chore: update Go dependencies

Co-authored-by: Andrei Regiani <andrei.cpp@gmail.com>

Makefile

@@ -53,6 +53,7 @@ PROMETHEUS_NAMESPACE=gotenberg
 PROMETHEUS_COLLECT_INTERVAL=1s
 PROMETHEUS_DISABLE_ROUTE_LOGGING=false
 PROMETHEUS_DISABLE_COLLECT=false
+UNOCONV_DISABLE_LISTENER=false
 WEBHOOK_ALLOW_LIST=
 WEBHOOK_DENY_LIST=
 WEBHOOK_ERROR_ALLOW_LIST=
@@ -95,6 +96,7 @@ run: ## Start a Gotenberg container
 	--prometheus-collect-interval=$(PROMETHEUS_COLLECT_INTERVAL) \
 	--prometheus-disable-route-logging=$(PROMETHEUS_DISABLE_ROUTE_LOGGING) \
 	--prometheus-disable-collect=$(PROMETHEUS_DISABLE_COLLECT) \
+	--unoconv-disable-listener=$(UNOCONV_DISABLE_LISTENER) \
 	--webhook-allow-list=$(WEBHOOK_ALLOW_LIST) \
 	--webhook-deny-list=$(WEBHOOK_DENY_LIST) \
 	--webhook-error-allow-list=$(WEBHOOK_ERROR_ALLOW_LIST) \

build/Dockerfile

@@ -131,6 +131,8 @@ RUN \
     # See https://github.com/gotenberg/gotenberg/pull/273.
     curl -o /usr/bin/pdftk-all.jar "https://gitlab.com/pdftk-java/pdftk/-/jobs/$PDFTK_VERSION/artifacts/raw/build/libs/pdftk-all.jar" &&\
     chmod a+x /usr/bin/pdftk-all.jar &&\
+    # Download QPDF.
+    DEBIAN_FRONTEND=noninteractive apt-get install -y -qq --no-install-recommends qpdf &&\
     # See https://github.com/nextcloud/docker/issues/380.
     mkdir -p /usr/share/man/man1mkdir -p /usr/share/man/man1 &&\
     # Cleanup.
@@ -142,7 +144,8 @@ RUN \
     chromium --version &&\
     libreoffice --version &&\
     unoconv --version &&\
-    pdftk --version
+    pdftk --version &&\
+    qpdf --version
 
 # Copy the Gotenberg binary from the builder stage.
 COPY --from=builder /home/gotenberg /usr/bin/
@@ -152,6 +155,7 @@ ENV GC_EXCLUDE_SUBSTR "hsperfdata_root,hsperfdata_gotenberg"
 ENV CHROMIUM_BIN_PATH /usr/bin/chromium
 ENV UNOCONV_BIN_PATH /usr/bin/unoconv
 ENV PDFTK_BIN_PATH /usr/bin/pdftk
+ENV QPDF_BIN_PATH /usr/bin/qpdf
 
 USER gotenberg
 WORKDIR /home/gotenberg

cmd/gotenberg.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/gotenberg/gotenberg/v7/pkg/gotenberg"
 	flag "github.com/spf13/pflag"
+	"golang.org/x/sync/errgroup"
 )
 
 // See https://patorjk.com/software/taag/#p=display&f=Small%20Slant&t=Gotenberg.
@@ -117,18 +118,40 @@ func Run() {
 	gracefulShutdownCtx, cancel := context.WithTimeout(context.Background(), gracefulShutdownDuration)
 	defer cancel()
 
+	forceQuit := make(chan os.Signal, 1)
+	signal.Notify(forceQuit, os.Interrupt)
+
+	go func() {
+		// In case of force quit, cancel the context.
+		<-forceQuit
+		cancel()
+	}()
+
 	fmt.Printf("[SYSTEM] graceful shutdown of %s\n", gracefulShutdownDuration)
 
+	eg, _ := errgroup.WithContext(gracefulShutdownCtx)
+
 	for _, a := range apps {
-		id := a.(gotenberg.Module).Descriptor().ID
-		app := a.(gotenberg.App)
+		eg.Go(func(app gotenberg.App) func() error {
+			return func() error {
+				id := app.(gotenberg.Module).Descriptor().ID
 
-		err = app.Stop(gracefulShutdownCtx)
-		if err != nil {
-			fmt.Printf("[ERROR] stopping %s: %s\n", id, err)
-		}
+				err = app.Stop(gracefulShutdownCtx)
+				if err != nil {
+					return fmt.Errorf("stopping %s: %w", id, err)
+				}
 
-		fmt.Printf("[SYSTEM] %s: application stopped\n", id)
+				fmt.Printf("[SYSTEM] %s: application stopped\n", id)
+
+				return nil
+			}
+		}(a.(gotenberg.App)))
+	}
+
+	err = eg.Wait()
+	if err != nil {
+		fmt.Printf("[FATAL] %v\n", err)
+		os.Exit(1)
 	}
 
 	os.Exit(0)

go.mod

@@ -5,8 +5,8 @@ go 1.17
 require (
 	github.com/alexliesenfeld/health v0.6.0
 	github.com/andybalholm/brotli v1.0.4 // indirect
-	github.com/chromedp/cdproto v0.0.0-20211112223831-e508c7bf6446
-	github.com/chromedp/chromedp v0.7.4
+	github.com/chromedp/cdproto v0.0.0-20211205231339-d2673e93eee4
+	github.com/chromedp/chromedp v0.7.6
 	github.com/golang/snappy v0.0.4 // indirect
 	github.com/google/uuid v1.3.0
 	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect
@@ -20,7 +20,7 @@ require (
 	github.com/microcosm-cc/bluemonday v1.0.16
 	github.com/nwaples/rardecode v1.1.2 // indirect
 	github.com/pdfcpu/pdfcpu v0.3.12
-	github.com/pierrec/lz4/v4 v4.1.11 // indirect
+	github.com/pierrec/lz4/v4 v4.1.12 // indirect
 	github.com/prometheus/client_golang v1.11.0
 	github.com/russross/blackfriday/v2 v2.1.0
 	github.com/spf13/pflag v1.0.5
@@ -32,7 +32,7 @@ require (
 	golang.org/x/image v0.0.0-20211028202545-6944b10bf410 // indirect
 	golang.org/x/net v0.0.0-20210913180222-943fd674d43e
 	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
-	golang.org/x/sys v0.0.0-20211103235746-7861aae1554b // indirect
+	golang.org/x/sys v0.0.0-20211124211545-fe61309f8881 // indirect
 	golang.org/x/term v0.0.0-20210615171337-6886f2dfbf5b
 	golang.org/x/text v0.3.7
 )

go.sum

@@ -55,11 +55,11 @@ github.com/census-instrumentation/opencensus-proto v0.2.1/go.mod h1:f6KPmirojxKA
 github.com/cespare/xxhash/v2 v2.1.1/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/cespare/xxhash/v2 v2.1.2 h1:YRXhKfTDauu4ajMg1TPgFO5jnlC2HCbmLXMcTG5cbYE=
 github.com/cespare/xxhash/v2 v2.1.2/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
-github.com/chromedp/cdproto v0.0.0-20210713064928-7d28b402946a/go.mod h1:At5TxYYdxkbQL0TSefRjhLE3Q0lgvqKKMSFUglJ7i1U=
-github.com/chromedp/cdproto v0.0.0-20211112223831-e508c7bf6446 h1:uG7ssld4pEDC5O+1UoKqMb0djjJb8qjuKZVnCskGY+s=
-github.com/chromedp/cdproto v0.0.0-20211112223831-e508c7bf6446/go.mod h1:At5TxYYdxkbQL0TSefRjhLE3Q0lgvqKKMSFUglJ7i1U=
-github.com/chromedp/chromedp v0.7.4 h1:U+0d3WbB/Oj4mDuBOI0P7S3PJEued5UZIl5AJ3QulwU=
-github.com/chromedp/chromedp v0.7.4/go.mod h1:dBj+SXuQHznp6ZPwZeDDEBZKwclUwDLbZ0hjMialMYs=
+github.com/chromedp/cdproto v0.0.0-20211126220118-81fa0469ad77/go.mod h1:At5TxYYdxkbQL0TSefRjhLE3Q0lgvqKKMSFUglJ7i1U=
+github.com/chromedp/cdproto v0.0.0-20211205231339-d2673e93eee4 h1:St4rQbn3gGWL59ygb4NBxchIeAIW0CTz5Kw4m5JTemU=
+github.com/chromedp/cdproto v0.0.0-20211205231339-d2673e93eee4/go.mod h1:At5TxYYdxkbQL0TSefRjhLE3Q0lgvqKKMSFUglJ7i1U=
+github.com/chromedp/chromedp v0.7.6 h1:2juGaktzjwULlsn+DnvIZXFUckEp5xs+GOBroaea+jA=
+github.com/chromedp/chromedp v0.7.6/go.mod h1:ayT4YU/MGAALNfOg9gNrpGSAdnU51PMx+FCeuT1iXzo=
 github.com/chromedp/sysutil v1.0.0 h1:+ZxhTpfpZlmchB58ih/LBHX52ky7w2VhQVKQMucy3Ic=
 github.com/chromedp/sysutil v1.0.0/go.mod h1:kgWmDdq8fTzXYcKIBqIYvRRTnYb9aNS9moAV0xufSww=
 github.com/chzyer/logex v1.1.10/go.mod h1:+Ywpsq7O8HXn0nuIou7OrIPyXbp3wmkHB+jjWRnGsAI=
@@ -233,8 +233,8 @@ github.com/orisano/pixelmatch v0.0.0-20210112091706-4fa4c7ba91d5/go.mod h1:nZgzb
 github.com/pdfcpu/pdfcpu v0.3.12 h1:B+MdKisilWNSk5OCO58Z9U6H93usH73xqk6hMOaZCls=
 github.com/pdfcpu/pdfcpu v0.3.12/go.mod h1:8XVBtVxuuIuSZL4Ez15Q4QoC+H8zeAaGnuiOEwAk8jA=
 github.com/pierrec/lz4/v4 v4.1.2/go.mod h1:gZWDp/Ze/IJXGXf23ltt2EXimqmTUXEy0GFuRQyBid4=
-github.com/pierrec/lz4/v4 v4.1.11 h1:LVs17FAZJFOjgmJXl9Tf13WfLUvZq7/RjfEJrnwZ9OE=
-github.com/pierrec/lz4/v4 v4.1.11/go.mod h1:gZWDp/Ze/IJXGXf23ltt2EXimqmTUXEy0GFuRQyBid4=
+github.com/pierrec/lz4/v4 v4.1.12 h1:44l88ehTZAUGW4VlO1QC4zkilL99M6Y9MXNwEs0uzP8=
+github.com/pierrec/lz4/v4 v4.1.12/go.mod h1:gZWDp/Ze/IJXGXf23ltt2EXimqmTUXEy0GFuRQyBid4=
 github.com/pkg/errors v0.8.0/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
@@ -442,14 +442,14 @@ golang.org/x/sys v0.0.0-20210124154548-22da62e12c0c/go.mod h1:h1NjWce9XRLGQEsW7w
 golang.org/x/sys v0.0.0-20210330210617-4fbd30eecc44/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210423082822-04245dca01da/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210510120138-977fb7262007/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20210525143221-35b2ab0089ea/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20210603081109-ebe580a85c40/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20210910150752-751e447fb3d0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20210927094055-39ccf1dd6fa6/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20211103235746-7861aae1554b h1:1VkfZQv42XQlA/jchYumAnv1UPo6RgF9rJFkTgZIxO4=
 golang.org/x/sys v0.0.0-20211103235746-7861aae1554b/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20211124211545-fe61309f8881 h1:TyHqChC80pFkXWraUUf6RuB5IqFdQieMLwwCJokV2pc=
+golang.org/x/sys v0.0.0-20211124211545-fe61309f8881/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
 golang.org/x/term v0.0.0-20210615171337-6886f2dfbf5b h1:9zKuko04nR4gjZ4+DNjHqRlAJqbJETHwiNKDqTfOjfE=
 golang.org/x/term v0.0.0-20210615171337-6886f2dfbf5b/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=

pkg/gotenberg/cmd.go

@@ -31,7 +31,7 @@ func Command(logger *zap.Logger, binPath string, args ...string) Cmd {
 
 	return Cmd{
 		ctx:     nil,
-		logger:  logger.Named("cmd"),
+		logger:  logger.Named(strings.ReplaceAll(binPath, "/", "")),
 		process: cmd,
 	}
 }
@@ -51,7 +51,7 @@ func CommandContext(ctx context.Context, logger *zap.Logger, binPath string, arg
 
 	return Cmd{
 		ctx:     ctx,
-		logger:  logger.Named("cmd"),
+		logger:  logger.Named(strings.ReplaceAll(binPath, "/", "")),
 		process: cmd,
 	}, nil
 }

pkg/modules/api/middlewares.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
 	"time"
 
 	"github.com/google/uuid"
@@ -147,10 +148,17 @@ func loggerMiddleware(logger *zap.Logger, disableLoggingForPaths []string) echo.
 		return func(c echo.Context) error {
 			startTime := c.Get("startTime").(time.Time)
 			trace := c.Get("trace").(string)
+			rootPath := c.Get("rootPath").(string)
 
 			// Create the request logger and add it to our locals.
 			reqLogger := logger.With(zap.String("trace", trace))
-			c.Set("logger", reqLogger)
+			c.Set("logger", reqLogger.Named(func() string {
+				return strings.ReplaceAll(
+					strings.ReplaceAll(c.Request().URL.Path, rootPath, ""),
+					"/",
+					"",
+				)
+			}()))
 
 			// Call the next middleware in the chain.
 			err := next(c)
@@ -159,7 +167,6 @@ func loggerMiddleware(logger *zap.Logger, disableLoggingForPaths []string) echo.
 			}
 
 			for _, path := range disableLoggingForPaths {
-				rootPath := c.Get("rootPath").(string)
 				URI := fmt.Sprintf("%s%s", rootPath, path)
 
 				if c.Request().RequestURI == URI {

pkg/modules/chromium/chromium.go

@@ -355,7 +355,7 @@ func (mod Chromium) Routes() ([]api.Route, error) {
 // drastically. In such a scenario, the given context may also be done before
 // the end of the conversion.
 func (mod Chromium) PDF(ctx context.Context, logger *zap.Logger, URL, outputPath string, options Options) error {
-	debug := debugLogger{logger: logger.Named("chromium.debug")}
+	debug := debugLogger{logger: logger.Named("browser")}
 	userProfileDirPath := gotenberg.NewDirPath()
 
 	args := append(chromedp.DefaultExecAllocatorOptions[:],

pkg/modules/chromium/chromium_test.go

@@ -176,11 +176,11 @@ func TestChromium_Validate(t *testing.T) {
 func TestChromium_Metrics(t *testing.T) {
 	metrics, err := new(Chromium).Metrics()
 	if err != nil {
-		t.Errorf("expected no error but got: %v", err)
+		t.Fatalf("expected no error but got: %v", err)
 	}
 
 	if len(metrics) != 1 {
-		t.Errorf("expected %d metrics, but got %d", 1, len(metrics))
+		t.Fatalf("expected %d metrics, but got %d", 1, len(metrics))
 	}
 
 	actual := metrics[0].Read()

pkg/modules/gc/gc.go

@@ -5,7 +5,6 @@ import (
 	"errors"
 	"fmt"
 	"os"
-	"os/signal"
 	"path/filepath"
 	"strings"
 	"sync"
@@ -202,19 +201,10 @@ func (gc *GarbageCollector) Stop(ctx context.Context) error {
 	}
 
 	// Block until the context is done so that other module may gracefully stop
-	// before we do a shutdown cleanup. We skip this step if we receive a
-	// SIGINT in the meantime.
+	// before we do a shutdown cleanup.
 	gc.logger.Debug("wait for the end of grace duration")
 
-	quit := make(chan os.Signal, 1)
-	signal.Notify(quit, os.Interrupt)
-
-	select {
-	case <-quit:
-		return nil
-	case <-ctx.Done():
-		break
-	}
+	<-ctx.Done()
 
 	gc.ticker.Stop()
 	gc.done <- true

pkg/modules/libreoffice/unoconv/freeport.go

@@ -0,0 +1,31 @@
+package unoconv
+
+import (
+	"fmt"
+	"net"
+	"strconv"
+
+	"go.uber.org/zap"
+)
+
+func freePort(logger *zap.Logger) (int, error) {
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		return 0, fmt.Errorf("listen on the local network address: %w", err)
+	}
+	defer func() {
+		err := listener.Close()
+		if err != nil {
+			logger.Error(fmt.Sprintf("close listener: %s", err.Error()))
+		}
+	}()
+
+	addr := listener.Addr().String()
+
+	_, portStr, err := net.SplitHostPort(addr)
+	if err != nil {
+		return 0, fmt.Errorf("get free port from host: %w", err)
+	}
+
+	return strconv.Atoi(portStr)
+}

pkg/modules/libreoffice/unoconv/unoconv.go

@@ -4,13 +4,12 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"net"
 	"os"
-	"strconv"
 	"strings"
 	"sync"
 
 	"github.com/gotenberg/gotenberg/v7/pkg/gotenberg"
+	flag "github.com/spf13/pflag"
 	"go.uber.org/zap"
 )
 
@@ -24,7 +23,12 @@ var ErrMalformedPageRanges = errors.New("page ranges are malformed")
 
 // Unoconv is a module which provides an API to interact with unoconv.
 type Unoconv struct {
-	binPath string
+	binPath         string
+	disableListener bool
+
+	listenerCmd  gotenberg.Cmd
+	listenerPort int
+	logger       *zap.Logger
 }
 
 // Options gathers available options when converting a document to PDF.
@@ -67,14 +71,23 @@ type Provider interface {
 // Descriptor returns a Unoconv's module descriptor.
 func (Unoconv) Descriptor() gotenberg.ModuleDescriptor {
 	return gotenberg.ModuleDescriptor{
-		ID:  "unoconv",
+		ID: "unoconv",
+		FlagSet: func() *flag.FlagSet {
+			fs := flag.NewFlagSet("unoconv", flag.ExitOnError)
+			fs.Bool("unoconv-disable-listener", false, "Do not start a unoconv listener - save resources in detriment of performance")
+
+			return fs
+		}(),
 		New: func() gotenberg.Module { return new(Unoconv) },
 	}
 }
 
 // Provision sets the module properties. It returns an error if the environment
 // variable UNOCONV_BIN_PATH is not set.
-func (mod *Unoconv) Provision(_ *gotenberg.Context) error {
+func (mod *Unoconv) Provision(ctx *gotenberg.Context) error {
+	flags := ctx.ParsedFlags()
+	mod.disableListener = flags.MustBool("unoconv-disable-listener")
+
 	binPath, ok := os.LookupEnv("UNOCONV_BIN_PATH")
 	if !ok {
 		return errors.New("UNOCONV_BIN_PATH environment variable is not set")
@@ -82,6 +95,18 @@ func (mod *Unoconv) Provision(_ *gotenberg.Context) error {
 
 	mod.binPath = binPath
 
+	loggerProvider, err := ctx.Module(new(gotenberg.LoggerProvider))
+	if err != nil {
+		return fmt.Errorf("get logger provider: %w", err)
+	}
+
+	logger, err := loggerProvider.(gotenberg.LoggerProvider).Logger(mod)
+	if err != nil {
+		return fmt.Errorf("get logger: %w", err)
+	}
+
+	mod.logger = logger
+
 	return nil
 }
 
@@ -95,12 +120,92 @@ func (mod Unoconv) Validate() error {
 	return nil
 }
 
+func (mod *Unoconv) Start() error {
+	if mod.disableListener {
+		return nil
+	}
+
+	port, err := freePort(mod.logger)
+	if err != nil {
+		return fmt.Errorf("get free port: %w", err)
+	}
+
+	mod.listenerPort = port
+
+	args := []string{
+		"--listener",
+		"--user-profile",
+		// Just to make sure LibreOffice does not leak files in an unknown
+		// directory. The directory will be removed anyway by the garbage
+		// collector.
+		fmt.Sprintf("//%s", gotenberg.NewDirPath()),
+		"--port",
+		fmt.Sprintf("%d", mod.listenerPort),
+	}
+
+	checkedEntry := mod.logger.Check(zap.DebugLevel, "check for debug level before setting high verbosity")
+	if checkedEntry != nil {
+		args = append(args, "-vvv")
+	}
+
+	mod.listenerCmd = gotenberg.Command(mod.logger, mod.binPath, args...)
+
+	err = mod.listenerCmd.Start()
+	if err != nil {
+		return fmt.Errorf("start unoconv listener: %w", err)
+	}
+
+	listenerActiveInstancesCountMu.Lock()
+	listenerActiveInstancesCount += 1
+	listenerActiveInstancesCountMu.Unlock()
+
+	return nil
+}
+
+// StartupMessage returns a custom startup message.
+func (mod Unoconv) StartupMessage() string {
+	if mod.disableListener {
+		return "listener disabled"
+	}
+
+	return fmt.Sprintf("listener started on port %d", mod.listenerPort)
+}
+
+// Stop stops the HTTP server.
+func (mod *Unoconv) Stop(ctx context.Context) error {
+	if mod.disableListener {
+		return nil
+	}
+
+	_, ok := ctx.Deadline()
+	if !ok {
+		return errors.New("no context dead line")
+	}
+
+	// Block until the context is done so that other module may gracefully stop
+	// before we do a shutdown cleanup.
+	mod.logger.Debug("wait for the end of grace duration")
+
+	<-ctx.Done()
+
+	err := mod.listenerCmd.Kill()
+	if err != nil {
+		return fmt.Errorf("kill unoconv listener: %w", err)
+	}
+
+	listenerActiveInstancesCountMu.Lock()
+	listenerActiveInstancesCount -= 1
+	listenerActiveInstancesCountMu.Unlock()
+
+	return nil
+}
+
 // Metrics returns the metrics.
 func (mod Unoconv) Metrics() ([]gotenberg.Metric, error) {
 	return []gotenberg.Metric{
 		{
 			Name:        "unoconv_active_instances_count",
-			Description: "Current number of active LibreOffice instances.",
+			Description: "Current number of active unoconv instances.",
 			Read: func() float64 {
 				activeInstancesCountMu.RLock()
 				defer activeInstancesCountMu.RUnlock()
@@ -108,57 +213,70 @@ func (mod Unoconv) Metrics() ([]gotenberg.Metric, error) {
 				return activeInstancesCount
 			},
 		},
+		{
+			Name:        "unoconv_listener_active_instances_count",
+			Description: "Current number of active unoconv listener instances.",
+			Read: func() float64 {
+				listenerActiveInstancesCountMu.RLock()
+				defer listenerActiveInstancesCountMu.RUnlock()
+
+				return listenerActiveInstancesCount
+			},
+		},
+		{
+			Name:        "unoconv_listener_queue_length",
+			Description: "Current number of processes in the queue.",
+			Read: func() float64 {
+				listenerQueueLengthMu.RLock()
+				defer listenerQueueLengthMu.RUnlock()
+
+				return listenerQueueLength
+			},
+		},
 	}, nil
 }
 
 // Unoconv returns an API for interacting with unoconv.
-func (mod Unoconv) Unoconv() (API, error) {
+func (mod *Unoconv) Unoconv() (API, error) {
 	return mod, nil
 }
 
-// PDF converts a document to PDF. It creates a dedicated LibreOffice instance
-// thanks to a custom user profile directory and a free port. Substantial calls
-// to this method may increase CPU and memory usage drastically. In such a
-// scenario, the given context may also be done before the end of the
-// conversion.
+// PDF converts a document to PDF.
+//
+// In stateless mode, it creates a dedicated LibreOffice instance thanks to a
+// custom user profile directory and a free port. Substantial calls to this
+// method may increase CPU and memory usage drastically. In such a scenario,
+// the given context may also be done before the end of the conversion.
+//
+// In listener mode, it calls the unoconv listener to interact with
+// LibreOffice, improving substantially the performance. However, it cannot
+// perform parallel operations and have to wait for the lock to be available.
 func (mod Unoconv) PDF(ctx context.Context, logger *zap.Logger, inputPath, outputPath string, options Options) error {
-	port, err := func() (int, error) {
-		listener, err := net.Listen("tcp", "127.0.0.1:0")
-		if err != nil {
-			return 0, fmt.Errorf("listen on the local network address: %w", err)
-		}
-		defer func() {
-			err := listener.Close()
-			if err != nil {
-				logger.Error(fmt.Sprintf("close listener: %s", err.Error()))
-			}
-		}()
-
-		addr := listener.Addr().String()
-
-		_, portStr, err := net.SplitHostPort(addr)
-		if err != nil {
-			return 0, fmt.Errorf("get free port from host: %w", err)
-		}
-
-		return strconv.Atoi(portStr)
-	}()
-
-	if err != nil {
-		return fmt.Errorf("get free port: %w", err)
-	}
-
-	userProfileDirPath := gotenberg.NewDirPath()
-
 	args := []string{
-		"--user-profile",
-		fmt.Sprintf("//%s", userProfileDirPath),
-		"--port",
-		fmt.Sprintf("%d", port),
 		"--format",
 		"pdf",
 	}
 
+	var userProfileDirPath string
+
+	if mod.disableListener {
+		port, err := freePort(logger)
+		if err != nil {
+			return fmt.Errorf("get free port: %w", err)
+		}
+
+		userProfileDirPath = gotenberg.NewDirPath()
+
+		args = append(args,
+			"--port",
+			fmt.Sprintf("%d", port),
+			"--user-profile",
+			fmt.Sprintf("//%s", userProfileDirPath),
+		)
+	} else {
+		args = append(args, "--port", fmt.Sprintf("%d", mod.listenerPort))
+	}
+
 	checkedEntry := logger.Check(zap.DebugLevel, "check for debug level before setting high verbosity")
 	if checkedEntry != nil {
 		args = append(args, "-vvv")
@@ -178,6 +296,36 @@ func (mod Unoconv) PDF(ctx context.Context, logger *zap.Logger, inputPath, outpu
 
 	args = append(args, "--output", outputPath, inputPath)
 
+	if !mod.disableListener {
+		listenerQueueLengthMu.Lock()
+		listenerQueueLength += 1
+		listenerQueueLengthMu.Unlock()
+
+		select {
+		case listenerLock <- struct{}{}:
+			logger.Debug("unoconv lock acquired")
+
+			listenerQueueLengthMu.Lock()
+			listenerQueueLength -= 1
+			listenerQueueLengthMu.Unlock()
+
+			break
+		case <-ctx.Done():
+			logger.Debug("failed to acquire the unoconv lock before deadline")
+
+			listenerQueueLengthMu.Lock()
+			listenerQueueLength -= 1
+			listenerQueueLengthMu.Unlock()
+
+			return fmt.Errorf("acquire unoconv lock: %w", ctx.Err())
+		}
+
+		defer func() {
+			<-listenerLock
+			logger.Debug("unoconv lock released")
+		}()
+	}
+
 	cmd, err := gotenberg.CommandContext(ctx, logger, mod.binPath, args...)
 	if err != nil {
 		return fmt.Errorf("create unoconv command: %w", err)
@@ -195,16 +343,18 @@ func (mod Unoconv) PDF(ctx context.Context, logger *zap.Logger, inputPath, outpu
 	activeInstancesCount -= 1
 	activeInstancesCountMu.Unlock()
 
-	// Always remove the user profile directory created by LibreOffice.
-	// See https://github.com/gotenberg/gotenberg/issues/192.
-	go func() {
-		logger.Debug(fmt.Sprintf("remove user profile directory '%s'", userProfileDirPath))
+	if mod.disableListener {
+		// Always remove the user profile directory created by LibreOffice.
+		// See https://github.com/gotenberg/gotenberg/issues/192.
+		go func() {
+			logger.Debug(fmt.Sprintf("remove user profile directory '%s'", userProfileDirPath))
 
-		err := os.RemoveAll(userProfileDirPath)
-		if err != nil {
-			logger.Error(fmt.Sprintf("remove user profile directory: %s", err))
-		}
-	}()
+			err := os.RemoveAll(userProfileDirPath)
+			if err != nil {
+				logger.Error(fmt.Sprintf("remove user profile directory: %s", err))
+			}
+		}()
+	}
 
 	if err == nil {
 		return nil
@@ -311,8 +461,13 @@ func (mod Unoconv) Extensions() []string {
 }
 
 var (
-	activeInstancesCount   float64
-	activeInstancesCountMu sync.RWMutex
+	listenerLock                   = make(chan struct{}, 1)
+	listenerQueueLength            float64
+	listenerQueueLengthMu          sync.RWMutex
+	listenerActiveInstancesCount   float64
+	listenerActiveInstancesCountMu sync.RWMutex
+	activeInstancesCount           float64
+	activeInstancesCountMu         sync.RWMutex
 )
 
 // Interface guards.
@@ -320,6 +475,7 @@ var (
 	_ gotenberg.Module          = (*Unoconv)(nil)
 	_ gotenberg.Provisioner     = (*Unoconv)(nil)
 	_ gotenberg.Validator       = (*Unoconv)(nil)
+	_ gotenberg.App             = (*Unoconv)(nil)
 	_ gotenberg.MetricsProvider = (*Unoconv)(nil)
 	_ API                       = (*Unoconv)(nil)
 	_ Provider                  = (*Unoconv)(nil)

pkg/modules/libreoffice/unoconv/unoconv_test.go

@@ -2,14 +2,42 @@ package unoconv
 
 import (
 	"context"
+	"errors"
 	"os"
 	"reflect"
 	"testing"
+	"time"
 
 	"github.com/gotenberg/gotenberg/v7/pkg/gotenberg"
 	"go.uber.org/zap"
 )
 
+type ProtoModule struct {
+	descriptor func() gotenberg.ModuleDescriptor
+}
+
+func (mod ProtoModule) Descriptor() gotenberg.ModuleDescriptor {
+	return mod.descriptor()
+}
+
+type ProtoValidator struct {
+	ProtoModule
+	validate func() error
+}
+
+func (mod ProtoValidator) Validate() error {
+	return mod.validate()
+}
+
+type ProtoLoggerProvider struct {
+	ProtoModule
+	logger func(mod gotenberg.Module) (*zap.Logger, error)
+}
+
+func (factory ProtoLoggerProvider) Logger(mod gotenberg.Module) (*zap.Logger, error) {
+	return factory.logger(mod)
+}
+
 func TestUnoconv_Descriptor(t *testing.T) {
 	descriptor := Unoconv{}.Descriptor()
 
@@ -22,12 +50,69 @@ func TestUnoconv_Descriptor(t *testing.T) {
 }
 
 func TestUnoconv_Provision(t *testing.T) {
-	mod := new(Unoconv)
-	ctx := gotenberg.NewContext(gotenberg.ParsedFlags{}, nil)
+	for i, tc := range []struct {
+		ctx       *gotenberg.Context
+		expectErr bool
+	}{
+		{
+			ctx: gotenberg.NewContext(
+				gotenberg.ParsedFlags{FlagSet: new(Unoconv).Descriptor().FlagSet},
+				make([]gotenberg.ModuleDescriptor, 0),
+			),
+			expectErr: true,
+		},
+		{
+			ctx: func() *gotenberg.Context {
+				mod := struct {
+					ProtoLoggerProvider
+				}{}
+				mod.descriptor = func() gotenberg.ModuleDescriptor {
+					return gotenberg.ModuleDescriptor{ID: "foo", New: func() gotenberg.Module { return mod }}
+				}
+				mod.logger = func(mod gotenberg.Module) (*zap.Logger, error) { return nil, errors.New("foo") }
 
-	err := mod.Provision(ctx)
-	if err != nil {
-		t.Errorf("expected no error but got: %v", err)
+				return gotenberg.NewContext(
+					gotenberg.ParsedFlags{
+						FlagSet: new(Unoconv).Descriptor().FlagSet,
+					},
+					[]gotenberg.ModuleDescriptor{
+						mod.Descriptor(),
+					},
+				)
+			}(),
+			expectErr: true,
+		},
+		{
+			ctx: func() *gotenberg.Context {
+				mod := struct {
+					ProtoLoggerProvider
+				}{}
+				mod.descriptor = func() gotenberg.ModuleDescriptor {
+					return gotenberg.ModuleDescriptor{ID: "foo", New: func() gotenberg.Module { return mod }}
+				}
+				mod.logger = func(mod gotenberg.Module) (*zap.Logger, error) { return zap.NewNop(), nil }
+
+				return gotenberg.NewContext(
+					gotenberg.ParsedFlags{
+						FlagSet: new(Unoconv).Descriptor().FlagSet,
+					},
+					[]gotenberg.ModuleDescriptor{
+						mod.Descriptor(),
+					},
+				)
+			}(),
+		},
+	} {
+		mod := new(Unoconv)
+		err := mod.Provision(tc.ctx)
+
+		if tc.expectErr && err == nil {
+			t.Errorf("test %d: expected error but got: %v", i, err)
+		}
+
+		if !tc.expectErr && err != nil {
+			t.Errorf("test %d: expected no error but got: %v", i, err)
+		}
 	}
 }
 
@@ -47,8 +132,10 @@ func TestUnoconv_Validate(t *testing.T) {
 			binPath: os.Getenv("UNOCONV_BIN_PATH"),
 		},
 	} {
-		mod := new(Unoconv)
-		mod.binPath = tc.binPath
+		mod := Unoconv{
+			binPath: tc.binPath,
+		}
+
 		err := mod.Validate()
 
 		if tc.expectErr && err == nil {
@@ -61,20 +148,149 @@ func TestUnoconv_Validate(t *testing.T) {
 	}
 }
 
-func TestChromium_Metrics(t *testing.T) {
+func TestUnoconv_Start(t *testing.T) {
+	for i, tc := range []struct {
+		mod       *Unoconv
+		expectErr bool
+	}{
+		{
+			mod: &Unoconv{
+				disableListener: true,
+				logger:          zap.NewNop(),
+			},
+		},
+		{
+			mod: &Unoconv{
+				binPath:         os.Getenv("UNOCONV_BIN_PATH"),
+				disableListener: false,
+				logger:          zap.NewExample(),
+			},
+		},
+	} {
+		func() {
+			err := tc.mod.Start()
+
+			if tc.expectErr && err == nil {
+				t.Errorf("test %d: expected error but got: %v", i, err)
+			}
+
+			if !tc.expectErr && err != nil {
+				t.Errorf("test %d: expected no error but got: %v", i, err)
+			}
+
+			ctx, cancel := context.WithTimeout(context.Background(), time.Duration(1)*time.Nanosecond)
+			defer cancel()
+
+			err = tc.mod.Stop(ctx)
+			if err != nil {
+				t.Errorf("test %d: expected not error but got: %v", i, err)
+			}
+		}()
+	}
+}
+
+func TestUnoconv_StartupMessage(t *testing.T) {
+	for i, tc := range []struct {
+		disableListener bool
+		expectMessage   string
+	}{
+		{
+			disableListener: true,
+			expectMessage:   "listener disabled",
+		},
+		{
+			expectMessage: "listener started on port 0",
+		},
+	} {
+		mod := Unoconv{
+			disableListener: tc.disableListener,
+		}
+
+		actual := mod.StartupMessage()
+		if actual != tc.expectMessage {
+			t.Errorf("test %d: expected '%s' but got '%s'", i, tc.expectMessage, actual)
+		}
+	}
+}
+
+func TestUnoconv_Stop(t *testing.T) {
+	for i, tc := range []struct {
+		start           bool
+		disableListener bool
+		timeout         time.Duration
+		expectErr       bool
+	}{
+		{
+			disableListener: true,
+		},
+		{
+			expectErr: true,
+		},
+		{
+			start:   true,
+			timeout: time.Duration(1) * time.Nanosecond,
+		},
+	} {
+		func() {
+			mod := &Unoconv{
+				binPath:         os.Getenv("UNOCONV_BIN_PATH"),
+				disableListener: tc.disableListener,
+				logger:          zap.NewNop(),
+			}
+
+			if tc.start {
+				err := mod.Start()
+				if err != nil {
+					t.Fatalf("test %d: expected no error but got: %v", i, err)
+				}
+			}
+
+			var err error
+
+			if tc.timeout == 0 {
+				err = mod.Stop(context.TODO())
+			} else {
+				ctx, cancel := context.WithTimeout(context.Background(), tc.timeout)
+				defer cancel()
+
+				err = mod.Stop(ctx)
+			}
+
+			if tc.expectErr && err == nil {
+				t.Errorf("test %d: expected error but got: %v", i, err)
+			}
+
+			if !tc.expectErr && err != nil {
+				t.Errorf("test %d: expected no error but got: %v", i, err)
+			}
+		}()
+	}
+}
+
+func TestUnoconv_Metrics(t *testing.T) {
 	metrics, err := new(Unoconv).Metrics()
 	if err != nil {
-		t.Errorf("expected no error but got: %v", err)
+		t.Fatalf("expected no error but got: %v", err)
 	}
 
-	if len(metrics) != 1 {
-		t.Errorf("expected %d metrics, but got %d", 1, len(metrics))
+	if len(metrics) != 3 {
+		t.Fatalf("expected %d metrics, but got %d", 1, len(metrics))
 	}
 
 	actual := metrics[0].Read()
 	if actual != 0 {
 		t.Errorf("expected %d unoconv instances, but got %f", 0, actual)
 	}
+
+	actual = metrics[1].Read()
+	if actual != 0 {
+		t.Errorf("expected %d unoconv listener instances, but got %f", 0, actual)
+	}
+
+	actual = metrics[2].Read()
+	if actual != 0 {
+		t.Errorf("expected %d processes in the queue, but got %f", 0, actual)
+	}
 }
 
 func TestUnoconv_Unoconv(t *testing.T) {
@@ -89,17 +305,26 @@ func TestUnoconv_Unoconv(t *testing.T) {
 func TestUnoconv_PDF(t *testing.T) {
 	for i, tc := range []struct {
 		ctx       context.Context
+		mod       Unoconv
 		logger    *zap.Logger
 		inputPath string
 		options   Options
 		expectErr bool
 	}{
 		{
+			mod: Unoconv{
+				binPath:         os.Getenv("UNOCONV_BIN_PATH"),
+				disableListener: true,
+			},
 			logger:    zap.NewNop(),
 			expectErr: true,
 		},
 		{
-			ctx:       context.Background(),
+			ctx: context.Background(),
+			mod: Unoconv{
+				binPath:         os.Getenv("UNOCONV_BIN_PATH"),
+				disableListener: true,
+			},
 			logger:    zap.NewExample(),
 			inputPath: "/tests/test/testdata/libreoffice/sample1.docx",
 			options: Options{
@@ -109,7 +334,11 @@ func TestUnoconv_PDF(t *testing.T) {
 			},
 		},
 		{
-			ctx:       context.Background(),
+			ctx: context.Background(),
+			mod: Unoconv{
+				binPath:         os.Getenv("UNOCONV_BIN_PATH"),
+				disableListener: true,
+			},
 			logger:    zap.NewNop(),
 			inputPath: "/tests/test/testdata/libreoffice/sample1.docx",
 			options: Options{
@@ -124,19 +353,40 @@ func TestUnoconv_PDF(t *testing.T) {
 
 				return ctx
 			}(),
+			mod: Unoconv{
+				binPath:         os.Getenv("UNOCONV_BIN_PATH"),
+				disableListener: true,
+			},
+			logger:    zap.NewNop(),
+			inputPath: "/tests/test/testdata/libreoffice/sample1.docx",
+			expectErr: true,
+		},
+		{
+			ctx: context.Background(),
+			mod: Unoconv{
+				binPath: os.Getenv("UNOCONV_BIN_PATH"),
+				logger:  zap.NewNop(),
+			},
+			logger:    zap.NewNop(),
+			inputPath: "/tests/test/testdata/libreoffice/sample1.docx",
+		},
+		{
+			ctx: func() context.Context {
+				ctx, cancel := context.WithCancel(context.TODO())
+				defer cancel()
+
+				return ctx
+			}(),
+			mod: Unoconv{
+				binPath: os.Getenv("UNOCONV_BIN_PATH"),
+				logger:  zap.NewNop(),
+			},
 			logger:    zap.NewNop(),
 			inputPath: "/tests/test/testdata/libreoffice/sample1.docx",
 			expectErr: true,
 		},
 	} {
 		func() {
-			mod := new(Unoconv)
-
-			err := mod.Provision(nil)
-			if err != nil {
-				t.Fatalf("test %d: expected error but got: %v", i, err)
-			}
-
 			outputDir, err := gotenberg.MkdirAll()
 			if err != nil {
 				t.Fatalf("test %d: expected error but got: %v", i, err)
@@ -149,7 +399,17 @@ func TestUnoconv_PDF(t *testing.T) {
 				}
 			}()
 
-			err = mod.PDF(tc.ctx, tc.logger, tc.inputPath, outputDir+"/foo.pdf", tc.options)
+			if !tc.mod.disableListener {
+				err = tc.mod.Start()
+				if err != nil {
+					t.Fatalf("test %d: expected no error but got: %v", i, err)
+				}
+
+				// Let's give it some room to start.
+				time.Sleep(time.Duration(1) * time.Second)
+			}
+
+			err = tc.mod.PDF(tc.ctx, tc.logger, tc.inputPath, outputDir+"/foo.pdf", tc.options)
 
 			if tc.expectErr && err == nil {
 				t.Errorf("test %d: expected error but got: %v", i, err)
@@ -158,6 +418,16 @@ func TestUnoconv_PDF(t *testing.T) {
 			if !tc.expectErr && err != nil {
 				t.Errorf("test %d: expected no error but got: %v", i, err)
 			}
+
+			if !tc.mod.disableListener {
+				ctx, cancel := context.WithTimeout(context.Background(), time.Duration(1)*time.Nanosecond)
+				defer cancel()
+
+				err = tc.mod.Stop(ctx)
+				if err != nil {
+					t.Fatalf("test %d: expected no error but got: %v", i, err)
+				}
+			}
 		}()
 	}
 }
@@ -173,3 +443,11 @@ func TestUnoconv_Extensions(t *testing.T) {
 		t.Errorf("expected %d extensions but got %d", expect, actual)
 	}
 }
+
+// Interface guards.
+var (
+	_ gotenberg.Module         = (*ProtoModule)(nil)
+	_ gotenberg.Validator      = (*ProtoValidator)(nil)
+	_ gotenberg.LoggerProvider = (*ProtoLoggerProvider)(nil)
+	_ gotenberg.Module         = (*ProtoLoggerProvider)(nil)
+)

pkg/modules/pdftk/pdftk_test.go

@@ -65,11 +65,11 @@ func TestPDFtk_Validate(t *testing.T) {
 func TestPDFtk_Metrics(t *testing.T) {
 	metrics, err := new(PDFtk).Metrics()
 	if err != nil {
-		t.Errorf("expected no error but got: %v", err)
+		t.Fatalf("expected no error but got: %v", err)
 	}
 
 	if len(metrics) != 1 {
-		t.Errorf("expected %d metrics, but got %d", 1, len(metrics))
+		t.Fatalf("expected %d metrics, but got %d", 1, len(metrics))
 	}
 
 	actual := metrics[0].Read()

pkg/modules/prometheus/prometheus.go

@@ -150,7 +150,7 @@ func (mod Prometheus) Start() error {
 // StartupMessage returns a custom startup message.
 func (mod Prometheus) StartupMessage() string {
 	if mod.disableCollect {
-		return "application not started (collect disabled by user)"
+		return "collect disabled"
 	}
 
 	return "collecting metrics"

pkg/modules/prometheus/prometheus_test.go

@@ -296,8 +296,8 @@ func TestPrometheus_StartupMessage(t *testing.T) {
 		expectMessage  string
 	}{
 		{
-			expectMessage:  "application not started (collect disabled by user)",
 			disableCollect: true,
+			expectMessage:  "collect disabled",
 		},
 		{
 			expectMessage: "collecting metrics",

pkg/modules/qpdf/doc.go

@@ -0,0 +1,3 @@
+// Package qpdf provides a module which abstracts the CLI tool QPDF and
+// implements the gotenberg.PDFEngine interface.
+package qpdf

pkg/modules/qpdf/qpdf.go

@@ -0,0 +1,117 @@
+package qpdf
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"sync"
+
+	"github.com/gotenberg/gotenberg/v7/pkg/gotenberg"
+	"go.uber.org/zap"
+)
+
+func init() {
+	gotenberg.MustRegisterModule(QPDF{})
+}
+
+// QPDF abstracts the CLI tool QPDF and implements the gotenberg.PDFEngine
+// interface.
+type QPDF struct {
+	binPath string
+}
+
+// Descriptor returns a QPDF's module descriptor.
+func (QPDF) Descriptor() gotenberg.ModuleDescriptor {
+	return gotenberg.ModuleDescriptor{
+		ID:  "qpdf",
+		New: func() gotenberg.Module { return new(QPDF) },
+	}
+}
+
+// Provision sets the modules properties. It returns an error if the
+// environment variable QPDF_BIN_PATH is not set.
+func (engine *QPDF) Provision(_ *gotenberg.Context) error {
+	binPath, ok := os.LookupEnv("QPDF_BIN_PATH")
+	if !ok {
+		return errors.New("QPDF_BIN_PATH environment variable is not set")
+	}
+
+	engine.binPath = binPath
+
+	return nil
+}
+
+// Validate validates the module properties.
+func (engine QPDF) Validate() error {
+	_, err := os.Stat(engine.binPath)
+	if os.IsNotExist(err) {
+		return fmt.Errorf("QPDF binary path does not exist: %w", err)
+	}
+
+	return nil
+}
+
+// Metrics returns the metrics.
+func (engine QPDF) Metrics() ([]gotenberg.Metric, error) {
+	return []gotenberg.Metric{
+		{
+			Name:        "qpdf_active_instances_count",
+			Description: "Current number of active QPDF instances.",
+			Read: func() float64 {
+				activeInstancesCountMu.RLock()
+				defer activeInstancesCountMu.RUnlock()
+
+				return activeInstancesCount
+			},
+		},
+	}, nil
+}
+
+// Merge merges the given PDFs into a unique PDF.
+func (engine QPDF) Merge(ctx context.Context, logger *zap.Logger, inputPaths []string, outputPath string) error {
+	var args []string
+	args = append(args, "--empty")
+	args = append(args, "--pages")
+	args = append(args, inputPaths...)
+	args = append(args, "--", outputPath)
+
+	cmd, err := gotenberg.CommandContext(ctx, logger, engine.binPath, args...)
+	if err != nil {
+		return fmt.Errorf("create command: %w", err)
+	}
+
+	activeInstancesCountMu.Lock()
+	activeInstancesCount += 1
+	activeInstancesCountMu.Unlock()
+
+	err = cmd.Exec()
+
+	activeInstancesCountMu.Lock()
+	activeInstancesCount -= 1
+	activeInstancesCountMu.Unlock()
+
+	if err == nil {
+		return nil
+	}
+
+	return fmt.Errorf("merge PDFs with QPDF: %w", err)
+}
+
+// Convert is not available for this PDF engine.
+func (engine QPDF) Convert(_ context.Context, _ *zap.Logger, format, _, _ string) error {
+	return fmt.Errorf("convert PDF to '%s' with QPDF: %w", format, gotenberg.ErrPDFEngineMethodNotAvailable)
+}
+
+var (
+	activeInstancesCount   float64
+	activeInstancesCountMu sync.RWMutex
+)
+
+var (
+	_ gotenberg.Module          = (*QPDF)(nil)
+	_ gotenberg.Provisioner     = (*QPDF)(nil)
+	_ gotenberg.Validator       = (*QPDF)(nil)
+	_ gotenberg.MetricsProvider = (*QPDF)(nil)
+	_ gotenberg.PDFEngine       = (*QPDF)(nil)
+)

pkg/modules/qpdf/qpdf_test.go

@@ -0,0 +1,152 @@
+package qpdf
+
+import (
+	"context"
+	"errors"
+	"os"
+	"reflect"
+	"testing"
+
+	"github.com/gotenberg/gotenberg/v7/pkg/gotenberg"
+	"go.uber.org/zap"
+)
+
+func TestQPDF_Descriptor(t *testing.T) {
+	descriptor := QPDF{}.Descriptor()
+
+	actual := reflect.TypeOf(descriptor.New())
+	expect := reflect.TypeOf(new(QPDF))
+
+	if actual != expect {
+		t.Errorf("expected '%s' but got '%s'", expect, actual)
+	}
+}
+
+func TestQPDF_Provision(t *testing.T) {
+	mod := new(QPDF)
+	ctx := gotenberg.NewContext(gotenberg.ParsedFlags{}, nil)
+
+	err := mod.Provision(ctx)
+	if err != nil {
+		t.Errorf("expected no error but got: %v", err)
+	}
+}
+
+func TestQPDF_Validate(t *testing.T) {
+	for i, tc := range []struct {
+		binPath   string
+		expectErr bool
+	}{
+		{
+			expectErr: true,
+		},
+		{
+			binPath:   "/foo",
+			expectErr: true,
+		},
+		{
+			binPath: os.Getenv("QPDF_BIN_PATH"),
+		},
+	} {
+		mod := new(QPDF)
+		mod.binPath = tc.binPath
+		err := mod.Validate()
+
+		if tc.expectErr && err == nil {
+			t.Errorf("test %d: expected error but got: %v", i, err)
+		}
+
+		if !tc.expectErr && err != nil {
+			t.Errorf("test %d: expected no error but got: %v", i, err)
+		}
+	}
+}
+
+func TestQPDF_Metrics(t *testing.T) {
+	metrics, err := new(QPDF).Metrics()
+	if err != nil {
+		t.Errorf("expected no error but got: %v", err)
+	}
+
+	if len(metrics) != 1 {
+		t.Errorf("expected %d metrics, but got %d", 1, len(metrics))
+	}
+
+	actual := metrics[0].Read()
+	if actual != 0 {
+		t.Errorf("expected %d QPDF instances, but got %f", 0, actual)
+	}
+}
+
+func TestQPDF_Merge(t *testing.T) {
+	for i, tc := range []struct {
+		ctx        context.Context
+		inputPaths []string
+		expectErr  bool
+	}{
+		{
+			ctx: context.TODO(),
+			inputPaths: []string{
+				"/tests/test/testdata/pdfengines/sample1.pdf",
+			},
+		},
+		{
+			ctx: context.TODO(),
+			inputPaths: []string{
+				"/tests/test/testdata/pdfengines/sample1.pdf",
+				"/tests/test/testdata/pdfengines/sample2.pdf",
+			},
+		},
+		{
+			ctx:       nil,
+			expectErr: true,
+		},
+		{
+			ctx: context.TODO(),
+			inputPaths: []string{
+				"foo",
+			},
+			expectErr: true,
+		},
+	} {
+		func() {
+			mod := new(QPDF)
+
+			err := mod.Provision(nil)
+			if err != nil {
+				t.Fatalf("test %d: expected error but got: %v", i, err)
+			}
+
+			outputDir, err := gotenberg.MkdirAll()
+			if err != nil {
+				t.Fatalf("test %d: expected error but got: %v", i, err)
+			}
+
+			defer func() {
+				err := os.RemoveAll(outputDir)
+				if err != nil {
+					t.Fatalf("test %d: expected no error but got: %v", i, err)
+				}
+			}()
+
+			err = mod.Merge(tc.ctx, zap.NewNop(), tc.inputPaths, outputDir+"/foo.pdf")
+
+			if tc.expectErr && err == nil {
+				t.Errorf("test %d: expected error but got: %v", i, err)
+			}
+
+			if !tc.expectErr && err != nil {
+				t.Errorf("test %d: expected no error but got: %v", i, err)
+			}
+		}()
+	}
+}
+
+func TestQPDF_Convert(t *testing.T) {
+	mod := new(QPDF)
+	err := mod.Convert(context.TODO(), zap.NewNop(), "", "", "")
+
+	if !errors.Is(err, gotenberg.ErrPDFEngineMethodNotAvailable) {
+		t.Errorf("expected error %v, but got: %v", gotenberg.ErrPDFEngineMethodNotAvailable, err)
+	}
+}

pkg/standard/imports.go

@@ -13,5 +13,6 @@ import (
 	_ "github.com/gotenberg/gotenberg/v7/pkg/modules/pdfengines"
 	_ "github.com/gotenberg/gotenberg/v7/pkg/modules/pdftk"
 	_ "github.com/gotenberg/gotenberg/v7/pkg/modules/prometheus"
+	_ "github.com/gotenberg/gotenberg/v7/pkg/modules/qpdf"
 	_ "github.com/gotenberg/gotenberg/v7/pkg/modules/webhook"
 )