Added integration support for DataDog APM Tracing

vendor/gopkg.in/DataDog/dd-trace-go.v1/ddtrace/tracer/payload.go

@@ -0,0 +1,116 @@
+package tracer
+
+import (
+	"bytes"
+	"encoding/binary"
+	"io"
+
+	"github.com/tinylib/msgp/msgp"
+)
+
+// payload is a wrapper on top of the msgpack encoder which allows constructing an
+// encoded array by pushing its entries sequentially, one at a time. It basically
+// allows us to encode as we would with a stream, except that the contents of the stream
+// can be read as a slice by the msgpack decoder at any time. It follows the guidelines
+// from the msgpack array spec:
+// https://github.com/msgpack/msgpack/blob/master/spec.md#array-format-family
+//
+// payload implements io.Reader and can be used with the decoder directly. To create
+// a new payload use the newPayload method.
+//
+// payload is not safe for concurrent use.
+//
+// This structure basically allows us to push traces into the payload one at a time
+// in order to always have knowledge of the payload size, but also making it possible
+// for the agent to decode it as an array.
+type payload struct {
+	// header specifies the first few bytes in the msgpack stream
+	// indicating the type of array (fixarray, array16 or array32)
+	// and the number of items contained in the stream.
+	header []byte
+
+	// off specifies the current read position on the header.
+	off int
+
+	// count specifies the number of items in the stream.
+	count uint64
+
+	// buf holds the sequence of msgpack-encoded items.
+	buf bytes.Buffer
+}
+
+var _ io.Reader = (*payload)(nil)
+
+// newPayload returns a ready to use payload.
+func newPayload() *payload {
+	p := &payload{
+		header: make([]byte, 8),
+		off:    8,
+	}
+	return p
+}
+
+// push pushes a new item into the stream.
+func (p *payload) push(t spanList) error {
+	if err := msgp.Encode(&p.buf, t); err != nil {
+		return err
+	}
+	p.count++
+	p.updateHeader()
+	return nil
+}
+
+// itemCount returns the number of items available in the srteam.
+func (p *payload) itemCount() int {
+	return int(p.count)
+}
+
+// size returns the payload size in bytes. After the first read the value becomes
+// inaccurate by up to 8 bytes.
+func (p *payload) size() int {
+	return p.buf.Len() + len(p.header) - p.off
+}
+
+// reset resets the internal buffer, counter and read offset.
+func (p *payload) reset() {
+	p.off = 8
+	p.count = 0
+	p.buf.Reset()
+}
+
+// https://github.com/msgpack/msgpack/blob/master/spec.md#array-format-family
+const (
+	msgpackArrayFix byte = 144  // up to 15 items
+	msgpackArray16       = 0xdc // up to 2^16-1 items, followed by size in 2 bytes
+	msgpackArray32       = 0xdd // up to 2^32-1 items, followed by size in 4 bytes
+)
+
+// updateHeader updates the payload header based on the number of items currently
+// present in the stream.
+func (p *payload) updateHeader() {
+	n := p.count
+	switch {
+	case n <= 15:
+		p.header[7] = msgpackArrayFix + byte(n)
+		p.off = 7
+	case n <= 1<<16-1:
+		binary.BigEndian.PutUint64(p.header, n) // writes 2 bytes
+		p.header[5] = msgpackArray16
+		p.off = 5
+	default: // n <= 1<<32-1
+		binary.BigEndian.PutUint64(p.header, n) // writes 4 bytes
+		p.header[3] = msgpackArray32
+		p.off = 3
+	}
+}
+
+// Read implements io.Reader. It reads from the msgpack-encoded stream.
+func (p *payload) Read(b []byte) (n int, err error) {
+	if p.off < len(p.header) {
+		// reading header
+		n = copy(b, p.header[p.off:])
+		p.off += n
+		return n, nil
+	}
+	return p.buf.Read(b)
+}