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watcher.go
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watcher.go
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package api
// This file implements a polling file watcher for esbuild (i.e. it detects
// when files are changed by repeatedly checking their contents). Polling is
// used instead of more efficient platform-specific file system APIs because:
//
// * Go's standard library doesn't have built-in APIs for file watching
// * Using platform-specific APIs means using cgo, which I want to avoid
// * Polling is cross-platform and esbuild needs to work on 20+ platforms
// * Platform-specific APIs might be unreliable and could introduce bugs
//
// That said, this polling system is designed to use relatively little CPU vs.
// a more traditional polling system that scans the whole directory tree at
// once. The file system is still scanned regularly but each scan only checks
// a random subset of your files, which means a change to a file will be picked
// up soon after the change is made but not necessarily instantly.
//
// With the current heuristics, large projects should be completely scanned
// around every 2 seconds so in the worst case it could take up to 2 seconds
// for a change to be noticed. However, after a change has been noticed the
// change's path goes on a short list of recently changed paths which are
// checked on every scan, so further changes to recently changed files should
// be noticed almost instantly.
import (
"fmt"
"math/rand"
"os"
"sync"
"sync/atomic"
"time"
"github.com/evanw/esbuild/internal/fs"
"github.com/evanw/esbuild/internal/logger"
"github.com/evanw/esbuild/internal/resolver"
)
// The time to wait between watch intervals
const watchIntervalSleep = 100 * time.Millisecond
// The maximum number of recently-edited items to check every interval
const maxRecentItemCount = 16
// The minimum number of non-recent items to check every interval
const minItemCountPerIter = 64
// The maximum number of intervals before a change is detected
const maxIntervalsBeforeUpdate = 20
type watcher struct {
data fs.WatchData
fs fs.FS
rebuild func() fs.WatchData
recentItems []string
itemsToScan []string
mutex sync.Mutex
itemsPerIteration int
shouldStop int32
}
func (w *watcher) setWatchData(data fs.WatchData) {
defer w.mutex.Unlock()
w.mutex.Lock()
w.data = data
w.itemsToScan = w.itemsToScan[:0] // Reuse memory
// Remove any recent items that weren't a part of the latest build
end := 0
for _, path := range w.recentItems {
if data.Paths[path] != nil {
w.recentItems[end] = path
end++
}
}
w.recentItems = w.recentItems[:end]
}
func (w *watcher) start(logLevel logger.LogLevel, useColor logger.UseColor) {
go func() {
shouldLog := logLevel == logger.LevelInfo || logLevel == logger.LevelDebug || logLevel == logger.LevelVerbose
// Note: Do not change these log messages without a breaking version change.
// People want to run regexes over esbuild's stderr stream to look for these
// messages instead of using esbuild's API.
if shouldLog {
logger.PrintTextWithColor(os.Stderr, useColor, func(colors logger.Colors) string {
return fmt.Sprintf("%s[watch] build finished, watching for changes...%s\n", colors.Dim, colors.Reset)
})
}
for atomic.LoadInt32(&w.shouldStop) == 0 {
// Sleep for the watch interval
time.Sleep(watchIntervalSleep)
// Rebuild if we're dirty
if absPath := w.tryToFindDirtyPath(); absPath != "" {
if shouldLog {
logger.PrintTextWithColor(os.Stderr, useColor, func(colors logger.Colors) string {
prettyPath := resolver.PrettyPath(w.fs, logger.Path{Text: absPath, Namespace: "file"})
return fmt.Sprintf("%s[watch] build started (change: %q)%s\n", colors.Dim, prettyPath, colors.Reset)
})
}
// Run the build
w.setWatchData(w.rebuild())
if shouldLog {
logger.PrintTextWithColor(os.Stderr, useColor, func(colors logger.Colors) string {
return fmt.Sprintf("%s[watch] build finished%s\n", colors.Dim, colors.Reset)
})
}
}
}
}()
}
func (w *watcher) stop() {
atomic.StoreInt32(&w.shouldStop, 1)
}
func (w *watcher) tryToFindDirtyPath() string {
defer w.mutex.Unlock()
w.mutex.Lock()
// If we ran out of items to scan, fill the items back up in a random order
if len(w.itemsToScan) == 0 {
items := w.itemsToScan[:0] // Reuse memory
for path := range w.data.Paths {
items = append(items, path)
}
rand.Seed(time.Now().UnixNano())
for i := int32(len(items) - 1); i > 0; i-- { // Fisher-Yates shuffle
j := rand.Int31n(i + 1)
items[i], items[j] = items[j], items[i]
}
w.itemsToScan = items
// Determine how many items to check every iteration, rounded up
perIter := (len(items) + maxIntervalsBeforeUpdate - 1) / maxIntervalsBeforeUpdate
if perIter < minItemCountPerIter {
perIter = minItemCountPerIter
}
w.itemsPerIteration = perIter
}
// Always check all recent items every iteration
for i, path := range w.recentItems {
if dirtyPath := w.data.Paths[path](); dirtyPath != "" {
// Move this path to the back of the list (i.e. the "most recent" position)
copy(w.recentItems[i:], w.recentItems[i+1:])
w.recentItems[len(w.recentItems)-1] = path
return dirtyPath
}
}
// Check a constant number of items every iteration
remainingCount := len(w.itemsToScan) - w.itemsPerIteration
if remainingCount < 0 {
remainingCount = 0
}
toCheck, remaining := w.itemsToScan[remainingCount:], w.itemsToScan[:remainingCount]
w.itemsToScan = remaining
// Check if any of the entries in this iteration have been modified
for _, path := range toCheck {
if dirtyPath := w.data.Paths[path](); dirtyPath != "" {
// Mark this item as recent by adding it to the back of the list
w.recentItems = append(w.recentItems, path)
if len(w.recentItems) > maxRecentItemCount {
// Remove items from the front of the list when we hit the limit
copy(w.recentItems, w.recentItems[1:])
w.recentItems = w.recentItems[:maxRecentItemCount]
}
return dirtyPath
}
}
return ""
}