Source: http.go in package google.golang.org/api/transport/http/internal/propagation

+build go1.8

Package propagation implements X-Cloud-Trace-Context header propagation used by Google Cloud products.

package propagation

import (
	"encoding/binary"
	"encoding/hex"
	"fmt"
	"net/http"
	"strconv"
	"strings"

	"go.opencensus.io/trace"
	"go.opencensus.io/trace/propagation"
)

const (
	httpHeaderMaxSize = 200
	httpHeader        = `X-Cloud-Trace-Context`
)

var _ propagation.HTTPFormat = (*HTTPFormat)(nil)

HTTPFormat implements propagation.HTTPFormat to propagate traces in HTTP headers for Google Cloud Platform and Stackdriver Trace.

type HTTPFormat struct{}

SpanContextFromRequest extracts a Stackdriver Trace span context from incoming requests.

func (f *HTTPFormat) SpanContextFromRequest(req *http.Request) (sc trace.SpanContext, ok bool) {

See https://cloud.google.com/trace/docs/faq for the header HTTPFormat. Return if the header is empty or missing, or if the header is unreasonably large, to avoid making unnecessary copies of a large string.

	if h == "" || len(h) > httpHeaderMaxSize {
		return trace.SpanContext{}, false
	}

Parse the trace id field.

	slash := strings.Index(h, `/`)
	if slash == -1 {
		return trace.SpanContext{}, false
	}
	tid, h := h[:slash], h[slash+1:]

	buf, err := hex.DecodeString(tid)
	if err != nil {
		return trace.SpanContext{}, false
	}
	copy(sc.TraceID[:], buf)

Parse the span id field.

	spanstr := h
	semicolon := strings.Index(h, `;`)
	if semicolon != -1 {
		spanstr, h = h[:semicolon], h[semicolon+1:]
	}
	sid, err := strconv.ParseUint(spanstr, 10, 64)
	if err != nil {
		return trace.SpanContext{}, false
	}
	binary.BigEndian.PutUint64(sc.SpanID[:], sid)

Parse the options field, options field is optional.

	if !strings.HasPrefix(h, "o=") {
		return sc, true
	}
	o, err := strconv.ParseUint(h[2:], 10, 64)
	if err != nil {
		return trace.SpanContext{}, false
	}
	sc.TraceOptions = trace.TraceOptions(o)
	return sc, true
}

SpanContextToRequest modifies the given request to include a Stackdriver Trace header.

func (f *HTTPFormat) SpanContextToRequest(sc trace.SpanContext, req *http.Request) {
	sid := binary.BigEndian.Uint64(sc.SpanID[:])
	header := fmt.Sprintf("%s/%d;o=%d", hex.EncodeToString(sc.TraceID[:]), sid, int64(sc.TraceOptions))
	req.Header.Set(httpHeader, header)