oauth2-proxy/oauth2-proxy

package watcher

import (
    "fmt"
    "os"
    "path/filepath"
    "time"

    "github.com/fsnotify/fsnotify"

    "github.com/oauth2-proxy/oauth2-proxy/v7/pkg/logger"
)

// WatchFileForUpdates performs an action every time a file on disk is updated
func WatchFileForUpdates(filename string, done <-chan bool, action func()) error {
    filename = filepath.Clean(filename)
    watcher, err := fsnotify.NewWatcher()
    if err != nil {
        return fmt.Errorf("failed to create watcher for '%s': %s", filename, err)
    }

    go func() {
        defer watcher.Close()

        for {
            select {
            case <-done:
                logger.Printf("shutting down watcher for: %s", filename)
                return
            case event := <-watcher.Events:
                filterEvent(watcher, event, filename, action)
            case err = <-watcher.Errors:
                logger.Errorf("error watching '%s': %s", filename, err)
            }
        }
    }()
    if err := watcher.Add(filename); err != nil {
        return fmt.Errorf("failed to add '%s' to watcher: %v", filename, err)
    }
    logger.Printf("watching '%s' for updates", filename)

    return nil
}

// Filter file operations based on the events sent by the watcher.
// Execute the action() function when the following conditions are met:
//   - the real path of the file was changed (Kubernetes ConfigMap/Secret)
//   - the file is modified or created
func filterEvent(watcher *fsnotify.Watcher, event fsnotify.Event, filename string, action func()) {
    switch filepath.Clean(event.Name) == filename {
    // In Kubernetes the file path is a symlink, so we should take action
    // when the ConfigMap/Secret is replaced.
    case event.Op&fsnotify.Remove != 0:
        logger.Printf("watching interrupted on event: %s", event)
        WaitForReplacement(filename, event.Op, watcher)
        action()
    case event.Op&(fsnotify.Create|fsnotify.Write) != 0:
        logger.Printf("reloading after event: %s", event)
        action()
    }
}

// WaitForReplacement waits for a file to exist on disk and then starts a watch
// for the file
func WaitForReplacement(filename string, op fsnotify.Op, watcher *fsnotify.Watcher) {
    const sleepInterval = 50 * time.Millisecond

    // Avoid a race when fsnofity.Remove is preceded by fsnotify.Chmod.
    if op&fsnotify.Chmod != 0 {
        time.Sleep(sleepInterval)
    }
    for {
        if _, err := os.Stat(filename); err == nil {
            if err := watcher.Add(filename); err == nil {
                logger.Printf("watching resumed for '%s'", filename)
                return
            }
        }
        time.Sleep(sleepInterval)
    }
}