLower(key)
+			field, ok = schema.byName[resolved]
 		}
 		if !ok {
 			if schema.embedMaps != nil {
@@ -279,6 +333,9 @@ func (d *decoder) assignStruct(tbl map[string]any, dst reflect.Value) error {
 			}
 			continue
 		}
+		if seen != nil {
+			seen[resolved] = true
+		}
 		fv, err := fieldByIndex(dst, field.index)
 		if err != nil {
 			return newDecodeError(key, err)
@@ -287,6 +344,11 @@ func (d *decoder) assignStruct(tbl map[string]any, dst reflect.Value) error {
 			return newDecodeError(key, err)
 		}
 	}
+	for _, key := range schema.required {
+		if !seen[key] {
+			return fmt.Errorf("interpres: missing required key %q", key)
+		}
+	}
 	return nil
 }
 
@@ -488,10 +550,12 @@ func setFloat(dst reflect.Value, v float64) error {
 
 // structFieldLoc locates one destination field by its index path from the
 // struct root and by the depth the field sits at, which breaks name clashes
-// in favour of the shallower field.
+// in favour of the shallower field. required records the tag option of the
+// field that won the name.
 type structFieldLoc struct {
-	index []int
-	depth int
+	index    []int
+	depth    int
+	required bool
 }
 
 // structSchema flattens the exported fields of t for decode, mirroring the
@@ -499,10 +563,11 @@ type structFieldLoc struct {
 // keys of the same table, and an untagged embedded map is recorded in
 // embedMaps (first declaration first) as the destination for leftover keys.
 // When two fields resolve to one name, the shallower wins, then the later
-// declaration.
+// declaration. required holds the keys a `toml:"...,required"` tag demands.
 type structSchema struct {
 	byName    map[string]structFieldLoc
 	embedMaps [][]int
+	required  []string
 }
 
 // structSchemaCache holds one schema per struct type. A schema is immutable
@@ -538,11 +603,20 @@ func newStructSchema(t reflect.Type) structSchema {
 			}
 			path := append(append([]int{}, prefix...), i)
 			name := ""
+			required := false
 			if tag, ok := f.Tag.Lookup("toml"); ok {
-				name, _, _ = strings.Cut(tag, ",")
+				var opts string
+				name, opts, _ = strings.Cut(tag, ",")
 				if name == "-" {
 					continue
 				}
+				for opts != "" {
+					var opt string
+					opt, opts, _ = strings.Cut(opts, ",")
+					if opt == "required" {
+						required = true
+					}
+				}
 			}
 			if f.Anonymous && name == "" {
 				ft := f.Type
@@ -566,11 +640,19 @@ func newStructSchema(t reflect.Type) structSchema {
 			}
 			key := strings.ToLower(name)
 			if existing, ok := s.byName[key]; !ok || depth <= existing.depth {
-				s.byName[key] = structFieldLoc{index: path, depth: depth}
+				s.byName[key] = structFieldLoc{index: path, depth: depth, required: required}
 			}
 		}
 	}
 	walk(t, nil, 0)
+	// The missing-key error must not depend on map order, so the demanded keys
+	// come out sorted.
+	for key, loc := range s.byName {
+		if loc.required {
+			s.required = append(s.required, key)
+		}
+	}
+	slices.Sort(s.required)
 	return s
 }
 

decode_test.go

@@ -1316,3 +1316,110 @@ func TestDecodeFixedArray(t *testing.T) {
 		}
 	})
 }
+
+func TestRequiredTag(t *testing.T) {
+	type Config struct {
+		Host    string `toml:"host,required"`
+		Radius  int    `toml:"radius"`
+	}
+	t.Run("a present key satisfies the tag", func(t *testing.T) {
+		var cfg Config
+		if err := Unmarshal([]byte("radius = 2\nhost = \"example.org\"\n"), &cfg); err != nil {
+			t.Fatal(err)
+		}
+		if cfg.Host != "example.org" || cfg.Radius != 2 {
+			t.Errorf("decoded %+v", cfg)
+		}
+	})
+	t.Run("a missing key is an error", func(t *testing.T) {
+		var cfg Config
+		err := Unmarshal([]byte("radius = 2\n"), &cfg)
+		want := `interpres: missing required key "host"`
+		if err == nil || err.Error() != want {
+			t.Errorf("err = %v, want %q", err, want)
+		}
+	})
+	t.Run("the error carries the key path", func(t *testing.T) {
+		var outer struct {
+			Server Config `toml:"server"`
+		}
+		err := Unmarshal([]byte("[server]\nradius = 1\n"), &outer)
+		want := `server: interpres: missing required key "host"`
+		if err == nil || err.Error() != want {
+			t.Errorf("err = %v, want %q", err, want)
+		}
+	})
+	t.Run("case-insensitive match satisfies the tag", func(t *testing.T) {
+		var cfg Config
+		if err := Unmarshal([]byte("HOST = \"x\"\n"), &cfg); err != nil {
+			t.Errorf("err = %v, want nil", err)
+		}
+	})
+}
+
+type ctxRecorder struct {
+	got   context.Context
+	value any
+}
+
+func (r *ctxRecorder) UnmarshalTOMLContext(ctx context.Context, data any) error {
+	r.got = ctx
+	r.value = data
+	return nil
+}
+
+func TestUnmarshalerContext(t *testing.T) {
+	t.Run("the context reaches the method", func(t *testing.T) {
+		type keyT struct{}
+		ctx := context.WithValue(context.Background(), keyT{}, "sentinel")
+		var r ctxRecorder
+		if err := UnmarshalContext(ctx, []byte("a = 1\n"), &r); err != nil {
+			t.Fatal(err)
+		}
+		if v, _ := r.got.Value(keyT{}).(string); v != "sentinel" {
+			t.Errorf("ctx = %v, want the caller's context", r.got)
+		}
+		tree, isMap := r.value.(map[string]any)
+		if !isMap || tree["a"] != int64(1) {
+			t.Errorf("value = %#v, want the tree with a = 1", r.value)
+		}
+	})
+	t.Run("the context wins over Unmarshaler", func(t *testing.T) {
+		var v struct {
+			R ctxBoth `toml:"r"`
+		}
+		if err := Unmarshal([]byte("r = 1\n"), &v); err != nil {
+			t.Fatal(err)
+		}
+		if !v.R.ctxCalled {
+			t.Error("UnmarshalTOMLContext was not called")
+		}
+		if v.R.plainCalled {
+			t.Error("UnmarshalTOML was called although the context method exists")
+		}
+	})
+	t.Run("a non-cancellable entry point hands in Background", func(t *testing.T) {
+		var r ctxRecorder
+		if err := Unmarshal([]byte("a = 1\n"), &r); err != nil {
+			t.Fatal(err)
+		}
+		if r.got != context.Background() {
+			t.Errorf("ctx = %v, want context.Background", r.got)
+		}
+	})
+}
+
+type ctxBoth struct {
+	ctxCalled   bool
+	plainCalled bool
+}
+
+func (b *ctxBoth) UnmarshalTOMLContext(ctx context.Context, data any) error {
+	b.ctxCalled = true
+	return nil
+}
+
+func (b *ctxBoth) UnmarshalTOML(data any) error {
+	b.plainCalled = true
+	return nil
+}

docs/API.md

@@ -225,6 +225,11 @@ one declared later wins.
 Unknown keys are ignored by default, landing in an untagged embedded map when
 the struct has one; [Strict decoding](#strict-decoding) rejects them instead.
 
+The tag may carry the `required` option, `toml:"host,required"`: the decode
+fails with `missing required key "host"` when no key of the document resolved
+to the field. The check runs after the table is read, so the other fields
+carry their values whether the required one is present or not.
+
 ### Numeric conversion
 
 The parser produces `int64` for every integer and `float64` for every float.
@@ -314,6 +319,21 @@ automatically, and a nil pointer destination is allocated first. An error
 returned from `UnmarshalTOML` halts the decode and propagates wrapped with the
 key path, for example `addr: unmarshal: not a string`.
 
+### Custom decoding: `UnmarshalerContext`
+
+`UnmarshalerContext` is `Unmarshaler` with the decode's context handed in:
+
+```go
+type UnmarshalerContext interface {
+	UnmarshalTOMLContext(ctx context.Context, data any) error
+}
+```
+
+A type that implements both gets `UnmarshalTOMLContext`, so a long custom
+decode can abort on cancellation instead of running to completion. The
+context a non-cancellable entry point carries is `context.Background`, never
+nil.
+
 ### Custom decoding: `encoding.TextUnmarshaler`
 
 A destination type that implements `encoding.TextUnmarshaler` receives a TOML
@@ -752,7 +772,9 @@ See [Custom encoding](#custom-encoding-marshaler).
 
 ### `type Unmarshaler interface{ UnmarshalTOML(data any) error }`
 
-See [Custom decoding](#custom-decoding-unmarshaler).
+See [Custom decoding](#custom-decoding-unmarshaler). `UnmarshalerContext`
+carries the decode's context through `UnmarshalTOMLContext(ctx, data)` and
+wins when a type implements both.
 
 ### `type Number string`
 

interpres.go

@@ -241,7 +241,9 @@ func UnmarshalContext(ctx context.Context, data []byte, v any) error {
 	if err != nil {
 		return err
 	}
-	return newDecoder().decode(tree, v)
+	dec := newDecoder()
+	dec.ctx = ctx
+	return dec.decode(tree, v)
 }
 
 // A Decoder decodes a TOML document into a Go value with configurable
@@ -315,6 +317,7 @@ func (d *Decoder) DecodeContext(ctx context.Context, data []byte, v any) error {
 	}
 	dec := newDecoder()
 	dec.disallowUnknown = d.disallowUnknown
+	dec.ctx = ctx
 	return dec.decode(tree, v)
 }
 
@@ -336,7 +339,8 @@ type Marshaler interface {
 // argument is whatever the parser produced for that key: one of string,
 // bool, int64, float64, OffsetDateTime, LocalDateTime, LocalDate, LocalTime,
 // []any, or map[string]any. A tree built by hand may carry a plain time.Time
-// where the parser would put an OffsetDateTime.
+// where the parser would put an OffsetDateTime, and a Decoder configured with
+// UseNumber a Number.
 //
 // UnmarshalTOML may parse, inspect, or transform the value however it likes,
 // then store the result by mutating its receiver through the standard
@@ -355,6 +359,15 @@ type Unmarshaler interface {
 	UnmarshalTOML(data any) error
 }
 
+// UnmarshalerContext is Unmarshaler with the decode's context handed in. A
+// type that implements both interfaces gets UnmarshalTOMLContext, so a long
+// custom decode can abort on cancellation instead of running to completion.
+// The context a non-cancellable entry point carries is context.Background,
+// never nil.
+type UnmarshalerContext interface {
+	UnmarshalTOMLContext(ctx context.Context, data any) error
+}
+
 // Marshal returns the TOML encoding of v. The output is valid TOML 1.1.
 //
 // Marshal traverses v using reflection and applies the following rules: