DVDRingBuffer: Unlock the ring buffer when waiting on state changes.

The DVD ringbuffer does not use the read ahead thread and hence holding
the writelock during safe_read is not strictly necessary (see
RingBuffer::ReadPriv). DVDRingBuffer::safe_read can also hold that lock
for tens of seconds when, for example, displaying a still frame (the dvd
device simply returns specific events, but not data, until it is told to
skip the still frame).

Under normal operation this is not a problem but events received from
external sources (such as network control) can trigger code that tries
to lock the ringbuffer from the main UI thread (e.g.
RingBuffer::GetFilename). The lock is not obtained, event processing in
the main thread is blocked and the wait state in the ringbuffer thread
is not cleared - and the frontend deadlocks as a result.

The solution is to release the lock while sleeping within
DVDRingBuffer::safe_read to allow other threads to obtain the lock
temporarily. I've also added a warning message should the read ahead
thread be running in the DVD ringbuffer.

Refs #9780

mythtv/libs/libmythtv/dvdringbuffer.cpp

@@ -502,7 +502,12 @@ void DVDRingBuffer::WaitForPlayer(void)
         m_playerWait = true;
         int count = 0;
         while (m_playerWait && count++ < 200)
+        {
+            rwlock.unlock();
             usleep(10000);
+            rwlock.lockForWrite();
+        }
+
         if (m_playerWait)
         {
             VERBOSE(VB_IMPORTANT, LOC_ERR +
@@ -530,6 +535,9 @@ int DVDRingBuffer::safe_read(void *data, uint sz)
         return -1;
     }
 
+    if (readaheadrunning)
+        VERBOSE(VB_IMPORTANT, LOC_ERR + "read ahead thread running.");
+
     while (needed)
     {
         blockBuf = m_dvdBlockWriteBuf;
@@ -886,7 +894,9 @@ int DVDRingBuffer::safe_read(void *data, uint sz)
 
                 // pause a little as the dvdnav VM will continue to return
                 // this event until it has been skipped
+                rwlock.unlock();
                 usleep(10000);
+                rwlock.lockForWrite();
 
                 // when scanning the file or exiting playback, skip immediately
                 // otherwise update the timeout in the player
@@ -919,7 +929,9 @@ int DVDRingBuffer::safe_read(void *data, uint sz)
                 else
                 {
                     m_dvdWaiting = true;
+                    rwlock.unlock();
                     usleep(10000);
+                    rwlock.lockForWrite();
                 }
 
                 // release buffer