Browsing services discoveries blocks the UI thread

Opening to the "browse" menu often freezes the UI.

The problem is the conjunction of the following two factors:

1. the SD are created synchronously from the main thread:

#1  0x00007ffff69ff3f4 in generic_start (func=0x7fffc9d51487 <Open>, forced=true, ap=0x7fffe283c4c0) at ../../src/modules/modules.c:275
#2  0x00007ffff69ff056 in vlc_module_load (log=0x602000000658, capability=0x7ffff6e5ee60 "services_discovery", name=0x602000247930 "udisks", strict=true, probe=0x7ffff69ff258 <generic_start>) at ../../src/modules/modules.c:243
#3  0x00007ffff69ff52a in module_need (obj=0x6080001cefa0, cap=0x7ffff6e5ee60 "services_discovery", name=0x602000247930 "udisks", strict=true) at ../../src/modules/modules.c:286
#4  0x00007ffff6ba979f in vlc_sd_Create (parent=0x606000043ac0, cfg=0x6020002455f0 "udisks", owner=0x7fffe283c6d0) at ../../src/input/services_discovery.c:118
#5  0x00007ffff69f709f in vlc_media_source_New (provider=0x606000043ac0, name=0x6020002455f0 "udisks") at ../../src/media_source/media_source.c:139
#6  0x00007ffff69f7e69 in vlc_media_source_provider_Add (provider=0x606000043ac0, name=0x6020002455f0 "udisks") at ../../src/media_source/media_source.c:255
#7  0x00007ffff69f7fef in vlc_media_source_provider_GetMediaSource (provider=0x606000043ac0, name=0x6020002455f0 "udisks") at ../../src/media_source/media_source.c:272
#8  0x00007fffea731202 in NetworkDeviceModel::initializeMediaSources() (this=0x61100057e580) at ../../modules/gui/qt/network/networkdevicemodel.cpp:287
#9  0x00007fffea72a178 in NetworkDeviceModel::setCtx(MainCtx*) (this=0x61100057e580, ctx=0x6110001033c0) at ../../modules/gui/qt/network/networkdevicemodel.cpp:88

2. the SD creation (their Open callback) may perform blocking calls.

Here are some measurements (in µs):

diff

diff --git a/src/input/services_discovery.c b/src/input/services_discovery.c
index b872c5b900..c1270cb2f6 100644
--- a/src/input/services_discovery.c
+++ b/src/input/services_discovery.c
@@ -115,8 +115,10 @@ services_discovery_t *vlc_sd_Create(vlc_object_t *parent, const char *cfg,
     sd->description = NULL;
     sd->owner = *owner;
 
+    vlc_tick_t t = vlc_tick_now();
     sd->p_module = module_need(sd, "services_discovery",
                                sd->psz_name, true);
+    fprintf(stderr, "=== %s: %ld\n", sd->psz_name, vlc_tick_now() - t);
     if (sd->p_module == NULL)
     {
         msg_Err(sd, "no suitable services discovery module");

=== upnp: 51961
=== avahi: 11577
=== sap: 249
=== pulse: 7857
=== udisks: 32402
=== v4l: 5613
=== disc: 8730
=== mtp: 211
=== xcb_apps: 4660

Here, the total is ~123ms, so the UI is frozen for that amount of time, so it feels laggy.