encoding/json: add JSON streaming parse API

This change adds new methods to Decoder.

 * Decoder.Token steps through a JSON document, returning a value for each token.
 * Decoder.Decode unmarshals the entire value at the token stream's current
   position (in addition to its existing function in a stream of JSON values)

Fixes #6050.
Fixes #6499.

Change-Id: Iff283e0e7b537221ae256392aca6529f06ebe211
Reviewed-on: https://go-review.googlesource.com/9073Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/json/bench_test.go

@@ -15,6 +15,7 @@ import (
 	"compress/gzip"
 	"io/ioutil"
 	"os"
+	"strings"
 	"testing"
 )
 
@@ -126,6 +127,28 @@ func BenchmarkCodeDecoder(b *testing.B) {
 	b.SetBytes(int64(len(codeJSON)))
 }
 
+func BenchmarkDecoderStream(b *testing.B) {
+	b.StopTimer()
+	var buf bytes.Buffer
+	dec := NewDecoder(&buf)
+	buf.WriteString(`"` + strings.Repeat("x", 1000000) + `"` + "\n\n\n")
+	var x interface{}
+	if err := dec.Decode(&x); err != nil {
+		b.Fatal("Decode:", err)
+	}
+	ones := strings.Repeat(" 1\n", 300000) + "\n\n\n"
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		if i%300000 == 0 {
+			buf.WriteString(ones)
+		}
+		x = nil
+		if err := dec.Decode(&x); err != nil || x != 1.0 {
+			b.Fatalf("Decode: %v after %d", err, i)
+		}
+	}
+}
+
 func BenchmarkCodeUnmarshal(b *testing.B) {
 	if codeJSON == nil {
 		b.StopTimer()

src/encoding/json/example_test.go

@@ -83,6 +83,97 @@ func ExampleDecoder() {
 	// Ed: Go fmt yourself!
 }
 
+// This example uses a Decoder to decode a stream of distinct JSON values.
+func ExampleDecoder_Token() {
+	const jsonStream = `
+		{"Message": "Hello", "Array": [1, 2, 3], "Null": null, "Number": 1.234}
+	`
+	dec := json.NewDecoder(strings.NewReader(jsonStream))
+	for {
+		t, err := dec.Token()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Printf("%T: %v", t, t)
+		if dec.More() {
+			fmt.Printf(" (more)")
+		}
+		fmt.Printf("\n")
+	}
+	// Output:
+	// json.Delim: { (more)
+	// string: Message (more)
+	// string: Hello (more)
+	// string: Array (more)
+	// json.Delim: [ (more)
+	// float64: 1 (more)
+	// float64: 2 (more)
+	// float64: 3
+	// json.Delim: ] (more)
+	// string: Null (more)
+	// <nil>: <nil> (more)
+	// string: Number (more)
+	// float64: 1.234
+	// json.Delim: }
+}
+
+// This example uses a Decoder to decode a streaming array of JSON objects.
+func ExampleDecoder_Decode_stream() {
+	const jsonStream = `
+		[
+			{"Name": "Ed", "Text": "Knock knock."},
+			{"Name": "Sam", "Text": "Who's there?"},
+			{"Name": "Ed", "Text": "Go fmt."},
+			{"Name": "Sam", "Text": "Go fmt who?"},
+			{"Name": "Ed", "Text": "Go fmt yourself!"}
+		]
+	`
+	type Message struct {
+		Name, Text string
+	}
+	dec := json.NewDecoder(strings.NewReader(jsonStream))
+
+	// read open bracket
+	t, err := dec.Token()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%T: %v\n", t, t)
+
+	var m Message
+	// while the array contains values
+	for dec.More() {
+
+		// decode an array value (Message)
+		err := dec.Decode(&m)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		fmt.Printf("%v: %v\n", m.Name, m.Text)
+	}
+
+	// read closing bracket
+	t, err = dec.Token()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%T: %v\n", t, t)
+
+	// Output:
+	// json.Delim: [
+	// Ed: Knock knock.
+	// Sam: Who's there?
+	// Ed: Go fmt.
+	// Sam: Go fmt who?
+	// Ed: Go fmt yourself!
+	// json.Delim: ]
+
+}
+
 // This example uses RawMessage to delay parsing part of a JSON message.
 func ExampleRawMessage() {
 	type Color struct {

src/encoding/json/stream.go

@@ -15,8 +15,12 @@ type Decoder struct {
 	r     io.Reader
 	buf   []byte
 	d     decodeState
+	scanp int // start of unread data in buf
 	scan  scanner
 	err   error
+
+	tokenState int
+	tokenStack []int
 }
 
 // NewDecoder returns a new decoder that reads from r.
@@ -41,20 +45,29 @@ func (dec *Decoder) Decode(v interface{}) error {
 		return dec.err
 	}
 
+	if err := dec.tokenPrepareForDecode(); err != nil {
+		return err
+	}
+
+	if !dec.tokenValueAllowed() {
+		return &SyntaxError{msg: "not at beginning of value"}
+	}
+
+	// Read whole value into buffer.
 	n, err := dec.readValue()
 	if err != nil {
 		return err
 	}
+	dec.d.init(dec.buf[dec.scanp : dec.scanp+n])
+	dec.scanp += n
 
 	// Don't save err from unmarshal into dec.err:
 	// the connection is still usable since we read a complete JSON
 	// object from it before the error happened.
-	dec.d.init(dec.buf[0:n])
 	err = dec.d.unmarshal(v)
 
-	// Slide rest of data down.
-	rest := copy(dec.buf, dec.buf[n:])
-	dec.buf = dec.buf[0:rest]
+	// fixup token streaming state
+	dec.tokenValueEnd()
 
 	return err
 }
@@ -62,7 +75,7 @@ func (dec *Decoder) Decode(v interface{}) error {
 // Buffered returns a reader of the data remaining in the Decoder's
 // buffer. The reader is valid until the next call to Decode.
 func (dec *Decoder) Buffered() io.Reader {
-	return bytes.NewReader(dec.buf)
+	return bytes.NewReader(dec.buf[dec.scanp:])
 }
 
 // readValue reads a JSON value into dec.buf.
@@ -70,7 +83,7 @@ func (dec *Decoder) Buffered() io.Reader {
 func (dec *Decoder) readValue() (int, error) {
 	dec.scan.reset()
 
-	scanp := 0
+	scanp := dec.scanp
 	var err error
 Input:
 	for {
@@ -111,7 +124,23 @@ Input:
 			return 0, err
 		}
 
+		n := scanp - dec.scanp
+		err = dec.refill()
+		scanp = dec.scanp + n
+	}
+	return scanp - dec.scanp, nil
+}
+
+func (dec *Decoder) refill() error {
 	// Make room to read more into the buffer.
+	// First slide down data already consumed.
+	if dec.scanp > 0 {
+		n := copy(dec.buf, dec.buf[dec.scanp:])
+		dec.buf = dec.buf[:n]
+		dec.scanp = 0
+	}
+
+	// Grow buffer if not large enough.
 	const minRead = 512
 	if cap(dec.buf)-len(dec.buf) < minRead {
 		newBuf := make([]byte, len(dec.buf), 2*cap(dec.buf)+minRead)
@@ -120,11 +149,10 @@ Input:
 	}
 
 	// Read.  Delay error for next iteration (after scan).
-		var n int
-		n, err = dec.r.Read(dec.buf[len(dec.buf):cap(dec.buf)])
+	n, err := dec.r.Read(dec.buf[len(dec.buf):cap(dec.buf)])
 	dec.buf = dec.buf[0 : len(dec.buf)+n]
-	}
-	return scanp, nil
+
+	return err
 }
 
 func nonSpace(b []byte) bool {
@@ -198,3 +226,245 @@ func (m *RawMessage) UnmarshalJSON(data []byte) error {
 
 var _ Marshaler = (*RawMessage)(nil)
 var _ Unmarshaler = (*RawMessage)(nil)
+
+// A Token holds a value of one of these types:
+//
+//	Delim, for the four JSON delimiters [ ] { }
+//	bool, for JSON booleans
+//	float64, for JSON numbers
+//	Number, for JSON numbers
+//	string, for JSON string literals
+//	nil, for JSON null
+//
+type Token interface{}
+
+const (
+	tokenTopValue = iota
+	tokenArrayStart
+	tokenArrayValue
+	tokenArrayComma
+	tokenObjectStart
+	tokenObjectKey
+	tokenObjectColon
+	tokenObjectValue
+	tokenObjectComma
+)
+
+// advance tokenstate from a separator state to a value state
+func (dec *Decoder) tokenPrepareForDecode() error {
+	c, err := dec.peek()
+	if err != nil {
+		return err
+	}
+	switch dec.tokenState {
+	case tokenArrayComma:
+		if c != ',' {
+			return &SyntaxError{"expected comma after array element", 0}
+		}
+		dec.scanp++
+		dec.tokenState = tokenArrayValue
+	case tokenObjectColon:
+		if c != ':' {
+			return &SyntaxError{"expected colon after object key", 0}
+		}
+		dec.scanp++
+		dec.tokenState = tokenObjectValue
+	}
+	return nil
+}
+
+func (dec *Decoder) tokenValueAllowed() bool {
+	switch dec.tokenState {
+	case tokenTopValue, tokenArrayStart, tokenArrayValue, tokenObjectValue:
+		return true
+	}
+	return false
+}
+
+func (dec *Decoder) tokenValueEnd() {
+	switch dec.tokenState {
+	case tokenArrayStart, tokenArrayValue:
+		dec.tokenState = tokenArrayComma
+	case tokenObjectValue:
+		dec.tokenState = tokenObjectComma
+	}
+}
+
+// A Delim is a JSON array or object delimiter, one of [ ] { or }.
+type Delim rune
+
+func (d Delim) String() string {
+	return string(d)
+}
+
+// Token returns the next JSON token in the input stream.
+// At the end of the input stream, Token returns nil, io.EOF.
+//
+// Token guarantees that the delimiters [ ] { } it returns are
+// properly nested and matched: if Token encounters an unexpected
+// delimiter in the input, it will return an error.
+//
+// The input stream consists of basic JSON values—bool, string,
+// number, and null—along with delimiters [ ] { } of type Delim
+// to mark the start and end of arrays and objects.
+// Commas and colons are elided.
+func (dec *Decoder) Token() (Token, error) {
+	for {
+		c, err := dec.peek()
+		if err != nil {
+			return nil, err
+		}
+		switch c {
+		case '[':
+			if !dec.tokenValueAllowed() {
+				return dec.tokenError(c)
+			}
+			dec.scanp++
+			dec.tokenStack = append(dec.tokenStack, dec.tokenState)
+			dec.tokenState = tokenArrayStart
+			return Delim('['), nil
+
+		case ']':
+			if dec.tokenState != tokenArrayStart && dec.tokenState != tokenArrayComma {
+				return dec.tokenError(c)
+			}
+			dec.scanp++
+			dec.tokenState = dec.tokenStack[len(dec.tokenStack)-1]
+			dec.tokenStack = dec.tokenStack[:len(dec.tokenStack)-1]
+			dec.tokenValueEnd()
+			return Delim(']'), nil
+
+		case '{':
+			if !dec.tokenValueAllowed() {
+				return dec.tokenError(c)
+			}
+			dec.scanp++
+			dec.tokenStack = append(dec.tokenStack, dec.tokenState)
+			dec.tokenState = tokenObjectStart
+			return Delim('{'), nil
+
+		case '}':
+			if dec.tokenState != tokenObjectStart && dec.tokenState != tokenObjectComma {
+				return dec.tokenError(c)
+			}
+			dec.scanp++
+			dec.tokenState = dec.tokenStack[len(dec.tokenStack)-1]
+			dec.tokenStack = dec.tokenStack[:len(dec.tokenStack)-1]
+			dec.tokenValueEnd()
+			return Delim('}'), nil
+
+		case ':':
+			if dec.tokenState != tokenObjectColon {
+				return dec.tokenError(c)
+			}
+			dec.scanp++
+			dec.tokenState = tokenObjectValue
+			continue
+
+		case ',':
+			if dec.tokenState == tokenArrayComma {
+				dec.scanp++
+				dec.tokenState = tokenArrayValue
+				continue
+			}
+			if dec.tokenState == tokenObjectComma {
+				dec.scanp++
+				dec.tokenState = tokenObjectKey
+				continue
+			}
+			return dec.tokenError(c)
+
+		case '"':
+			if dec.tokenState == tokenObjectStart || dec.tokenState == tokenObjectKey {
+				var x string
+				old := dec.tokenState
+				dec.tokenState = tokenTopValue
+				err := dec.Decode(&x)
+				dec.tokenState = old
+				if err != nil {
+					clearOffset(err)
+					return nil, err
+				}
+				dec.tokenState = tokenObjectColon
+				return x, nil
+			}
+			fallthrough
+
+		default:
+			if !dec.tokenValueAllowed() {
+				return dec.tokenError(c)
+			}
+			var x interface{}
+			if err := dec.Decode(&x); err != nil {
+				clearOffset(err)
+				return nil, err
+			}
+			return x, nil
+		}
+	}
+}
+
+func clearOffset(err error) {
+	if s, ok := err.(*SyntaxError); ok {
+		s.Offset = 0
+	}
+}
+
+func (dec *Decoder) tokenError(c byte) (Token, error) {
+	var context string
+	switch dec.tokenState {
+	case tokenTopValue:
+		context = " looking for beginning of value"
+	case tokenArrayStart, tokenArrayValue, tokenObjectValue:
+		context = " looking for beginning of value"
+	case tokenArrayComma:
+		context = " after array element"
+	case tokenObjectKey:
+		context = " looking for beginning of object key string"
+	case tokenObjectColon:
+		context = " after object key"
+	case tokenObjectComma:
+		context = " after object key:value pair"
+	}
+	return nil, &SyntaxError{"invalid character " + quoteChar(int(c)) + " " + context, 0}
+}
+
+// More reports whether there is another element in the
+// current array or object being parsed.
+func (dec *Decoder) More() bool {
+	c, err := dec.peek()
+	return err == nil && c != ']' && c != '}'
+}
+
+func (dec *Decoder) peek() (byte, error) {
+	for {
+		for i := dec.scanp; i < len(dec.buf); i++ {
+			c := dec.buf[i]
+			if isSpace(rune(c)) {
+				continue
+			}
+			dec.scanp = i
+			return c, nil
+		}
+		if err := dec.refill(); err != nil {
+			return 0, err
+		}
+	}
+}
+
+/*
+TODO
+
+// EncodeToken writes the given JSON token to the stream.
+// It returns an error if the delimiters [ ] { } are not properly used.
+//
+// EncodeToken does not call Flush, because usually it is part of
+// a larger operation such as Encode, and those will call Flush when finished.
+// Callers that create an Encoder and then invoke EncodeToken directly,
+// without using Encode, need to call Flush when finished to ensure that
+// the JSON is written to the underlying writer.
+func (e *Encoder) EncodeToken(t Token) error  {
+	...
+}
+
+*/

src/encoding/json/stream_test.go

@@ -6,6 +6,7 @@ package json
 
 import (
 	"bytes"
+	"io"
 	"io/ioutil"
 	"net"
 	"reflect"
@@ -204,3 +205,113 @@ func BenchmarkEncoderEncode(b *testing.B) {
 		}
 	}
 }
+
+type tokenStreamCase struct {
+	json      string
+	expTokens []interface{}
+}
+
+type decodeThis struct {
+	v interface{}
+}
+
+var tokenStreamCases []tokenStreamCase = []tokenStreamCase{
+	// streaming token cases
+	{json: `10`, expTokens: []interface{}{float64(10)}},
+	{json: ` [10] `, expTokens: []interface{}{
+		Delim('['), float64(10), Delim(']')}},
+	{json: ` [false,10,"b"] `, expTokens: []interface{}{
+		Delim('['), false, float64(10), "b", Delim(']')}},
+	{json: `{ "a": 1 }`, expTokens: []interface{}{
+		Delim('{'), "a", float64(1), Delim('}')}},
+	{json: `{"a": 1, "b":"3"}`, expTokens: []interface{}{
+		Delim('{'), "a", float64(1), "b", "3", Delim('}')}},
+	{json: ` [{"a": 1},{"a": 2}] `, expTokens: []interface{}{
+		Delim('['),
+		Delim('{'), "a", float64(1), Delim('}'),
+		Delim('{'), "a", float64(2), Delim('}'),
+		Delim(']')}},
+	{json: `{"obj": {"a": 1}}`, expTokens: []interface{}{
+		Delim('{'), "obj", Delim('{'), "a", float64(1), Delim('}'),
+		Delim('}')}},
+	{json: `{"obj": [{"a": 1}]}`, expTokens: []interface{}{
+		Delim('{'), "obj", Delim('['),
+		Delim('{'), "a", float64(1), Delim('}'),
+		Delim(']'), Delim('}')}},
+
+	// streaming tokens with intermittent Decode()
+	{json: `{ "a": 1 }`, expTokens: []interface{}{
+		Delim('{'), "a",
+		decodeThis{float64(1)},
+		Delim('}')}},
+	{json: ` [ { "a" : 1 } ] `, expTokens: []interface{}{
+		Delim('['),
+		decodeThis{map[string]interface{}{"a": float64(1)}},
+		Delim(']')}},
+	{json: ` [{"a": 1},{"a": 2}] `, expTokens: []interface{}{
+		Delim('['),
+		decodeThis{map[string]interface{}{"a": float64(1)}},
+		decodeThis{map[string]interface{}{"a": float64(2)}},
+		Delim(']')}},
+	{json: `{ "obj" : [ { "a" : 1 } ] }`, expTokens: []interface{}{
+		Delim('{'), "obj", Delim('['),
+		decodeThis{map[string]interface{}{"a": float64(1)}},
+		Delim(']'), Delim('}')}},
+
+	{json: `{"obj": {"a": 1}}`, expTokens: []interface{}{
+		Delim('{'), "obj",
+		decodeThis{map[string]interface{}{"a": float64(1)}},
+		Delim('}')}},
+	{json: `{"obj": [{"a": 1}]}`, expTokens: []interface{}{
+		Delim('{'), "obj",
+		decodeThis{[]interface{}{
+			map[string]interface{}{"a": float64(1)},
+		}},
+		Delim('}')}},
+	{json: ` [{"a": 1} {"a": 2}] `, expTokens: []interface{}{
+		Delim('['),
+		decodeThis{map[string]interface{}{"a": float64(1)}},
+		decodeThis{&SyntaxError{"expected comma after array element", 0}},
+	}},
+	{json: `{ "a" 1 }`, expTokens: []interface{}{
+		Delim('{'), "a",
+		decodeThis{&SyntaxError{"expected colon after object key", 0}},
+	}},
+}
+
+func TestDecodeInStream(t *testing.T) {
+
+	for ci, tcase := range tokenStreamCases {
+
+		dec := NewDecoder(strings.NewReader(tcase.json))
+		for i, etk := range tcase.expTokens {
+
+			var tk interface{}
+			var err error
+
+			if dt, ok := etk.(decodeThis); ok {
+				etk = dt.v
+				err = dec.Decode(&tk)
+			} else {
+				tk, err = dec.Token()
+			}
+			if experr, ok := etk.(error); ok {
+				if err == nil || err.Error() != experr.Error() {
+					t.Errorf("case %v: Expected error %v in %q, but was %v", ci, experr, tcase.json, err)
+				}
+				break
+			} else if err == io.EOF {
+				t.Errorf("case %v: Unexpected EOF in %q", ci, tcase.json)
+				break
+			} else if err != nil {
+				t.Errorf("case %v: Unexpected error '%v' in %q", ci, err, tcase.json)
+				break
+			}
+			if !reflect.DeepEqual(tk, etk) {
+				t.Errorf(`case %v: %q @ %v expected %T(%v) was %T(%v)`, ci, tcase.json, i, etk, etk, tk, tk)
+				break
+			}
+		}
+	}
+
+}