src/internal/trace/v2/event.go

// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package trace

import (
	"fmt"
	"math"
	"strings"
	"time"

	"internal/trace/v2/event"
	"internal/trace/v2/event/go122"
	"internal/trace/v2/version"
)

// EventKind indicates the kind of event this is.
//
// Use this information to obtain a more specific event that
// allows access to more detailed information.
type EventKind uint16

const (
	EventBad EventKind = iota

	// EventKindSync is an event that indicates a global synchronization
	// point in the trace. At the point of a sync event, the
	// trace reader can be certain that all resources (e.g. threads,
	// goroutines) that have existed until that point have been enumerated.
	EventSync

	// EventMetric is an event that represents the value of a metric at
	// a particular point in time.
	EventMetric

	// EventLabel attaches a label to a resource.
	EventLabel

	// EventStackSample represents an execution sample, indicating what a
	// thread/proc/goroutine was doing at a particular point in time via
	// its backtrace.
	//
	// Note: Samples should be considered a close approximation of
	// what a thread/proc/goroutine was executing at a given point in time.
	// These events may slightly contradict the situation StateTransitions
	// describe, so they should only be treated as a best-effort annotation.
	EventStackSample

	// EventRangeBegin and EventRangeEnd are a pair of generic events representing
	// a special range of time. Ranges are named and scoped to some resource
	// (identified via ResourceKind). A range that has begun but has not ended
	// is considered active.
	//
	// EvRangeBegin and EvRangeEnd will share the same name, and an End will always
	// follow a Begin on the same instance of the resource. The associated
	// resource ID can be obtained from the Event. ResourceNone indicates the
	// range is globally scoped. That is, any goroutine/proc/thread can start or
	// stop, but only one such range may be active at any given time.
	//
	// EventRangeActive is like EventRangeBegin, but indicates that the range was
	// already active. In this case, the resource referenced may not be in the current
	// context.
	EventRangeBegin
	EventRangeActive
	EventRangeEnd

	// EvTaskBegin and EvTaskEnd are a pair of events representing a runtime/trace.Task.
	EventTaskBegin
	EventTaskEnd

	// EventRegionBegin and EventRegionEnd are a pair of events represent a runtime/trace.Region.
	EventRegionBegin
	EventRegionEnd

	// EventLog represents a runtime/trace.Log call.
	EventLog

	// Transitions in state for some resource.
	EventStateTransition
)

// String returns a string form of the EventKind.
func (e EventKind) String() string {
	if int(e) >= len(eventKindStrings) {
		return eventKindStrings[0]
	}
	return eventKindStrings[e]
}

var eventKindStrings = [...]string{
	EventBad:             "Bad",
	EventSync:            "Sync",
	EventMetric:          "Metric",
	EventLabel:           "Label",
	EventStackSample:     "StackSample",
	EventRangeBegin:      "RangeBegin",
	EventRangeActive:     "RangeActive",
	EventRangeEnd:        "RangeEnd",
	EventTaskBegin:       "TaskBegin",
	EventTaskEnd:         "TaskEnd",
	EventRegionBegin:     "RegionBegin",
	EventRegionEnd:       "RegionEnd",
	EventLog:             "Log",
	EventStateTransition: "StateTransition",
}

const maxTime = Time(math.MaxInt64)

// Time is a timestamp in nanoseconds.
//
// It corresponds to the monotonic clock on the platform that the
// trace was taken, and so is possible to correlate with timestamps
// for other traces taken on the same machine using the same clock
// (i.e. no reboots in between).
//
// The actual absolute value of the timestamp is only meaningful in
// relation to other timestamps from the same clock.
//
// BUG: Timestamps coming from traces on Windows platforms are
// only comparable with timestamps from the same trace. Timestamps
// across traces cannot be compared, because the system clock is
// not used as of Go 1.22.
//
// BUG: Traces produced by Go versions 1.21 and earlier cannot be
// compared with timestamps from other traces taken on the same
// machine. This is because the system clock was not used at all
// to collect those timestamps.
type Time int64

// Sub subtracts t0 from t, returning the duration in nanoseconds.
func (t Time) Sub(t0 Time) time.Duration {
	return time.Duration(int64(t) - int64(t0))
}

// Metric provides details about a Metric event.
type Metric struct {
	// Name is the name of the sampled metric.
	//
	// Names follow the same convention as metric names in the
	// runtime/metrics package, meaning they include the unit.
	// Names that match with the runtime/metrics package represent
	// the same quantity. Note that this corresponds to the
	// runtime/metrics package for the Go version this trace was
	// collected for.
	Name string

	// Value is the sampled value of the metric.
	//
	// The Value's Kind is tied to the name of the metric, and so is
	// guaranteed to be the same for metric samples for the same metric.
	Value Value
}

// Label provides details about a Label event.
type Label struct {
	// Label is the label applied to some resource.
	Label string

	// Resource is the resource to which this label should be applied.
	Resource ResourceID
}

// Range provides details about a Range event.
type Range struct {
	// Name is a human-readable name for the range.
	//
	// This name can be used to identify the end of the range for the resource
	// its scoped to, because only one of each type of range may be active on
	// a particular resource. The relevant resource should be obtained from the
	// Event that produced these details. The corresponding RangeEnd will have
	// an identical name.
	Name string

	// Scope is the resource that the range is scoped to.
	//
	// For example, a ResourceGoroutine scope means that the same goroutine
	// must have a start and end for the range, and that goroutine can only
	// have one range of a particular name active at any given time. The
	// ID that this range is scoped to may be obtained via Event.Goroutine.
	//
	// The ResourceNone scope means that the range is globally scoped. As a
	// result, any goroutine/proc/thread may start or end the range, and only
	// one such named range may be active globally at any given time.
	//
	// For RangeBegin and RangeEnd events, this will always reference some
	// resource ID in the current execution context. For RangeActive events,
	// this may reference a resource not in the current context. Prefer Scope
	// over the current execution context.
	Scope ResourceID
}

// RangeAttributes provides attributes about a completed Range.
type RangeAttribute struct {
	// Name is the human-readable name for the range.
	Name string

	// Value is the value of the attribute.
	Value Value
}

// TaskID is the internal ID of a task used to disambiguate tasks (even if they
// are of the same type).
type TaskID uint64

const (
	// NoTask indicates the lack of a task.
	NoTask = TaskID(^uint64(0))

	// BackgroundTask is the global task that events are attached to if there was
	// no other task in the context at the point the event was emitted.
	BackgroundTask = TaskID(0)
)

// Task provides details about a Task event.
type Task struct {
	// ID is a unique identifier for the task.
	//
	// This can be used to associate the beginning of a task with its end.
	ID TaskID

	// ParentID is the ID of the parent task.
	Parent TaskID

	// Type is the taskType that was passed to runtime/trace.NewTask.
	//
	// May be "" if a task's TaskBegin event isn't present in the trace.
	Type string
}

// Region provides details about a Region event.
type Region struct {
	// Task is the ID of the task this region is associated with.
	Task TaskID

	// Type is the regionType that was passed to runtime/trace.StartRegion or runtime/trace.WithRegion.
	Type string
}

// Log provides details about a Log event.
type Log struct {
	// Task is the ID of the task this region is associated with.
	Task TaskID

	// Category is the category that was passed to runtime/trace.Log or runtime/trace.Logf.
	Category string

	// Message is the message that was passed to runtime/trace.Log or runtime/trace.Logf.
	Message string
}

// Stack represents a stack. It's really a handle to a stack and it's trivially comparable.
//
// If two Stacks are equal then their Frames are guaranteed to be identical. If they are not
// equal, however, their Frames may still be equal.
type Stack struct {
	table *evTable
	id    stackID
}

// Frames is an iterator over the frames in a Stack.
func (s Stack) Frames(yield func(f StackFrame) bool) bool {
	if s.id == 0 {
		return true
	}
	stk := s.table.stacks.mustGet(s.id)
	for _, f := range stk.frames {
		sf := StackFrame{
			PC:   f.pc,
			Func: s.table.strings.mustGet(f.funcID),
			File: s.table.strings.mustGet(f.fileID),
			Line: f.line,
		}
		if !yield(sf) {
			return false
		}
	}
	return true
}

// NoStack is a sentinel value that can be compared against any Stack value, indicating
// a lack of a stack trace.
var NoStack = Stack{}

// StackFrame represents a single frame of a stack.
type StackFrame struct {
	// PC is the program counter of the function call if this
	// is not a leaf frame. If it's a leaf frame, it's the point
	// at which the stack trace was taken.
	PC uint64

	// Func is the name of the function this frame maps to.
	Func string

	// File is the file which contains the source code of Func.
	File string

	// Line is the line number within File which maps to PC.
	Line uint64
}

// Event represents a single event in the trace.
type Event struct {
	table *evTable
	ctx   schedCtx
	base  baseEvent
}

// Kind returns the kind of event that this is.
func (e Event) Kind() EventKind {
	return go122Type2Kind[e.base.typ]
}

// Time returns the timestamp of the event.
func (e Event) Time() Time {
	return e.base.time
}

// Goroutine returns the ID of the goroutine that was executing when
// this event happened. It describes part of the execution context
// for this event.
//
// Note that for goroutine state transitions this always refers to the
// state before the transition. For example, if a goroutine is just
// starting to run on this thread and/or proc, then this will return
// NoGoroutine. In this case, the goroutine starting to run will be
// can be found at Event.StateTransition().Resource.
func (e Event) Goroutine() GoID {
	return e.ctx.G
}

// Proc returns the ID of the proc this event event pertains to.
//
// Note that for proc state transitions this always refers to the
// state before the transition. For example, if a proc is just
// starting to run on this thread, then this will return NoProc.
func (e Event) Proc() ProcID {
	return e.ctx.P
}

// Thread returns the ID of the thread this event pertains to.
//
// Note that for thread state transitions this always refers to the
// state before the transition. For example, if a thread is just
// starting to run, then this will return NoThread.
//
// Note: tracking thread state is not currently supported, so this
// will always return a valid thread ID. However thread state transitions
// may be tracked in the future, and callers must be robust to this
// possibility.
func (e Event) Thread() ThreadID {
	return e.ctx.M
}

// Stack returns a handle to a stack associated with the event.
//
// This represents a stack trace at the current moment in time for
// the current execution context.
func (e Event) Stack() Stack {
	if e.base.typ == evSync {
		return NoStack
	}
	if e.base.typ == go122.EvCPUSample {
		return Stack{table: e.table, id: stackID(e.base.args[0])}
	}
	spec := go122.Specs()[e.base.typ]
	if len(spec.StackIDs) == 0 {
		return NoStack
	}
	// The stack for the main execution context is always the
	// first stack listed in StackIDs. Subtract one from this
	// because we've peeled away the timestamp argument.
	id := stackID(e.base.args[spec.StackIDs[0]-1])
	if id == 0 {
		return NoStack
	}
	return Stack{table: e.table, id: id}
}

// Metric returns details about a Metric event.
//
// Panics if Kind != EventMetric.
func (e Event) Metric() Metric {
	if e.Kind() != EventMetric {
		panic("Metric called on non-Metric event")
	}
	var m Metric
	switch e.base.typ {
	case go122.EvProcsChange:
		m.Name = "/sched/gomaxprocs:threads"
		m.Value = Value{kind: ValueUint64, scalar: e.base.args[0]}
	case go122.EvHeapAlloc:
		m.Name = "/memory/classes/heap/objects:bytes"
		m.Value = Value{kind: ValueUint64, scalar: e.base.args[0]}
	case go122.EvHeapGoal:
		m.Name = "/gc/heap/goal:bytes"
		m.Value = Value{kind: ValueUint64, scalar: e.base.args[0]}
	default:
		panic(fmt.Sprintf("internal error: unexpected event type for Metric kind: %s", go122.EventString(e.base.typ)))
	}
	return m
}

// Label returns details about a Label event.
//
// Panics if Kind != EventLabel.
func (e Event) Label() Label {
	if e.Kind() != EventLabel {
		panic("Label called on non-Label event")
	}
	if e.base.typ != go122.EvGoLabel {
		panic(fmt.Sprintf("internal error: unexpected event type for Label kind: %s", go122.EventString(e.base.typ)))
	}
	return Label{
		Label:    e.table.strings.mustGet(stringID(e.base.args[0])),
		Resource: ResourceID{Kind: ResourceGoroutine, id: int64(e.ctx.G)},
	}
}

// Range returns details about an EventRangeBegin, EventRangeActive, or EventRangeEnd event.
//
// Panics if Kind != EventRangeBegin, Kind != EventRangeActive, and Kind != EventRangeEnd.
func (e Event) Range() Range {
	if kind := e.Kind(); kind != EventRangeBegin && kind != EventRangeActive && kind != EventRangeEnd {
		panic("Range called on non-Range event")
	}
	var r Range
	switch e.base.typ {
	case go122.EvSTWBegin, go122.EvSTWEnd:
		// N.B. ordering.advance smuggles in the STW reason as e.base.args[0]
		// for go122.EvSTWEnd (it's already there for Begin).
		r.Name = "stop-the-world (" + e.table.strings.mustGet(stringID(e.base.args[0])) + ")"
		r.Scope = ResourceID{Kind: ResourceGoroutine, id: int64(e.Goroutine())}
	case go122.EvGCBegin, go122.EvGCActive, go122.EvGCEnd:
		r.Name = "GC concurrent mark phase"
		r.Scope = ResourceID{Kind: ResourceNone}
	case go122.EvGCSweepBegin, go122.EvGCSweepActive, go122.EvGCSweepEnd:
		r.Name = "GC incremental sweep"
		r.Scope = ResourceID{Kind: ResourceProc}
		if e.base.typ == go122.EvGCSweepActive {
			r.Scope.id = int64(e.base.args[0])
		} else {
			r.Scope.id = int64(e.Proc())
		}
		r.Scope.id = int64(e.Proc())
	case go122.EvGCMarkAssistBegin, go122.EvGCMarkAssistActive, go122.EvGCMarkAssistEnd:
		r.Name = "GC mark assist"
		r.Scope = ResourceID{Kind: ResourceGoroutine}
		if e.base.typ == go122.EvGCMarkAssistActive {
			r.Scope.id = int64(e.base.args[0])
		} else {
			r.Scope.id = int64(e.Goroutine())
		}
	default:
		panic(fmt.Sprintf("internal error: unexpected event type for Range kind: %s", go122.EventString(e.base.typ)))
	}
	return r
}

// RangeAttributes returns attributes for a completed range.
//
// Panics if Kind != EventRangeEnd.
func (e Event) RangeAttributes() []RangeAttribute {
	if e.Kind() != EventRangeEnd {
		panic("Range called on non-Range event")
	}
	if e.base.typ != go122.EvGCSweepEnd {
		return nil
	}
	return []RangeAttribute{
		{
			Name:  "bytes swept",
			Value: Value{kind: ValueUint64, scalar: e.base.args[0]},
		},
		{
			Name:  "bytes reclaimed",
			Value: Value{kind: ValueUint64, scalar: e.base.args[1]},
		},
	}
}

// Task returns details about a TaskBegin or TaskEnd event.
//
// Panics if Kind != EventTaskBegin and Kind != EventTaskEnd.
func (e Event) Task() Task {
	if kind := e.Kind(); kind != EventTaskBegin && kind != EventTaskEnd {
		panic("Task called on non-Task event")
	}
	parentID := NoTask
	var typ string
	switch e.base.typ {
	case go122.EvUserTaskBegin:
		parentID = TaskID(e.base.args[1])
		typ = e.table.strings.mustGet(stringID(e.base.args[2]))
	case go122.EvUserTaskEnd:
		parentID = TaskID(e.base.extra(version.Go122)[0])
		typ = e.table.getExtraString(extraStringID(e.base.extra(version.Go122)[1]))
	default:
		panic(fmt.Sprintf("internal error: unexpected event type for Task kind: %s", go122.EventString(e.base.typ)))
	}
	return Task{
		ID:     TaskID(e.base.args[0]),
		Parent: parentID,
		Type:   typ,
	}
}

// Region returns details about a RegionBegin or RegionEnd event.
//
// Panics if Kind != EventRegionBegin and Kind != EventRegionEnd.
func (e Event) Region() Region {
	if kind := e.Kind(); kind != EventRegionBegin && kind != EventRegionEnd {
		panic("Region called on non-Region event")
	}
	if e.base.typ != go122.EvUserRegionBegin && e.base.typ != go122.EvUserRegionEnd {
		panic(fmt.Sprintf("internal error: unexpected event type for Region kind: %s", go122.EventString(e.base.typ)))
	}
	return Region{
		Task: TaskID(e.base.args[0]),
		Type: e.table.strings.mustGet(stringID(e.base.args[1])),
	}
}

// Log returns details about a Log event.
//
// Panics if Kind != EventLog.
func (e Event) Log() Log {
	if e.Kind() != EventLog {
		panic("Log called on non-Log event")
	}
	if e.base.typ != go122.EvUserLog {
		panic(fmt.Sprintf("internal error: unexpected event type for Log kind: %s", go122.EventString(e.base.typ)))
	}
	return Log{
		Task:     TaskID(e.base.args[0]),
		Category: e.table.strings.mustGet(stringID(e.base.args[1])),
		Message:  e.table.strings.mustGet(stringID(e.base.args[2])),
	}
}

// StateTransition returns details about a StateTransition event.
//
// Panics if Kind != EventStateTransition.
func (e Event) StateTransition() StateTransition {
	if e.Kind() != EventStateTransition {
		panic("StateTransition called on non-StateTransition event")
	}
	var s StateTransition
	switch e.base.typ {
	case go122.EvProcStart:
		s = procStateTransition(ProcID(e.base.args[0]), ProcIdle, ProcRunning)
	case go122.EvProcStop:
		s = procStateTransition(e.ctx.P, ProcRunning, ProcIdle)
	case go122.EvProcSteal:
		// N.B. ordering.advance populates e.base.extra.
		beforeState := ProcRunning
		if go122.ProcStatus(e.base.extra(version.Go122)[0]) == go122.ProcSyscallAbandoned {
			// We've lost information because this ProcSteal advanced on a
			// SyscallAbandoned state. Treat the P as idle because ProcStatus
			// treats SyscallAbandoned as Idle. Otherwise we'll have an invalid
			// transition.
			beforeState = ProcIdle
		}
		s = procStateTransition(ProcID(e.base.args[0]), beforeState, ProcIdle)
	case go122.EvProcStatus:
		// N.B. ordering.advance populates e.base.extra.
		s = procStateTransition(ProcID(e.base.args[0]), ProcState(e.base.extra(version.Go122)[0]), go122ProcStatus2ProcState[e.base.args[1]])
	case go122.EvGoCreate:
		s = goStateTransition(GoID(e.base.args[0]), GoNotExist, GoRunnable)
		s.Stack = Stack{table: e.table, id: stackID(e.base.args[1])}
	case go122.EvGoCreateSyscall:
		s = goStateTransition(GoID(e.base.args[0]), GoNotExist, GoSyscall)
	case go122.EvGoStart:
		s = goStateTransition(GoID(e.base.args[0]), GoRunnable, GoRunning)
	case go122.EvGoDestroy:
		s = goStateTransition(e.ctx.G, GoRunning, GoNotExist)
		s.Stack = e.Stack() // This event references the resource the event happened on.
	case go122.EvGoDestroySyscall:
		s = goStateTransition(e.ctx.G, GoSyscall, GoNotExist)
	case go122.EvGoStop:
		s = goStateTransition(e.ctx.G, GoRunning, GoRunnable)
		s.Reason = e.table.strings.mustGet(stringID(e.base.args[0]))
		s.Stack = e.Stack() // This event references the resource the event happened on.
	case go122.EvGoBlock:
		s = goStateTransition(e.ctx.G, GoRunning, GoWaiting)
		s.Reason = e.table.strings.mustGet(stringID(e.base.args[0]))
		s.Stack = e.Stack() // This event references the resource the event happened on.
	case go122.EvGoUnblock:
		s = goStateTransition(GoID(e.base.args[0]), GoWaiting, GoRunnable)
	case go122.EvGoSyscallBegin:
		s = goStateTransition(e.ctx.G, GoRunning, GoSyscall)
		s.Stack = e.Stack() // This event references the resource the event happened on.
	case go122.EvGoSyscallEnd:
		s = goStateTransition(e.ctx.G, GoSyscall, GoRunning)
		s.Stack = e.Stack() // This event references the resource the event happened on.
	case go122.EvGoSyscallEndBlocked:
		s = goStateTransition(e.ctx.G, GoSyscall, GoRunnable)
		s.Stack = e.Stack() // This event references the resource the event happened on.
	case go122.EvGoStatus:
		// N.B. ordering.advance populates e.base.extra.
		s = goStateTransition(GoID(e.base.args[0]), GoState(e.base.extra(version.Go122)[0]), go122GoStatus2GoState[e.base.args[2]])
	default:
		panic(fmt.Sprintf("internal error: unexpected event type for StateTransition kind: %s", go122.EventString(e.base.typ)))
	}
	return s
}

const evSync = ^event.Type(0)

var go122Type2Kind = [...]EventKind{
	go122.EvCPUSample:           EventStackSample,
	go122.EvProcsChange:         EventMetric,
	go122.EvProcStart:           EventStateTransition,
	go122.EvProcStop:            EventStateTransition,
	go122.EvProcSteal:           EventStateTransition,
	go122.EvProcStatus:          EventStateTransition,
	go122.EvGoCreate:            EventStateTransition,
	go122.EvGoCreateSyscall:     EventStateTransition,
	go122.EvGoStart:             EventStateTransition,
	go122.EvGoDestroy:           EventStateTransition,
	go122.EvGoDestroySyscall:    EventStateTransition,
	go122.EvGoStop:              EventStateTransition,
	go122.EvGoBlock:             EventStateTransition,
	go122.EvGoUnblock:           EventStateTransition,
	go122.EvGoSyscallBegin:      EventStateTransition,
	go122.EvGoSyscallEnd:        EventStateTransition,
	go122.EvGoSyscallEndBlocked: EventStateTransition,
	go122.EvGoStatus:            EventStateTransition,
	go122.EvSTWBegin:            EventRangeBegin,
	go122.EvSTWEnd:              EventRangeEnd,
	go122.EvGCActive:            EventRangeActive,
	go122.EvGCBegin:             EventRangeBegin,
	go122.EvGCEnd:               EventRangeEnd,
	go122.EvGCSweepActive:       EventRangeActive,
	go122.EvGCSweepBegin:        EventRangeBegin,
	go122.EvGCSweepEnd:          EventRangeEnd,
	go122.EvGCMarkAssistActive:  EventRangeActive,
	go122.EvGCMarkAssistBegin:   EventRangeBegin,
	go122.EvGCMarkAssistEnd:     EventRangeEnd,
	go122.EvHeapAlloc:           EventMetric,
	go122.EvHeapGoal:            EventMetric,
	go122.EvGoLabel:             EventLabel,
	go122.EvUserTaskBegin:       EventTaskBegin,
	go122.EvUserTaskEnd:         EventTaskEnd,
	go122.EvUserRegionBegin:     EventRegionBegin,
	go122.EvUserRegionEnd:       EventRegionEnd,
	go122.EvUserLog:             EventLog,
	evSync:                      EventSync,
}

var go122GoStatus2GoState = [...]GoState{
	go122.GoRunnable: GoRunnable,
	go122.GoRunning:  GoRunning,
	go122.GoWaiting:  GoWaiting,
	go122.GoSyscall:  GoSyscall,
}

var go122ProcStatus2ProcState = [...]ProcState{
	go122.ProcRunning:          ProcRunning,
	go122.ProcIdle:             ProcIdle,
	go122.ProcSyscall:          ProcRunning,
	go122.ProcSyscallAbandoned: ProcIdle,
}

// String returns the event as a human-readable string.
//
// The format of the string is intended for debugging and is subject to change.
func (e Event) String() string {
	var sb strings.Builder
	fmt.Fprintf(&sb, "M=%d P=%d G=%d", e.Thread(), e.Proc(), e.Goroutine())
	fmt.Fprintf(&sb, " %s Time=%d", e.Kind(), e.Time())
	// Kind-specific fields.
	switch kind := e.Kind(); kind {
	case EventMetric:
		m := e.Metric()
		fmt.Fprintf(&sb, " Name=%q Value=%s", m.Name, valueAsString(m.Value))
	case EventLabel:
		l := e.Label()
		fmt.Fprintf(&sb, " Label=%q Resource=%s", l.Label, l.Resource)
	case EventRangeBegin, EventRangeActive, EventRangeEnd:
		r := e.Range()
		fmt.Fprintf(&sb, " Name=%q Scope=%s", r.Name, r.Scope)
		if kind == EventRangeEnd {
			fmt.Fprintf(&sb, " Attributes=[")
			for i, attr := range e.RangeAttributes() {
				if i != 0 {
					fmt.Fprintf(&sb, " ")
				}
				fmt.Fprintf(&sb, "%q=%s", attr.Name, valueAsString(attr.Value))
			}
			fmt.Fprintf(&sb, "]")
		}
	case EventTaskBegin, EventTaskEnd:
		t := e.Task()
		fmt.Fprintf(&sb, " ID=%d Parent=%d Type=%q", t.ID, t.Parent, t.Type)
	case EventRegionBegin, EventRegionEnd:
		r := e.Region()
		fmt.Fprintf(&sb, " Task=%d Type=%q", r.Task, r.Type)
	case EventLog:
		l := e.Log()
		fmt.Fprintf(&sb, " Task=%d Category=%q Message=%q", l.Task, l.Category, l.Message)
	case EventStateTransition:
		s := e.StateTransition()
		fmt.Fprintf(&sb, " Resource=%s Reason=%q", s.Resource, s.Reason)
		switch s.Resource.Kind {
		case ResourceGoroutine:
			id := s.Resource.Goroutine()
			old, new := s.Goroutine()
			fmt.Fprintf(&sb, " GoID=%d %s->%s", id, old, new)
		case ResourceProc:
			id := s.Resource.Proc()
			old, new := s.Proc()
			fmt.Fprintf(&sb, " ProcID=%d %s->%s", id, old, new)
		}
		if s.Stack != NoStack {
			fmt.Fprintln(&sb)
			fmt.Fprintln(&sb, "TransitionStack=")
			s.Stack.Frames(func(f StackFrame) bool {
				fmt.Fprintf(&sb, "\t%s @ 0x%x\n", f.Func, f.PC)
				fmt.Fprintf(&sb, "\t\t%s:%d\n", f.File, f.Line)
				return true
			})
		}
	}
	if stk := e.Stack(); stk != NoStack {
		fmt.Fprintln(&sb)
		fmt.Fprintln(&sb, "Stack=")
		stk.Frames(func(f StackFrame) bool {
			fmt.Fprintf(&sb, "\t%s @ 0x%x\n", f.Func, f.PC)
			fmt.Fprintf(&sb, "\t\t%s:%d\n", f.File, f.Line)
			return true
		})
	}
	return sb.String()
}

// validateTableIDs checks to make sure lookups in e.table
// will work.
func (e Event) validateTableIDs() error {
	if e.base.typ == evSync {
		return nil
	}
	spec := go122.Specs()[e.base.typ]

	// Check stacks.
	for _, i := range spec.StackIDs {
		id := stackID(e.base.args[i-1])
		_, ok := e.table.stacks.get(id)
		if !ok {
			return fmt.Errorf("found invalid stack ID %d for event %s", id, spec.Name)
		}
	}
	// N.B. Strings referenced by stack frames are validated
	// early on, when reading the stacks in to begin with.

	// Check strings.
	for _, i := range spec.StringIDs {
		id := stringID(e.base.args[i-1])
		_, ok := e.table.strings.get(id)
		if !ok {
			return fmt.Errorf("found invalid string ID %d for event %s", id, spec.Name)
		}
	}
	return nil
}

func syncEvent(table *evTable, ts Time) Event {
	return Event{
		table: table,
		ctx: schedCtx{
			G: NoGoroutine,
			P: NoProc,
			M: NoThread,
		},
		base: baseEvent{
			typ:  evSync,
			time: ts,
		},
	}
}