aja: Replace global semaphore with per-device flock()

The global semaphore was never closed/unlinked, causing permission
denied issue if the device is later used by another user. Properly
removing the semaphore when stopping the pipeline would still leave it
open in case of a crash.

With a GStreamer specific name, it was also not preventing other apps to access
the device concurrently.

Finally, if the system has multiple cards, the lock should be per card
and not global (to be confirmed).

Fixes: #3283.

Sponsored-by: Netflix Inc.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6117>

subprojects/gst-plugins-bad/sys/aja/gstajacommon.cpp

@@ -22,7 +22,7 @@
 #endif
 
 #include <fcntl.h>
-#include <semaphore.h>
+#include <sys/file.h>
 
 #include "gstajacommon.h"
 
@@ -713,8 +713,20 @@ GstAjaNtv2Device *gst_aja_ntv2_device_obtain(const gchar *device_identifier) {
     return NULL;
   }
 
+  gchar *path = g_strdup_printf("/dev/ajantv2%d", device->GetIndexNumber());
+  int fd = open(path, O_RDONLY);
+  if (fd < 0) {
+    GST_ERROR("Failed to open device node %s: %s", path, g_strerror(errno));
+    delete device;
+    g_free(path);
+    return NULL;
+  }
+
   GstAjaNtv2Device *dev = g_atomic_rc_box_new0(GstAjaNtv2Device);
   dev->device = device;
+  dev->fd = fd;
+
+  g_free(path);
 
   return dev;
 }
@@ -728,35 +740,18 @@ void gst_aja_ntv2_device_unref(GstAjaNtv2Device *device) {
     GstAjaNtv2Device *dev = (GstAjaNtv2Device *)data;
 
     delete dev->device;
+    close(dev->fd);
   });
 }
 
-static gpointer init_setup_mutex(gpointer data) {
+GstAjaNtv2DeviceLocker::GstAjaNtv2DeviceLocker(GstAjaNtv2Device *device) {
-  sem_t *s = SEM_FAILED;
+  _device = gst_aja_ntv2_device_ref(device);
-  s = sem_open("/gstreamer-aja-sem", O_CREAT, S_IRUSR | S_IWUSR, 1);
+  flock(_device->fd, LOCK_EX);
-  if (s == SEM_FAILED) {
-    g_critical("Failed to create SHM semaphore for GStreamer AJA plugin: %s",
-               g_strerror(errno));
-  }
-  return s;
 }
 
-static sem_t *get_setup_mutex(void) {
+GstAjaNtv2DeviceLocker::~GstAjaNtv2DeviceLocker() {
-  static GOnce once = G_ONCE_INIT;
+  flock(_device->fd, LOCK_UN);
-
+  gst_aja_ntv2_device_unref(_device);
-  g_once(&once, init_setup_mutex, NULL);
-
-  return (sem_t *)once.retval;
-}
-
-ShmMutexLocker::ShmMutexLocker() {
-  sem_t *s = get_setup_mutex();
-  if (s != SEM_FAILED) sem_wait(s);
-}
-
-ShmMutexLocker::~ShmMutexLocker() {
-  sem_t *s = get_setup_mutex();
-  if (s != SEM_FAILED) sem_post(s);
 }
 
 static guint gst_aja_device_get_frame_multiplier(GstAjaNtv2Device *device,

subprojects/gst-plugins-bad/sys/aja/gstajacommon.h

@@ -56,6 +56,7 @@ GstAjaAudioMeta *gst_buffer_add_aja_audio_meta(GstBuffer *buffer,
 
 typedef struct {
   CNTV2Card *device;
+  int fd;
 } GstAjaNtv2Device;
 
 G_GNUC_INTERNAL
@@ -319,10 +320,12 @@ void gst_aja_common_init(void);
 
 G_END_DECLS
 
-class ShmMutexLocker {
+class GstAjaNtv2DeviceLocker {
  public:
-  ShmMutexLocker();
+  GstAjaNtv2DeviceLocker(GstAjaNtv2Device *device);
-  ~ShmMutexLocker();
+  ~GstAjaNtv2DeviceLocker();
+ private:
+  GstAjaNtv2Device *_device;
 };
 
 G_GNUC_INTERNAL

subprojects/gst-plugins-bad/sys/aja/gstajasink.cpp

@@ -724,7 +724,7 @@ static gboolean gst_aja_sink_set_caps(GstBaseSink *bsink, GstCaps *caps) {
 
   // Make sure to globally lock here as the routing settings and others are
   // global shared state
-  ShmMutexLocker locker;
+  GstAjaNtv2DeviceLocker locker(self->device);
 
   if (!::NTV2DeviceCanDoVideoFormat(self->device_id, video_format)) {
     GST_ERROR_OBJECT(self, "Device does not support mode %d",
@@ -1931,7 +1931,7 @@ restart:
   {
     // Make sure to globally lock here as the routing settings and others are
     // global shared state
-    ShmMutexLocker locker;
+    GstAjaNtv2DeviceLocker locker(self->device);
 
     self->device->device->AutoCirculateStop(self->channel);
 
@@ -2238,7 +2238,7 @@ restart:
 out: {
   // Make sure to globally lock here as the routing settings and others are
   // global shared state
-  ShmMutexLocker locker;
+  GstAjaNtv2DeviceLocker locker(self->device);
 
   self->device->device->AutoCirculateStop(self->channel);
   self->device->device->UnsubscribeOutputVerticalEvent(self->channel);

subprojects/gst-plugins-bad/sys/aja/gstajasrc.cpp

@@ -623,7 +623,7 @@ static gboolean gst_aja_src_close(GstAjaSrc *self) {
   return TRUE;
 }
 
-// Must be called with ShmMutexLocker
+// Must be called with GstAjaNtv2DeviceLocker
 static gboolean gst_aja_src_configure(GstAjaSrc *self) {
   GST_DEBUG_OBJECT(self, "Starting");
 
@@ -2301,7 +2301,7 @@ restart:
 
       // Make sure to globally lock here as the routing settings and others are
       // global shared state
-      ShmMutexLocker locker;
+      GstAjaNtv2DeviceLocker locker(self->device);
 
       if (!gst_aja_src_configure(self)) {
         g_mutex_lock(&self->queue_lock);
@@ -2788,7 +2788,7 @@ restart:
 out: {
   // Make sure to globally lock here as the routing settings and others are
   // global shared state
-  ShmMutexLocker locker;
+  GstAjaNtv2DeviceLocker locker(self->device);
 
   self->device->device->AutoCirculateStop(self->channel);
   self->device->device->UnsubscribeInputVerticalEvent(self->channel);