SoundFonts cannot be unloaded if polyphony is ever exceeded

If polyphony is exceeded and FluidSynth has to allocate a voice by
calling fluid_synth_free_voice_by_kill_LOCAL(), two problems occur:

 1)The value returned by fluid_synth_get_active_voice_count() never
   returns back to 0.
 2)SoundFont samples are not unref'd properly, and therefore if an attempt is
   made to unload the SoundFont, the deferred unload timer is started, and
   fluid_synth_sfunload_callback() unsuccessfully tries	to unload the SoundFont forever.

These 2 issues are fixed by this commit.

src/synth/fluid_synth.c

@@ -1067,6 +1067,12 @@ delete_fluid_synth(fluid_synth_t *synth)
                 continue;
             }
 
+            /* WARNING: A this point we must ensure that the reference counter
+               of any soundfont sample owned by any rvoice belonging to the voice
+               are correctly decremented. This is the contrary part to
+               to fluid_voice_init() where the sample's reference counter is
+               incremented.
+            */
             fluid_voice_unlock_rvoice(voice);
             fluid_voice_overflow_rvoice_finished(voice);
 
@@ -4289,7 +4295,21 @@ fluid_synth_check_finished_voices(fluid_synth_t *synth)
             }
             else if(synth->voice[j]->overflow_rvoice == fv)
             {
+                /* Unlock the overflow_rvoice of the voice.
+                   Decrement the reference count of the sample owned by this
+                   rvoice.
+                */
                 fluid_voice_overflow_rvoice_finished(synth->voice[j]);
+
+                /* Decrement synth active voice count. Must not be incorporated
+                   in fluid_voice_overflow_rvoice_finished() because
+                   fluid_voice_overflow_rvoice_finished() is called also
+                   at synth destruction and in this case the variable should be
+                   accessed via voice->channel->synth->active_voice_count.
+                   And for certain voices which are not playing, the field
+                   voice->channel is NULL.
+                */
+                synth->active_voice_count--;
                 break;
             }
         }

src/synth/fluid_voice.c

@@ -321,12 +321,13 @@ fluid_voice_init(fluid_voice_t *voice, fluid_sample_t *sample,
     voice->has_noteoff = 0;
     UPDATE_RVOICE0(fluid_rvoice_reset);
 
-    /* Increment the reference count of the sample to prevent the
+    /*
-       unloading of the soundfont while this voice is playing,
+       We increment the reference count of the sample to indicate that this
-       once for us and once for the rvoice. */
+       sample is about to be owned by the rvoice. This will prevent the
+       unloading of the soundfont while this rvoice is playing.
+    */
     fluid_sample_incr_ref(sample);
     fluid_rvoice_eventhandler_push_ptr(voice->eventhandler, fluid_rvoice_set_sample, voice->rvoice, sample);
-    fluid_sample_incr_ref(sample);
     voice->sample = sample;
 
     i = fluid_channel_get_interp_method(channel);
@@ -1406,11 +1407,19 @@ fluid_voice_kill_excl(fluid_voice_t *voice)
 }
 
 /*
- * Called by fluid_synth when the overflow rvoice can be reclaimed.
+ * Unlock the overflow rvoice of the voice.
+ * Decrement the reference count of the sample owned by this rvoice.
+ *
+ * Called by fluid_synth when the overflow rvoice has finished by itself.
+ * Must be called also explicitly at synth destruction to ensure that
+ * the soundfont be unloaded successfully.
  */
 void fluid_voice_overflow_rvoice_finished(fluid_voice_t *voice)
 {
     voice->can_access_overflow_rvoice = 1;
+
+    /* Decrement the reference count of the sample to indicate
+       that this sample isn't owned by the rvoice anymore */
     fluid_voice_sample_unref(&voice->overflow_rvoice->dsp.sample);
 }
 
@@ -1439,17 +1448,19 @@ fluid_voice_stop(fluid_voice_t *voice)
 
     voice->chan = NO_CHANNEL;
 
+    /* Decrement the reference count of the sample, to indicate
+       that this sample isn't owned by the rvoice anymore.
+    */
     if(voice->can_access_rvoice)
     {
         fluid_voice_sample_unref(&voice->rvoice->dsp.sample);
     }
 
+    voice->sample = NULL;
+
     voice->status = FLUID_VOICE_OFF;
     voice->has_noteoff = 1;
 
-    /* Decrement the reference count of the sample. */
-    fluid_voice_sample_unref(&voice->sample);
-
     /* Decrement voice count */
     voice->channel->synth->active_voice_count--;
 }