[gtk] Let printer enumeration run to completion, to avoid a GTK bug.

widget/gtk/nsDeviceContextSpecG.cpp

@@ -250,6 +250,12 @@ gboolean nsDeviceContextSpecGTK::PrinterEnumerator(GtkPrinter *aPrinter,
                                                    gpointer aData) {
   nsDeviceContextSpecGTK *spec = (nsDeviceContextSpecGTK*)aData;
 
+  if (spec->mHasEnumerationFoundAMatch) {
+    // We're already done, but we're letting the enumeration run its course,
+    // to avoid a GTK bug.
+    return FALSE;
+  }
+
   // Find the printer whose name matches the one inside the settings.
   nsXPIDLString printerName;
   nsresult rv =
@@ -266,7 +272,14 @@ gboolean nsDeviceContextSpecGTK::PrinterEnumerator(GtkPrinter *aPrinter,
       // GTK bug (https://bugzilla.gnome.org/show_bug.cgi?id=753041). We
       // sidestep this by deferring the print to the next tick.
       NS_DispatchToCurrentThread(NewRunnableMethod(spec, &nsDeviceContextSpecGTK::StartPrintJob));
-      return TRUE;
+
+      // We're already done, but we need to let the enumeration run its course,
+      // to avoid a GTK bug. So we record that we've found a match and
+      // then return FALSE.
+      // TODO: If/when we can be sure that GTK handles this OK, we could
+      // return TRUE here to avoid some needless enumeration.
+      spec->mHasEnumerationFoundAMatch = true;
+      return FALSE;
     }
   }
 
@@ -290,6 +303,7 @@ void nsDeviceContextSpecGTK::StartPrintJob() {
 void
 nsDeviceContextSpecGTK::EnumeratePrinters()
 {
+  mHasEnumerationFoundAMatch = false;
   gtk_enumerate_printers(&nsDeviceContextSpecGTK::PrinterEnumerator, this,
                          nullptr, TRUE);
 }

widget/gtk/nsDeviceContextSpecG.h

@@ -54,6 +54,8 @@ protected:
   nsCString         mSpoolName;
   nsCOMPtr<nsIFile> mSpoolFile;
   nsCString         mTitle;
+  // Helper for EnumeratePrinters / PrinterEnumerator:
+  bool mHasEnumerationFoundAMatch = false;
 
 private:
   void EnumeratePrinters();