src

counter.go (11526B)

---

 // 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"
 )
 
 var (
 	// Note: not using internal/godebug, so that internal/godebug can use
 	// internal/counter.
 	debugCounter = strings.Contains(os.Getenv("GODEBUG"), "countertrace=1")
 	CrashOnBugs  = false // for testing; if set, exit on fatal log messages
 )
 
 // debugPrintf formats a debug message if GODEBUG=countertrace=1.
 func debugPrintf(format string, args ...any) {
 	if debugCounter {
 		if len(format) == 0 || format[len(format)-1] != '\n' {
 			format += "\n"
 		}
 		fmt.Fprintf(os.Stderr, "counter: "+format, args...)
 	}
 }
 
 // debugFatalf logs a fatal error if GODEBUG=countertrace=1.
 func debugFatalf(format string, args ...any) {
 	if debugCounter || CrashOnBugs {
 		if len(format) == 0 || format[len(format)-1] != '\n' {
 			format += "\n"
 		}
 		fmt.Fprintf(os.Stderr, "counter bug: "+format, args...)
 		os.Exit(1)
 	}
 }
 
 // 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
 	}
 }
 
 // add wraps the atomic.Uint64.Add operation to handle integer overflow.
 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 := ReadMapped(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.ReadFile.
 func ReadFile(name string) (counters, stackCounters map[string]uint64, _ error) {
 	// TODO: Document the format of the stackCounters names.
 
 	data, err := ReadMapped(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
 }
 
 // ReadMapped reads the contents of the given file by memory mapping.
 //
 // This avoids file synchronization issues.
 func ReadMapped(name string) ([]byte, error) {
 	f, err := os.OpenFile(name, os.O_RDWR, 0666)
 	if err != nil {
 		return nil, err
 	}
 	defer f.Close()
 	fi, err := f.Stat()
 	if err != nil {
 		return nil, err
 	}
 	mapping, err := memmap(f)
 	if err != nil {
 		return nil, err
 	}
 	data := make([]byte, fi.Size())
 	copy(data, mapping.Data)
 	munmap(mapping)
 	return data, nil
 }