audio/filter: remove reentrancy flag

This flag was used by some filters and made sure none of these filters
were inserted twice. This triggers only if the user explicitly tries to
add multiple filters (and not e.g. due to auto-insertion), so at best
this warned the user from doing something potentially pointless. At
worst, it blocked some (mildly) legitimate use-cases. Get rid of it.

Also see #2322.

audio/filter/af.c

@@ -159,24 +159,11 @@ static struct af_instance *af_create(struct af_stream *s, char *name,
         MP_ERR(s, "Couldn't find audio filter '%s'.\n", name);
         return NULL;
     }
-    const struct af_info *info = desc.p;
-    /* Make sure that the filter is not already in the list if it is
-       non-reentrant */
-    if (info->flags & AF_FLAGS_NOT_REENTRANT) {
-        for (struct af_instance *cur = s->first; cur; cur = cur->next) {
-            if (cur->info == info) {
-                MP_ERR(s, "There can only be one "
-                       "instance of the filter '%s' in each stream\n", name);
-                return NULL;
-            }
-        }
-    }
-
     MP_VERBOSE(s, "Adding filter %s \n", name);
 
     struct af_instance *af = talloc_zero(NULL, struct af_instance);
     *af = (struct af_instance) {
-        .info = info,
+        .info = desc.p,
         .data = talloc_zero(af, struct mp_audio),
         .log = mp_log_new(af, s->log, name),
         .replaygain_data = s->replaygain_data,

audio/filter/af.h

@@ -35,10 +35,6 @@ struct mpv_global;
 // Number of channels
 #define AF_NCH MP_NUM_CHANNELS
 
-// Flags used for defining the behavior of an audio filter
-#define AF_FLAGS_REENTRANT      0x00000000
-#define AF_FLAGS_NOT_REENTRANT  0x00000001
-
 // Flags for af->filter()
 #define AF_FILTER_FLAG_EOF 1
 
@@ -47,7 +43,6 @@ struct mpv_global;
 struct af_info {
     const char *info;
     const char *name;
-    const int flags;
     int (*open)(struct af_instance *vf);
     int priv_size;
     const void *priv_defaults;

audio/filter/af_drc.c

@@ -324,7 +324,6 @@ static int af_open(struct af_instance* af){
 const struct af_info af_info_drc = {
     .info = "Dynamic range compression filter",
     .name = "drc",
-    .flags = AF_FLAGS_NOT_REENTRANT,
     .open = af_open,
     .priv_size = sizeof(af_drc_t),
     .options = (const struct m_option[]) {

audio/filter/af_equalizer.c

@@ -203,7 +203,6 @@ static int af_open(struct af_instance* af){
 const struct af_info af_info_equalizer = {
   .info = "Equalizer audio filter",
   .name = "equalizer",
-  .flags = AF_FLAGS_NOT_REENTRANT,
   .open = af_open,
   .priv_size = sizeof(af_equalizer_t),
   .options = (const struct m_option[]) {

audio/filter/af_volume.c

@@ -168,7 +168,6 @@ static int af_open(struct af_instance *af)
 const struct af_info af_info_volume = {
     .info = "Volume control audio filter",
     .name = "volume",
-    .flags = AF_FLAGS_NOT_REENTRANT,
     .open = af_open,
     .priv_size = sizeof(struct priv),
     .options = (const struct m_option[]) {