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openbar.go
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openbar.go
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// Package openbar is a simple status command for SwayWM.
// It implements sway-protocol(7).
package openbar
import (
"context"
"encoding/json"
"io"
"log"
"math/rand"
"os"
"os/signal"
"sync"
"syscall"
"time"
)
// Header is the bar header according to sway-protocol(7).
type Header struct {
Version int `json:"version"`
ClickEvents bool `json:"click_events"`
ContSignal int `json:"cont_signal"`
StopSignal int `json:"stop_signal"`
}
var defaultHeader = Header{
Version: 1,
ClickEvents: false,
ContSignal: int(syscall.SIGCONT),
StopSignal: int(syscall.SIGSTOP),
}
// Block is one entry of the bar body according to sway-protocol(7).
// Only the required field is implemented.
type Block struct {
FullText string `json:"full_text"`
}
// Module is a bar module that emits the content of a block.
type Module interface {
FullText() (string, error)
}
// ModuleFunc is a function for the single-method interface Module.
type ModuleFunc func() (string, error)
// FullText implements Module for ModuleFunc.
func (f ModuleFunc) FullText() (string, error) {
return f()
}
// Run starts emitting the JSON infinite array with the given configuration.
func Run(ctx context.Context, opts ...Option) error {
cfg := new(config)
// Parse configuration options.
for _, opt := range opts {
opt(cfg)
}
// If we can't print headers, exit early to avoid having already started
// multiple goroutines that will leak.
if err := write(cfg.out, defaultHeader, 0x0A, 0x5B); err != nil {
return err
}
n := len(cfg.cells)
// Create the scheduler and wait for all workers to terminate before
// closing the output channel.
scheduler := bootstrap(n)
// Start one worker per module. This allows us to have variable refresh rate
// for each and every one of them.
for i, c := range cfg.cells {
go scheduler.update(ctx, i, c.module, c.interval, jitter(cfg.jitter))
}
b := make([]Block, n)
// Each time a screen update is required, mutate the bar body and print the new
// output inside the infinite JSON array. No error handling here because we
// don't want to prevent other modules from working.
for res := range scheduler.out {
b[res.idx].FullText = res.out
debug(res.err)
debug(write(cfg.out, b, 0x2C))
}
return nil
}
// A scheduler is responsible for coordination of the asynchronous updates for each
// module. Each time an update occurs, it is written to the scheduler's output channel.
type scheduler struct {
wg *sync.WaitGroup
out chan result
}
// The result of a module update holding the module index and data to be
// printed as well as any processing error.
type result struct {
idx int
out string
err error
}
// Create a scheduler of the given size.
func bootstrap(size int) scheduler {
wg := new(sync.WaitGroup)
wg.Add(size)
out := make(chan result, size)
go func() {
defer close(out)
wg.Wait()
}()
return scheduler{wg, out}
}
const (
broadcast = syscall.SIGUSR1 // Reload all modules.
sigRtMin = 0x22 // Minimum reload signal value for a single module.
sigRtMax = 0x40 // Maximum reload signal value for a single module.
)
// The function responsible for periodically updating cells. It performs an
// initial execution delayed with a random jitter to spread the load upon booting
// Sway. Then, modules are updated according to their respective intervals or when
// a signal is received. A SIGUSR1 signal will trigger a refresh for all modules
// whereas each module can be individually reloaded with SIGRTMIN+i.
func (s scheduler) update(ctx context.Context, i int, m Module, d, j time.Duration) {
defer s.wg.Done()
s.wait(i)
t1 := time.NewTimer(j)
defer t1.Stop()
// Initialize the ticker with a higher interval than the jitter timer to allow
// first paint to only be triggered by the timer. Then, receiving on the timer
// channel will reset the ticker's duration to its normal value.
t2 := time.NewTicker(j + 1)
defer t2.Stop()
sigc, id := make(chan os.Signal, 1), sigRtMin+((i+1)%sigRtMax)
signal.Notify(sigc, broadcast, syscall.Signal(id))
defer close(sigc)
defer signal.Stop(sigc)
for {
select {
case <-ctx.Done():
return
// A normal tick occurs.
case <-t2.C:
// When the jitter timer finishes, reset the ticker so the jitter offset
// affects future updates. This avoids having modules with the same interval
// updating exactly at the same time (and also sets the correct ticker interval
// which was temporarily overridden at initialization phase).
case <-t1.C:
t2.Reset(d)
// When activating a manual refresh for all modules, spread execution with
// jitter and cancel upcoming ticks by resetting the timer. This avoids performing
// the update twice in a row. Since jitter can span a few seconds, display a text
// showing to the user the module is reloading. For single module reloads,
// simply execute as fast as possible to minimize the time to visual feedback
// as this feature is often used to match another action that happened in the
// system (ie. user changed volume, we want to update the volume cell without any
// other visual artifact, we don't care about doing this twice).
case sig := <-sigc:
if sig != broadcast {
break
}
s.wait(i)
time.Sleep(j)
t2.Reset(d)
}
s.do(i, m)
}
}
// Process module output and write the result to the output channel.
func (s scheduler) do(idx int, m Module) {
out, err := m.FullText()
s.out <- result{idx, out, err}
}
const placeholder = "..."
// Display a placeholder to inform user refresh instruction has been received.
func (s scheduler) wait(idx int) {
s.out <- result{idx, placeholder, nil}
}
var initRand sync.Once
// Return a random duration lesser than the given maximum.
func jitter(max int) time.Duration {
if max == 0 {
return 0
}
initRand.Do(func() {
rand.Seed(time.Now().UnixNano())
})
//nolint:gosec
return time.Duration(rand.Intn(max)) * time.Millisecond
}
// Print a log entry if there is an error.
func debug(err error) {
if err != nil {
log.Println(err)
}
}
// Marshal the given value to JSON, concatenate additional trailing bytes and
// write them to the writer.
func write(w io.Writer, v interface{}, glue ...byte) error {
json, err := json.Marshal(v)
if err != nil {
return err
}
if _, err := w.Write(append(json, glue...)); err != nil {
return err
}
return nil
}
// This struct holds the global configuration.
type config struct {
out io.Writer
jitter int
cells []cell
}
// A cell is a module and the interval at which it must be updated.
type cell struct {
module Module
interval time.Duration
}
// Option is an application setting.
type Option func(*config)
// WithOutput configures the output for the JSON data.
func WithOutput(w io.Writer) Option {
return func(cfg *config) {
cfg.out = w
}
}
// WithError configures the output for the log entries.
func WithError(w io.Writer) Option {
return func(cfg *config) {
log.SetOutput(w)
}
}
// WithModule configures a module. Modules are printed in the order they are
// passed through this function.
func WithModule(module Module, interval time.Duration) Option {
return func(cfg *config) {
cfg.cells = append(cfg.cells, cell{module, interval})
}
}
// WithModuleFunc configures a module from an anonymous function.
func WithModuleFunc(f func() (string, error), interval time.Duration) Option {
return WithModule(ModuleFunc(f), interval)
}
// WithJitter configures the maximum time (in ms) over which modules will delay
// their updates.
func WithJitter(jitter int) Option {
return func(cfg *config) {
cfg.jitter = jitter
}
}