Introduce priority amongst plugins

+ a bit more logging

main.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"errors"
+	"fmt"
 	"image"
 	"image/jpeg"
 	"log"
@@ -10,19 +12,43 @@ import (
 	"plugin"
 )
 
-func loadPlugin(path string) (func(image.Image) image.Image, error) {
+type Transformer struct {
+	name     string
+	t        func(image.Image) image.Image
+	priority int // priority can be 0 or 1
+}
+
+func loadPlugin(path string) (*Transformer, error) {
+	fmt.Println("Loading", path)
 	p, err := plugin.Open(path)
 	if err != nil {
 		return nil, err
 	}
-	transform, err := p.Lookup("Transform")
+
+	transformSymbol, err := p.Lookup("Transform")
 	if err != nil {
 		return nil, err
 	}
-	return transform.(func(image.Image) image.Image), nil
+	transform, ok := transformSymbol.(func(image.Image) image.Image)
+	if !ok {
+		return nil, errors.New(fmt.Sprintf("%s: Transform not a func(image.Image) image.Image", path))
+	}
+
+	var priority int
+	prioritySymbol, err := p.Lookup("Priority")
+	if err != nil {
+		fmt.Println("Error looking up Priority:", err)
+	} else {
+		var ok bool
+		priority, ok = prioritySymbol.(int)
+		if !ok {
+			fmt.Println("Error casting Priority to int")
+		}
+	}
+	return &Transformer{path, transform, priority}, nil
 }
 
-func getPlugins(pluginDir string) ([]func(image.Image) image.Image, error) {
+func getPlugins(pluginDir string) (map[int][]*Transformer, error) {
 	pDir, err := os.Open(pluginDir)
 	if err != nil {
 		return nil, err
@@ -32,7 +58,7 @@ func getPlugins(pluginDir string) ([]func(image.Image) image.Image, error) {
 		return nil, err
 	}
 
-	var ret []func(image.Image) image.Image
+	ret := make(map[int][]*Transformer)
 	for _, pFile := range pFiles {
 		if pFile.IsDir() {
 			continue
@@ -42,7 +68,7 @@ func getPlugins(pluginDir string) ([]func(image.Image) image.Image, error) {
 		if err != nil {
 			log.Printf("Failed to load pluging %s", p)
 		} else {
-			ret = append(ret, p)
+			ret[p.priority] = append(ret[p.priority], p)
 		}
 	}
 	return ret, nil
@@ -57,8 +83,13 @@ func handler(w http.ResponseWriter, r *http.Request) {
 		panic(err)
 	}
 
-	for _, p := range plugins {
-		i = p(i)
+	for _, p := range plugins[1] {
+		fmt.Println("Apply", p.name)
+		i = p.t(i)
+	}
+	for _, p := range plugins[0] {
+		fmt.Println("Apply", p.name)
+		i = p.t(i)
 	}
 
 	jpeg.Encode(w, i, nil)

pluginSrc/grayscale/plugin.go

@@ -2,6 +2,8 @@ package main
 
 import "image"
 
+var Priority int = 1
+
 // Convert to grayscale image
 func Transform(src image.Image) image.Image {
 	bounds := src.Bounds()