fix(pdfengines): selection of PDF engines and add a new module interface for modules that want to print system message on startup

README.md

@@ -1,8 +1,8 @@
 <p align="center">
     <img src="https://user-images.githubusercontent.com/8983173/130322857-185831e2-f041-46eb-a17f-0a69d066c4e5.png" alt="Gotenberg Logo" width="150" height="150" />
-<h3 align="center">Gotenberg</h3>
+    <h3 align="center">Gotenberg</h3>
-<p align="center">A Docker-powered stateless API for PDF files</p>
+    <p align="center">A Docker-powered stateless API for PDF files</p>
-<p align="center"><a href="https://gotenberg.dev/docs/about">Documentation</a> &#183; 🔥 <a href="https://gotenberg.dev/docs/get-started/live-demo">Live Demo</a></p>
+    <p align="center"><a href="https://gotenberg.dev/docs/about">Documentation</a> &#183; 🔥 <a href="https://gotenberg.dev/docs/get-started/live-demo">Live Demo</a></p>
 </p>
 
 ---

cmd/gotenberg.go

@@ -33,7 +33,7 @@ var Version = "snapshot"
 func Run() {
 	fmt.Printf(banner, Version)
 
-	// Creates the roo` FlagSet and adds the modules flags to it.
+	// Create the root FlagSet and adds the modules flags to it.
 	fs := flag.NewFlagSet("gotenberg", flag.ExitOnError)
 	fs.Duration("gotenberg-graceful-shutdown-duration", time.Duration(30)*time.Second, "Set the graceful shutdown duration")
 
@@ -46,14 +46,14 @@ func Run() {
 
 	fmt.Printf("[SYSTEM] modules: %s\n", modsInfo)
 
-	// Parses the flags...
+	// Parse the flags...
 	err := fs.Parse(os.Args[1:])
 	if err != nil {
 		fmt.Println(err)
 		os.Exit(1)
 	}
 
-	// ...and creates a wrapper around those.
+	// ...and create a wrapper around those.
 	parsedFlags := gotenberg.ParsedFlags{FlagSet: fs}
 
 	// Get the graceful shutdown duration.
@@ -61,7 +61,7 @@ func Run() {
 
 	ctx := gotenberg.NewContext(parsedFlags, descriptors)
 
-	// Starts application modules.
+	// Start application modules.
 	apps, err := ctx.Modules(new(gotenberg.App))
 	if err != nil {
 		fmt.Printf("[FATAL] %s\n", err)
@@ -87,7 +87,23 @@ func Run() {
 
 			fmt.Printf("[SYSTEM] %s: %s\n", id, startupMessage)
 		}(a.(gotenberg.App))
+	}
 
+	// Get modules that want to print system messages.
+	sysLoggers, err := ctx.Modules(new(gotenberg.SystemLogger))
+	if err != nil {
+		fmt.Printf("[FATAL] %s\n", err)
+		os.Exit(1)
+	}
+
+	for _, l := range sysLoggers {
+		go func(logger gotenberg.SystemLogger) {
+			id := logger.(gotenberg.Module).Descriptor().ID
+
+			for _, message := range logger.SystemMessages() {
+				fmt.Printf("[SYSTEM] %s: %s\n", id, message)
+			}
+		}(l.(gotenberg.SystemLogger))
 	}
 
 	quit := make(chan os.Signal, 1)

pkg/gotenberg/modules.go

@@ -69,6 +69,12 @@ type App interface {
 	Stop(ctx context.Context) error
 }
 
+// SystemLogger is a module interface for modules which want to display
+// messages on startup.
+type SystemLogger interface {
+	SystemMessages() []string
+}
+
 // MustRegisterModule registers a module.
 //
 // To register a module, create an init() method in the module main go file:

pkg/modules/pdfengines/pdfengines.go

@@ -3,6 +3,7 @@ package pdfengines
 import (
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/gotenberg/gotenberg/v7/pkg/gotenberg"
 	"github.com/gotenberg/gotenberg/v7/pkg/modules/api"
@@ -36,7 +37,7 @@ func (PDFEngines) Descriptor() gotenberg.ModuleDescriptor {
 		ID: "pdfengines",
 		FlagSet: func() *flag.FlagSet {
 			fs := flag.NewFlagSet("pdfengines", flag.ExitOnError)
-			fs.StringSlice("pdfengines-engines", make([]string, 0), "Set the PDF engines - all by default")
+			fs.StringSlice("pdfengines-engines", make([]string, 0), "Set the PDF engines and their order - all by default")
 			fs.Bool("pdfengines-disable-routes", false, "Disable the routes")
 
 			return fs
@@ -118,14 +119,25 @@ func (mod PDFEngines) Validate() error {
 	return fmt.Errorf("non-existing PDF engine(s): %s - available PDF engine(s): %s", nonExistingEngines, availableEngines)
 }
 
+// SystemMessages returns one message with the selected gotenberg.PDFEngine
+// modules.
+func (mod PDFEngines) SystemMessages() []string {
+	return []string{
+		strings.Join(mod.names[:], " "),
+	}
+}
+
 // PDFEngine returns a gotenberg.PDFEngine.
 func (mod PDFEngines) PDFEngine() (gotenberg.PDFEngine, error) {
-	engines := make([]gotenberg.PDFEngine, len(mod.engines))
+	engines := make([]gotenberg.PDFEngine, len(mod.names))
 
-	i := 0
+	for i, name := range mod.names {
-	for _, engine := range mod.engines {
+		for _, engine := range mod.engines {
-		engines[i] = engine
+			if name == engine.(gotenberg.Module).Descriptor().ID {
-		i++
+				engines[i] = engine
+				break
+			}
+		}
 	}
 
 	return newMultiPDFEngines(engines...), nil
@@ -155,6 +167,7 @@ var (
 	_ gotenberg.Module            = (*PDFEngines)(nil)
 	_ gotenberg.Provisioner       = (*PDFEngines)(nil)
 	_ gotenberg.Validator         = (*PDFEngines)(nil)
+	_ gotenberg.SystemLogger      = (*PDFEngines)(nil)
 	_ gotenberg.PDFEngineProvider = (*PDFEngines)(nil)
 	_ api.Router                  = (*PDFEngines)(nil)
 )

pkg/modules/pdfengines/pdfengines_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"reflect"
+	"strings"
 	"testing"
 
 	"github.com/gotenberg/gotenberg/v7/pkg/gotenberg"
@@ -252,11 +253,40 @@ func TestPDFEngine_Validate(t *testing.T) {
 	}
 }
 
+func TestPDFEngines_SystemMessages(t *testing.T) {
+	mod := new(PDFEngines)
+	mod.names = []string{"foo", "bar"}
+
+	messages := mod.SystemMessages()
+	if len(messages) != 1 {
+		t.Errorf("expected one and only one message but got %d", len(messages))
+	}
+
+	expect := strings.Join(mod.names[:], " ")
+	if messages[0] != expect {
+		t.Errorf("expected message '%s' but got '%s'", expect, messages[0])
+	}
+}
+
 func TestPDFEngine_PDFEngine(t *testing.T) {
 	mod := new(PDFEngines)
-	mod.engines = []gotenberg.PDFEngine{
+	mod.names = []string{"foo", "bar"}
-		struct{ ProtoPDFEngine }{},
+	mod.engines = func() []gotenberg.PDFEngine {
-	}
+		engine1 := struct{ ProtoPDFEngine }{}
+		engine1.descriptor = func() gotenberg.ModuleDescriptor {
+			return gotenberg.ModuleDescriptor{ID: "foo", New: func() gotenberg.Module { return engine1 }}
+		}
+
+		engine2 := struct{ ProtoPDFEngine }{}
+		engine2.descriptor = func() gotenberg.ModuleDescriptor {
+			return gotenberg.ModuleDescriptor{ID: "bar", New: func() gotenberg.Module { return engine2 }}
+		}
+
+		return []gotenberg.PDFEngine{
+			engine1,
+			engine2,
+		}
+	}()
 
 	_, err := mod.PDFEngine()
 	if err != nil {