Add polling as a fallback to native filesystem events in server watch

Fixes #8720
Fixes #6849
Fixes #7930

watcher/filenotify/filenotify.go

@@ -0,0 +1,49 @@
+// Package filenotify provides a mechanism for watching file(s) for changes.
+// Generally leans on fsnotify, but provides a poll-based notifier which fsnotify does not support.
+// These are wrapped up in a common interface so that either can be used interchangeably in your code.
+//
+// This package is adapted from https://github.com/moby/moby/tree/master/pkg/filenotify, Apache-2.0 License.
+// Hopefully this can be replaced with an external package sometime in the future, see https://github.com/fsnotify/fsnotify/issues/9
+package filenotify
+
+import (
+	"time"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+// FileWatcher is an interface for implementing file notification watchers
+type FileWatcher interface {
+	Events() <-chan fsnotify.Event
+	Errors() <-chan error
+	Add(name string) error
+	Remove(name string) error
+	Close() error
+}
+
+// New tries to use an fs-event watcher, and falls back to the poller if there is an error
+func New(interval time.Duration) (FileWatcher, error) {
+	if watcher, err := NewEventWatcher(); err == nil {
+		return watcher, nil
+	}
+	return NewPollingWatcher(interval), nil
+}
+
+// NewPollingWatcher returns a poll-based file watcher
+func NewPollingWatcher(interval time.Duration) FileWatcher {
+	return &filePoller{
+		interval: interval,
+		done:     make(chan struct{}),
+		events:   make(chan fsnotify.Event),
+		errors:   make(chan error),
+	}
+}
+
+// NewEventWatcher returns an fs-event based file watcher
+func NewEventWatcher() (FileWatcher, error) {
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		return nil, err
+	}
+	return &fsNotifyWatcher{watcher}, nil
+}