Fix file watching in pixlet serve (#1068)

File watching was not working when using `pixlet serve` for a file in
the working directory, or for `pixlet serve .`. This was due to a bug in
how we handle file paths in the watcher.

Fixes #1067.

cmd/serve.go

@@ -1,6 +1,8 @@
 package cmd
 
 import (
+	"fmt"
+
 	"github.com/spf13/cobra"
 
 	"tidbyt.dev/pixlet/server"
@@ -33,6 +35,10 @@ containing multiple Starlark files and resources.`,
 }
 
 func serve(cmd *cobra.Command, args []string) error {
+	if watch && cmd.Flags().Changed("watch") {
+		fmt.Printf("explicitly setting --watch is unnecessary, since it's the default\n\n")
+	}
+
 	s, err := server.NewServer(host, port, watch, args[0], maxDuration, timeout)
 	if err != nil {
 		return err

server/server.go

@@ -29,7 +29,7 @@ func NewServer(host string, port int, watch bool, path string, maxDuration int,
 	// check if path exists, and whether it is a directory or a file
 	info, err := os.Stat(path)
 	if err != nil {
-		return nil, fmt.Errorf("failed to stat %s: %w", path, err)
+		return nil, fmt.Errorf("stat'ing %s: %w", path, err)
 	}
 
 	var fs fs.FS
@@ -43,7 +43,7 @@ func NewServer(host string, port int, watch bool, path string, maxDuration int,
 		}
 
 		fs = tools.NewSingleFileFS(path)
-		w = NewWatcher(filepath.Dir(path), fileChanges)
+		w = NewWatcher(path, fileChanges)
 	}
 
 	updatesChan := make(chan loader.Update, 100)

server/watcher.go

@@ -2,9 +2,8 @@ package server
 
 import (
 	"fmt"
-	"log"
+	"os"
 	"path/filepath"
-	"strings"
 
 	"github.com/fsnotify/fsnotify"
 )
@@ -29,37 +28,53 @@ func NewWatcher(filename string, fileChanges chan bool) *Watcher {
 // If there is an error, it's returned. It's up to the caller to respawn the
 // watcher if it's desireable to keep watching.
 //
-// The reason it watches a directory is becausde some editers like VIM write
+// The reason it watches a directory is becausde some editors like VIM write
 // to a swap file and recreate the original file. So we can't simply watch the
 // original file, we have to watch the directory. This is also why we check both
 // the WRITE and CREATE events since VIM will write to a swap and then create
 // the file on save. VSCode does a WRITE and then a CHMOD, so tracking WRITE
 // catches the changes for VSCode exactly once.
 func (w *Watcher) Run() error {
+	// check if path exists, and whether it is a directory or a file
+	info, err := os.Stat(w.path)
+	if err != nil {
+		return fmt.Errorf("stat'ing %s: %w", w.path, err)
+	}
+	isDir := info.IsDir()
+
 	watcher, err := fsnotify.NewWatcher()
 	if err != nil {
-		return fmt.Errorf("error watching for changes: %w", err)
+		return fmt.Errorf("watching for changes: %w", err)
 	}
 	defer watcher.Close()
 
-	watcher.Add(w.path)
+	if isDir {
+		watcher.Add(w.path)
+	} else {
+		watcher.Add(filepath.Dir(w.path))
+	}
 
 	for {
 		select {
 		case event, ok := <-watcher.Events:
 			if !ok {
-				return fmt.Errorf("something is weird with the file watcher")
+				return fmt.Errorf("watcher events channel closed unexpectedly")
 			}
-			log.Println(event.Name)
-			if strings.HasPrefix(event.Name, w.path) && shouldNotify(event.Op) {
+
+			if !isDir && event.Name != w.path {
+				// if we're watching a single file, ignore changes to other files
+				continue
+			}
+
+			if shouldNotify(event.Op) {
 				w.fileChanges <- true
 			}
 
 		case err, ok := <-watcher.Errors:
 			if !ok {
-				return fmt.Errorf("something is weird with the file watcher around error handling")
+				return fmt.Errorf("watcher errors channel closed unexpectedly")
 			}
-			return fmt.Errorf("error in file watcher: %w", err)
+			return fmt.Errorf("watcher: %w", err)
 		}
 	}
 }