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// 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 internal/counter implements the internals of the public counter package.
// In addition to the public API, this package also includes APIs to parse and
// manage the counter files, needed by the upload package.
package counter
import (
"fmt"
"os"
"runtime"
"strings"
"sync/atomic"
)
// Note: not using internal/godebug, so that internal/godebug can use internal/counter.
var debugCounter = strings.Contains(os.Getenv("GODEBUG"), "countertrace=1")
func debugPrintf(format string, args ...interface{}) {
if debugCounter {
if len(format) == 0 || format[len(format)-1] != '\n' {
format += "\n"
}
fmt.Fprintf(os.Stderr, "counter: "+format, args...)
}
}
// A Counter is a single named event counter.
// A Counter is safe for use by multiple goroutines simultaneously.
//
// Counters should typically be created using New
// and stored as global variables, like:
//
// package mypackage
// var errorCount = counter.New("mypackage/errors")
//
// (The initialization of errorCount in this example is handled
// entirely by the compiler and linker; this line executes no code
// at program startup.)
//
// Then code can call Add to increment the counter
// each time the corresponding event is observed.
//
// Although it is possible to use New to create
// a Counter each time a particular event needs to be recorded,
// that usage fails to amortize the construction cost over
// multiple calls to Add, so it is more expensive and not recommended.
type Counter struct {
name string
file *file
next atomic.Pointer[Counter]
state counterState
ptr counterPtr
}
func (c *Counter) Name() string {
return c.name
}
type counterPtr struct {
m *mappedFile
count *atomic.Uint64
}
type counterState struct {
bits atomic.Uint64
}
func (s *counterState) load() counterStateBits {
return counterStateBits(s.bits.Load())
}
func (s *counterState) update(old *counterStateBits, new counterStateBits) bool {
if s.bits.CompareAndSwap(uint64(*old), uint64(new)) {
*old = new
return true
}
return false
}
type counterStateBits uint64
const (
stateReaders counterStateBits = 1<<30 - 1
stateLocked counterStateBits = stateReaders
stateHavePtr counterStateBits = 1 << 30
stateExtraShift = 31
stateExtra counterStateBits = 1<<64 - 1<<stateExtraShift
)
func (b counterStateBits) readers() int { return int(b & stateReaders) }
func (b counterStateBits) locked() bool { return b&stateReaders == stateLocked }
func (b counterStateBits) havePtr() bool { return b&stateHavePtr != 0 }
func (b counterStateBits) extra() uint64 { return uint64(b&stateExtra) >> stateExtraShift }
func (b counterStateBits) incReader() counterStateBits { return b + 1 }
func (b counterStateBits) decReader() counterStateBits { return b - 1 }
func (b counterStateBits) setLocked() counterStateBits { return b | stateLocked }
func (b counterStateBits) clearLocked() counterStateBits { return b &^ stateLocked }
func (b counterStateBits) setHavePtr() counterStateBits { return b | stateHavePtr }
func (b counterStateBits) clearHavePtr() counterStateBits { return b &^ stateHavePtr }
func (b counterStateBits) clearExtra() counterStateBits { return b &^ stateExtra }
func (b counterStateBits) addExtra(n uint64) counterStateBits {
const maxExtra = uint64(stateExtra) >> stateExtraShift // 0x1ffffffff
x := b.extra()
if x+n < x || x+n > maxExtra {
x = maxExtra
} else {
x += n
}
return b.clearExtra() | counterStateBits(x)<<stateExtraShift
}
// New returns a counter with the given name.
// New can be called in global initializers and will be compiled down to
// linker-initialized data. That is, calling New to initialize a global
// has no cost at program startup.
func New(name string) *Counter {
// Note: not calling defaultFile.New in order to keep this
// function something the compiler can inline and convert
// into static data initializations, with no init-time footprint.
return &Counter{name: name, file: &defaultFile}
}
// Inc adds 1 to the counter.
func (c *Counter) Inc() {
c.Add(1)
}
// Add adds n to the counter. n cannot be negative, as counts cannot decrease.
func (c *Counter) Add(n int64) {
debugPrintf("Add %q += %d", c.name, n)
if n < 0 {
panic("Counter.Add negative")
}
if n == 0 {
return
}
c.file.register(c)
state := c.state.load()
for ; ; state = c.state.load() {
switch {
case !state.locked() && state.havePtr():
if !c.state.update(&state, state.incReader()) {
continue
}
// Counter unlocked or counter shared; has an initialized count pointer; acquired shared lock.
if c.ptr.count == nil {
for !c.state.update(&state, state.addExtra(uint64(n))) {
// keep trying - we already took the reader lock
state = c.state.load()
}
debugPrintf("Add %q += %d: nil extra=%d\n", c.name, n, state.extra())
} else {
sum := c.add(uint64(n))
debugPrintf("Add %q += %d: count=%d\n", c.name, n, sum)
}
c.releaseReader(state)
return
case state.locked():
if !c.state.update(&state, state.addExtra(uint64(n))) {
continue
}
debugPrintf("Add %q += %d: locked extra=%d\n", c.name, n, state.extra())
return
case !state.havePtr():
if !c.state.update(&state, state.addExtra(uint64(n)).setLocked()) {
continue
}
debugPrintf("Add %q += %d: noptr extra=%d\n", c.name, n, state.extra())
c.releaseLock(state)
return
}
}
}
func (c *Counter) releaseReader(state counterStateBits) {
for ; ; state = c.state.load() {
// If we are the last reader and havePtr was cleared
// while this batch of readers was using c.ptr,
// it's our job to update c.ptr by upgrading to a full lock
// and letting releaseLock do the work.
// Note: no new reader will attempt to add itself now that havePtr is clear,
// so we are only racing against possible additions to extra.
if state.readers() == 1 && !state.havePtr() {
if !c.state.update(&state, state.setLocked()) {
continue
}
debugPrintf("releaseReader %s: last reader, need ptr\n", c.name)
c.releaseLock(state)
return
}
// Release reader.
if !c.state.update(&state, state.decReader()) {
continue
}
debugPrintf("releaseReader %s: released (%d readers now)\n", c.name, state.readers())
return
}
}
func (c *Counter) releaseLock(state counterStateBits) {
for ; ; state = c.state.load() {
if !state.havePtr() {
// Set havePtr before updating ptr,
// to avoid race with the next clear of havePtr.
if !c.state.update(&state, state.setHavePtr()) {
continue
}
debugPrintf("releaseLock %s: reset havePtr (extra=%d)\n", c.name, state.extra())
// Optimization: only bother loading a new pointer
// if we have a value to add to it.
c.ptr = counterPtr{nil, nil}
if state.extra() != 0 {
c.ptr = c.file.lookup(c.name)
debugPrintf("releaseLock %s: ptr=%v\n", c.name, c.ptr)
}
}
if extra := state.extra(); extra != 0 && c.ptr.count != nil {
if !c.state.update(&state, state.clearExtra()) {
continue
}
sum := c.add(extra)
debugPrintf("releaseLock %s: flush extra=%d -> count=%d\n", c.name, extra, sum)
}
// Took care of refreshing ptr and flushing extra.
// Now we can release the lock, unless of course
// another goroutine cleared havePtr or added to extra,
// in which case we go around again.
if !c.state.update(&state, state.clearLocked()) {
continue
}
debugPrintf("releaseLock %s: unlocked\n", c.name)
return
}
}
func (c *Counter) add(n uint64) uint64 {
count := c.ptr.count
for {
old := count.Load()
sum := old + n
if sum < old {
sum = ^uint64(0)
}
if count.CompareAndSwap(old, sum) {
runtime.KeepAlive(c.ptr.m)
return sum
}
}
}
func (c *Counter) invalidate() {
for {
state := c.state.load()
if !state.havePtr() {
debugPrintf("invalidate %s: no ptr\n", c.name)
return
}
if c.state.update(&state, state.clearHavePtr()) {
debugPrintf("invalidate %s: cleared havePtr\n", c.name)
return
}
}
}
func (c *Counter) refresh() {
for {
state := c.state.load()
if state.havePtr() || state.readers() > 0 || state.extra() == 0 {
debugPrintf("refresh %s: havePtr=%v readers=%d extra=%d\n", c.name, state.havePtr(), state.readers(), state.extra())
return
}
if c.state.update(&state, state.setLocked()) {
debugPrintf("refresh %s: locked havePtr=%v readers=%d extra=%d\n", c.name, state.havePtr(), state.readers(), state.extra())
c.releaseLock(state)
return
}
}
}
// Read reads the given counter.
// This is the implementation of x/telemetry/counter/countertest.ReadCounter.
func Read(c *Counter) (uint64, error) {
if c.file.current.Load() == nil {
return c.state.load().extra(), nil
}
pf, err := readFile(c.file)
if err != nil {
return 0, err
}
v, ok := pf.Count[DecodeStack(c.Name())]
if !ok {
return v, fmt.Errorf("not found:%q", DecodeStack(c.Name()))
}
return v, nil
}
func readFile(f *file) (*File, error) {
if f == nil {
debugPrintf("No file")
return nil, fmt.Errorf("counter is not initialized - was Open called?")
}
// Note: don't call f.rotate here as this will enqueue a follow-up rotation.
f.rotate1()
if f.err != nil {
return nil, fmt.Errorf("failed to rotate mapped file - %v", f.err)
}
current := f.current.Load()
if current == nil {
return nil, fmt.Errorf("counter has no mapped file")
}
name := current.f.Name()
data, err := os.ReadFile(name)
if err != nil {
return nil, fmt.Errorf("failed to read from file: %v", err)
}
pf, err := Parse(name, data)
if err != nil {
return nil, fmt.Errorf("failed to parse: %v", err)
}
return pf, nil
}
// ReadFile reads the counters and stack counters from the given file.
// This is the implementation of x/telemetry/counter/countertest.Read
func ReadFile(name string) (counters, stackCounters map[string]uint64, _ error) {
// TODO: Document the format of the stackCounters names.
data, err := os.ReadFile(name)
if err != nil {
return nil, nil, fmt.Errorf("failed to read from file: %v", err)
}
pf, err := Parse(name, data)
if err != nil {
return nil, nil, fmt.Errorf("failed to parse: %v", err)
}
counters = make(map[string]uint64)
stackCounters = make(map[string]uint64)
for k, v := range pf.Count {
if IsStackCounter(k) {
stackCounters[DecodeStack(k)] = v
} else {
counters[k] = v
}
}
return counters, stackCounters, nil
}