First

Dockerfile

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+## syntax=docker/dockerfile:1
+FROM golang:1.23
+WORKDIR /app
+COPY go.mod go.sum ./
+RUN go mod download
+COPY *.go ./
+RUN mkdir -p /images
+RUN CGO_ENABLED=0 GOOS=linux go build -o /random-image-server
+CMD ["/random-image-server"]

go.mod

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+module github.com/gabehf/random-image-server
+
+go 1.23.0
+
+require github.com/fsnotify/fsnotify v1.8.0
+
+require golang.org/x/sys v0.13.0 // indirect

go.sum

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+github.com/fsnotify/fsnotify v1.8.0 h1:dAwr6QBTBZIkG8roQaJjGof0pp0EeF+tNV7YBP3F/8M=
+github.com/fsnotify/fsnotify v1.8.0/go.mod h1:8jBTzvmWwFyi3Pb8djgCCO5IBqzKJ/Jwo8TRcHyHii0=
+golang.org/x/sys v0.13.0 h1:Af8nKPmuFypiUBjVoU9V20FiaFXOcuZI21p0ycVYYGE=
+golang.org/x/sys v0.13.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

main.go

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+package main
+
+import (
+	"fmt"
+	"log"
+	"math/rand"
+	"net/http"
+	"os"
+	"path"
+	"path/filepath"
+	"slices"
+	"strings"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+var images []string
+var validExtensions = getValidExtensions()
+
+func main() {
+	dirPath := os.Getenv("IMAGE_DIR")
+	if dirPath == "" {
+		dirPath = "/images"
+	}
+	files, err := os.ReadDir(dirPath)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("Image directory: %s\n", dirPath)
+	for _, file := range files {
+		if fileIsValid(path.Join(dirPath, file.Name())) {
+			images = appendIfNotExists(images, path.Join(dirPath, file.Name()))
+		} else {
+			log.Printf("File %s unreadable or not an image, ignoring.\n", file.Name())
+		}
+	}
+
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer watcher.Close()
+
+	go func() {
+		for {
+			select {
+			case event, ok := <-watcher.Events:
+				if !ok {
+					return
+				}
+				file := filenameFromEvent(event)
+				if event.Has(fsnotify.Create) {
+					// happens on rename and new file add
+					if fileIsValid(file) {
+						images = appendIfNotExists(images, file)
+					}
+				} else if event.Has(fsnotify.Remove) || event.Has(fsnotify.Rename) {
+					// happens on rm or old file rename
+					images = remove(images, file)
+				} else if event.Has(fsnotify.Chmod) {
+					// happens on chmod and rm if file descriptors are open
+					// file is deleted or unreadable, remove
+					if !fileIsValid(file) {
+						images = remove(images, file)
+					} else {
+						// file was unreadable but now is, add
+						images = appendIfNotExists(images, file)
+					}
+				}
+			case err, ok := <-watcher.Errors:
+				if !ok {
+					return
+				}
+				log.Println("error:", err)
+			}
+		}
+	}()
+
+	err = watcher.Add(dirPath)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	http.HandleFunc("/", serveImage)
+	log.Println("Listening on :8080")
+	log.Fatal(http.ListenAndServe(":8080", nil))
+}
+
+func serveImage(w http.ResponseWriter, r *http.Request) {
+	http.ServeFile(w, r, images[rand.Intn(len(images))])
+}
+
+// not fast at all but whatever
+func remove(s []string, item string) []string {
+	for i, fn := range s {
+		if fn == item {
+			log.Printf("REMOVE %s\n", item)
+			return append(s[:i], s[i+1:]...)
+		}
+	}
+	return s
+}
+
+func appendIfNotExists(slice []string, element string) []string {
+	for _, ele := range slice {
+		if ele == element {
+			return slice
+		}
+	}
+	log.Printf("ADD %s\n", element)
+	return append(slice, element)
+}
+
+func filenameFromEvent(event fsnotify.Event) string {
+	return strings.Split(event.String(), "\"")[1]
+}
+
+func fileIsValid(file string) bool {
+	return slices.Contains(validExtensions, filepath.Ext(file)) && isReadable(file)
+}
+
+func getValidExtensions() []string {
+	env := os.Getenv("ALLOWED_EXTENSIONS")
+	if env == "" {
+		return []string{".png", ".jpg", ".jpeg", ".webp"}
+	} else {
+		return strings.Split(env, ",")
+	}
+}
+
+// maybe expensive but works
+// tried file info mode perms, syscall.Access already, didnt work
+func isReadable(file string) bool {
+	_, err := os.Open(file)
+	return err == nil
+}