watch: increase the windows watch i/o buffer (#3620)

fixes https://github.com/tilt-dev/tilt/issues/3556

pkg/watch/notify.go

@@ -3,7 +3,11 @@ package watch
 import (
 	"expvar"
 	"fmt"
+	"os"
 	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
 
 	"github.com/tilt-dev/tilt/pkg/logger"
 )
@@ -67,3 +71,22 @@ var _ PathMatcher = EmptyMatcher{}
 func NewWatcher(paths []string, ignore PathMatcher, l logger.Logger) (Notify, error) {
 	return newWatcher(paths, ignore, l)
 }
+
+const WindowsBufferSizeEnvVar = "TILT_WATCH_WINDOWS_BUFFER_SIZE"
+
+const defaultBufferSize int = 65536
+
+func DesiredWindowsBufferSize() int {
+	envVar := os.Getenv(WindowsBufferSizeEnvVar)
+	if envVar != "" {
+		size, err := strconv.Atoi(envVar)
+		if err != nil {
+			return size
+		}
+	}
+	return defaultBufferSize
+}
+
+func IsWindowsShortReadError(err error) bool {
+	return runtime.GOOS == "windows" && err != nil && strings.Contains(err.Error(), "short read")
+}

pkg/watch/watcher_naive.go

@@ -136,6 +136,12 @@ func (d *naiveNotify) Errors() chan error {
 func (d *naiveNotify) loop() {
 	defer close(d.wrappedEvents)
 	for e := range d.events {
+		// The Windows fsnotify event stream sometimes gets events with empty names
+		// that are also sent to the error stream. Hmmmm...
+		if e.Name == "" {
+			continue
+		}
+
 		if e.Op&fsnotify.Create != fsnotify.Create {
 			if d.shouldNotify(e.Name) {
 				d.wrappedEvents <- FileEvent{e.Name}
@@ -251,6 +257,7 @@ func newWatcher(paths []string, ignore PathMatcher, l logger.Logger) (*naiveNoti
 	if err != nil {
 		return nil, err
 	}
+	MaybeIncreaseBufferSize(fsw)
 
 	err = fsw.SetRecursive()
 	isWatcherRecursive := err == nil

pkg/watch/watcher_nonwin.go

@@ -0,0 +1,9 @@
+// +build !windows
+
+package watch
+
+import "github.com/tilt-dev/fsnotify"
+
+func MaybeIncreaseBufferSize(w *fsnotify.Watcher) {
+	// Not needed on non-windows
+}

pkg/watch/watcher_windows.go

@@ -0,0 +1,19 @@
+// +build windows
+
+package watch
+
+import (
+	"github.com/tilt-dev/fsnotify"
+)
+
+// TODO(nick): I think the ideal API would be to automatically increase the
+// size of the buffer when we exceed capacity. But this gets messy,
+// because each time we get a short read error, we need to invalidate
+// everything we know about the currently changed files. So for now,
+// we just provide a way for people to increase the buffer ourselves.
+//
+// It might also pay to be clever about sizing the buffer
+// relative the number of files in the directory we're watching.
+func MaybeIncreaseBufferSize(w *fsnotify.Watcher) {
+	w.SetBufferSize(DesiredWindowsBufferSize())
+}