fix(chromium): serialize browser starts to prevent pinning proxy latch after a start timeout

pkg/modules/chromium/browser.go

@@ -58,6 +58,14 @@ type chromiumBrowser struct {
 	userProfileDirPath string
 	ctxMu              sync.RWMutex
 	isStarted          atomic.Bool
+	// startMu serializes Start calls. The supervisor's runWithDeadline
+	// abandons a Start goroutine when the request deadline expires while
+	// Chromium's startup handshake is still hanging; the abandoned goroutine
+	// keeps running, holding the resources it acquired (the pinning proxy).
+	// Serializing here prevents a second, overlapping Start from colliding
+	// with the in-flight one on the shared pinning proxy.
+	// See https://github.com/gotenberg/gotenberg/issues/1599.
+	startMu sync.Mutex
 
 	arguments    browserArguments
 	fs           *gotenberg.FileSystem
@@ -77,6 +85,19 @@ func newChromiumBrowser(arguments browserArguments) browser {
 }
 
 func (b *chromiumBrowser) Start(logger *slog.Logger) error {
+	// Refuse to run while a previous Start is still in flight. That previous
+	// Start may be a goroutine the supervisor abandoned after the request
+	// deadline expired while the Chromium startup handshake was hanging; it
+	// still holds the pinning proxy it started. An abandoned goroutine keeps
+	// holding startMu until it unwinds (bounded by --chromium-start-timeout),
+	// so no overlapping Start can collide with it on the shared pinning proxy
+	// and latch Chromium into a permanent "pinning proxy already started"
+	// state. See https://github.com/gotenberg/gotenberg/issues/1599.
+	if !b.startMu.TryLock() {
+		return errors.New("browser start already in progress")
+	}
+	defer b.startMu.Unlock()
+
 	if b.isStarted.Load() {
 		return errors.New("browser is already started")
 	}

pkg/modules/chromium/browser_test.go

@@ -0,0 +1,39 @@
+package chromium
+
+import (
+	"context"
+	"log/slog"
+	"strings"
+	"testing"
+)
+
+// TestChromiumBrowser_Start_rejectsOverlappingStart guards against the latch
+// reported in https://github.com/gotenberg/gotenberg/issues/1599. When the
+// supervisor abandons a Start goroutine on request-deadline expiry, that
+// goroutine keeps running and holds startMu (and the pinning proxy it started)
+// until it unwinds. A second Start must be refused rather than proceeding to
+// start the pinning proxy a second time.
+func TestChromiumBrowser_Start_rejectsOverlappingStart(t *testing.T) {
+	b := &chromiumBrowser{initialCtx: context.Background()}
+
+	// Simulate a Start still in flight.
+	b.startMu.Lock()
+	defer b.startMu.Unlock()
+
+	err := b.Start(slog.New(slog.DiscardHandler))
+	if err == nil {
+		t.Fatal("expected an error when a start is already in progress, got nil")
+	}
+	if !strings.Contains(err.Error(), "already in progress") {
+		t.Fatalf("expected an 'already in progress' error, got %q", err)
+	}
+
+	// The guard must return before touching any startup resource, so no user
+	// profile directory is created and the browser stays not started.
+	if b.userProfileDirPath != "" {
+		t.Fatalf("expected no user profile directory to be created, got %q", b.userProfileDirPath)
+	}
+	if b.isStarted.Load() {
+		t.Fatal("expected the browser to stay not started")
+	}
+}