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spancontext.go
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spancontext.go
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// Unless explicitly stated otherwise all files in this repository are licensed
// under the Apache License Version 2.0.
// This product includes software developed at Datadog (https://www.datadoghq.com/).
// Copyright 2016 Datadog, Inc.
package tracer
import (
"strconv"
"sync"
"sync/atomic"
"gopkg.in/DataDog/dd-trace-go.v1/ddtrace"
"gopkg.in/DataDog/dd-trace-go.v1/ddtrace/internal"
"gopkg.in/DataDog/dd-trace-go.v1/internal/log"
"gopkg.in/DataDog/dd-trace-go.v1/internal/samplernames"
)
var _ ddtrace.SpanContext = (*spanContext)(nil)
// SpanContext represents a span state that can propagate to descendant spans
// and across process boundaries. It contains all the information needed to
// spawn a direct descendant of the span that it belongs to. It can be used
// to create distributed tracing by propagating it using the provided interfaces.
type spanContext struct {
// the below group should propagate only locally
trace *trace // reference to the trace that this span belongs too
span *span // reference to the span that hosts this context
errors int64 // number of spans with errors in this trace
// the below group should propagate cross-process
traceID uint64
spanID uint64
mu sync.RWMutex // guards below fields
baggage map[string]string
hasBaggage int32 // atomic int for quick checking presence of baggage. 0 indicates no baggage, otherwise baggage exists.
origin string // e.g. "synthetics"
}
// newSpanContext creates a new SpanContext to serve as context for the given
// span. If the provided parent is not nil, the context will inherit the trace,
// baggage and other values from it. This method also pushes the span into the
// new context's trace and as a result, it should not be called multiple times
// for the same span.
func newSpanContext(span *span, parent *spanContext) *spanContext {
context := &spanContext{
traceID: span.TraceID,
spanID: span.SpanID,
span: span,
}
if parent != nil {
context.trace = parent.trace
context.origin = parent.origin
context.errors = parent.errors
parent.ForeachBaggageItem(func(k, v string) bool {
context.setBaggageItem(k, v)
return true
})
}
if context.trace == nil {
context.trace = newTrace()
}
if context.trace.root == nil {
// first span in the trace can safely be assumed to be the root
context.trace.root = span
}
// put span in context's trace
context.trace.push(span)
return context
}
// SpanID implements ddtrace.SpanContext.
func (c *spanContext) SpanID() uint64 { return c.spanID }
// TraceID implements ddtrace.SpanContext.
func (c *spanContext) TraceID() uint64 { return c.traceID }
// ForeachBaggageItem implements ddtrace.SpanContext.
func (c *spanContext) ForeachBaggageItem(handler func(k, v string) bool) {
if atomic.LoadInt32(&c.hasBaggage) == 0 {
return
}
c.mu.RLock()
defer c.mu.RUnlock()
for k, v := range c.baggage {
if !handler(k, v) {
break
}
}
}
func (c *spanContext) setSamplingPriority(p int, sampler samplernames.SamplerName, rate float64) {
if c.trace == nil {
c.trace = newTrace()
}
c.trace.setSamplingPriority(p, sampler, rate, c.span)
}
func (c *spanContext) samplingPriority() (p int, ok bool) {
if c.trace == nil {
return 0, false
}
return c.trace.samplingPriority()
}
func (c *spanContext) setBaggageItem(key, val string) {
c.mu.Lock()
defer c.mu.Unlock()
if c.baggage == nil {
atomic.StoreInt32(&c.hasBaggage, 1)
c.baggage = make(map[string]string, 1)
}
c.baggage[key] = val
}
func (c *spanContext) baggageItem(key string) string {
if atomic.LoadInt32(&c.hasBaggage) == 0 {
return ""
}
c.mu.RLock()
defer c.mu.RUnlock()
return c.baggage[key]
}
func (c *spanContext) meta(key string) (val string, ok bool) {
c.span.RLock()
defer c.span.RUnlock()
val, ok = c.span.Meta[key]
return val, ok
}
// finish marks this span as finished in the trace.
func (c *spanContext) finish() { c.trace.finishedOne(c.span) }
// samplingDecision is the decision to send a trace to the agent or not.
type samplingDecision int64
const (
// decisionNone is the default state of a trace.
// If no decision is made about the trace, the trace won't be sent to the agent.
decisionNone samplingDecision = iota
// decisionDrop prevents the trace from being sent to the agent.
decisionDrop
// decisionKeep ensures the trace will be sent to the agent.
decisionKeep
)
// trace contains shared context information about a trace, such as sampling
// priority, the root reference and a buffer of the spans which are part of the
// trace, if these exist.
type trace struct {
mu sync.RWMutex // guards below fields
spans []*span // all the spans that are part of this trace
tags map[string]string // trace level tags
propagatingTags map[string]string // trace level tags that will be propagated across service boundaries
finished int // the number of finished spans
full bool // signifies that the span buffer is full
priority *float64 // sampling priority
locked bool // specifies if the sampling priority can be altered
samplingDecision samplingDecision // samplingDecision indicates whether to send the trace to the agent.
// root specifies the root of the trace, if known; it is nil when a span
// context is extracted from a carrier, at which point there are no spans in
// the trace yet.
root *span
}
var (
// traceStartSize is the initial size of our trace buffer,
// by default we allocate for a handful of spans within the trace,
// reasonable as span is actually way bigger, and avoids re-allocating
// over and over. Could be fine-tuned at runtime.
traceStartSize = 10
// traceMaxSize is the maximum number of spans we keep in memory for a
// single trace. This is to avoid memory leaks. If more spans than this
// are added to a trace, then the trace is dropped and the spans are
// discarded. Adding additional spans after a trace is dropped does
// nothing.
traceMaxSize = int(1e5)
)
// newTrace creates a new trace using the given callback which will be called
// upon completion of the trace.
func newTrace() *trace {
return &trace{spans: make([]*span, 0, traceStartSize)}
}
func (t *trace) samplingPriorityLocked() (p int, ok bool) {
if t.priority == nil {
return 0, false
}
return int(*t.priority), true
}
func (t *trace) samplingPriority() (p int, ok bool) {
t.mu.RLock()
defer t.mu.RUnlock()
return t.samplingPriorityLocked()
}
func (t *trace) setSamplingPriority(p int, sampler samplernames.SamplerName, rate float64, span *span) {
t.mu.Lock()
defer t.mu.Unlock()
t.setSamplingPriorityLocked(p, sampler)
}
func (t *trace) keep() {
atomic.CompareAndSwapInt64((*int64)(&t.samplingDecision), int64(decisionNone), int64(decisionKeep))
}
func (t *trace) drop() {
atomic.CompareAndSwapInt64((*int64)(&t.samplingDecision), int64(decisionNone), int64(decisionDrop))
}
func (t *trace) setTag(key, value string) {
if t.tags == nil {
t.tags = make(map[string]string, 1)
}
t.tags[key] = value
}
// setPropagatingTag sets the key/value pair as a trace propagating tag.
func (t *trace) setPropagatingTag(key, value string) {
t.mu.Lock()
defer t.mu.Unlock()
t.setPropagatingTagLocked(key, value)
}
// setPropagatingTagLocked sets the key/value pair as a trace propagating tag.
// Not safe for concurrent use, setPropagatingTag should be used instead in that case.
func (t *trace) setPropagatingTagLocked(key, value string) {
if t.propagatingTags == nil {
t.propagatingTags = make(map[string]string, 1)
}
t.propagatingTags[key] = value
}
// unsetPropagatingTag deletes the key/value pair from the trace's propagated tags.
func (t *trace) unsetPropagatingTag(key string) {
t.mu.Lock()
defer t.mu.Unlock()
delete(t.propagatingTags, key)
}
func (t *trace) setSamplingPriorityLocked(p int, sampler samplernames.SamplerName) {
if t.locked {
return
}
if t.priority == nil {
t.priority = new(float64)
}
*t.priority = float64(p)
_, ok := t.propagatingTags[keyDecisionMaker]
if p > 0 && !ok && sampler != samplernames.Unknown {
// We have a positive priority and the sampling mechanism isn't set.
// Send nothing when sampler is `Unknown` for RFC compliance.
t.setPropagatingTagLocked(keyDecisionMaker, "-"+strconv.Itoa(int(sampler)))
}
if p <= 0 && ok {
delete(t.propagatingTags, keyDecisionMaker)
}
}
// push pushes a new span into the trace. If the buffer is full, it returns
// a errBufferFull error.
func (t *trace) push(sp *span) {
t.mu.Lock()
defer t.mu.Unlock()
if t.full {
return
}
tr, haveTracer := internal.GetGlobalTracer().(*tracer)
if len(t.spans) >= traceMaxSize {
// capacity is reached, we will not be able to complete this trace.
t.full = true
t.spans = nil // GC
log.Error("trace buffer full (%d), dropping trace", traceMaxSize)
if haveTracer {
atomic.AddInt64(&tr.tracesDropped, 1)
}
return
}
if v, ok := sp.Metrics[keySamplingPriority]; ok {
t.setSamplingPriorityLocked(int(v), samplernames.Unknown)
}
t.spans = append(t.spans, sp)
if haveTracer {
atomic.AddInt64(&tr.spansStarted, 1)
}
}
// finishedOne acknowledges that another span in the trace has finished, and checks
// if the trace is complete, in which case it calls the onFinish function. It uses
// the given priority, if non-nil, to mark the root span.
func (t *trace) finishedOne(s *span) {
t.mu.Lock()
defer t.mu.Unlock()
if t.full {
// capacity has been reached, the buffer is no longer tracking
// all the spans in the trace, so the below conditions will not
// be accurate and would trigger a pre-mature flush, exposing us
// to a race condition where spans can be modified while flushing.
return
}
t.finished++
if s == t.root && t.priority != nil {
// after the root has finished we lock down the priority;
// we won't be able to make changes to a span after finishing
// without causing a race condition.
t.root.setMetric(keySamplingPriority, *t.priority)
t.locked = true
}
if len(t.spans) > 0 && s == t.spans[0] {
// first span in chunk finished, lock down the tags
//
// TODO(barbayar): make sure this doesn't happen in vain when switching to
// the new wire format. We won't need to set the tags on the first span
// in the chunk there.
for k, v := range t.tags {
s.setMeta(k, v)
}
for k, v := range t.propagatingTags {
s.setMeta(k, v)
}
}
if len(t.spans) != t.finished {
return
}
defer func() {
t.spans = nil
t.finished = 0 // important, because a buffer can be used for several flushes
}()
tr, ok := internal.GetGlobalTracer().(*tracer)
if !ok {
return
}
// we have a tracer that can receive completed traces.
atomic.AddInt64(&tr.spansFinished, int64(len(t.spans)))
tr.pushTrace(&finishedTrace{
spans: t.spans,
decision: samplingDecision(atomic.LoadInt64((*int64)(&t.samplingDecision))),
})
}