Disallow seeking in http:// streams.

Seeking backwards in http streams will always fail and seeking forward
only works on a small minority of files, with no obvious rationale for
success. Attempting to allow seeking will almost certainly lead to a
raft of user issues once UPnP Media Server support is added.

A further fix is needed to disable rewinding and limit fast forward to
3x or less (i.e. avoid actually seeking).

Refs #9722

mythtv/libs/libmythtv/ringbuffer.h

@@ -69,8 +69,9 @@ class MTV_PUBLIC RingBuffer : protected QThread
     bool      IsNearEnd(double fps, uint vvf) const;
     /// \brief Returns true if open for either reading or writing.
     virtual bool IsOpen(void) const = 0;
-    virtual bool IsStreamed(void)     { return LiveMode(); }
-    virtual int  BestBufferSize(void) { return 32768; }
+    virtual bool IsStreamed(void)       { return LiveMode(); }
+    virtual bool IsSeekingAllowed(void) { return true;  }
+    virtual int  BestBufferSize(void)   { return 32768; }
     static QString BitrateToString(uint64_t rate);
 
     // DVD and bluray methods

mythtv/libs/libmythtv/streamingringbuffer.h

@@ -19,7 +19,8 @@ class StreamingRingBuffer : public RingBuffer
                           uint retry_ms = kDefaultOpenTimeout);
     virtual long long Seek(long long pos, int whence, bool has_lock);
     virtual long long GetRealFileSize(void) const;
-    virtual bool IsStreamed(void) { return true; }
+    virtual bool IsStreamed(void)       { return true;  }
+    virtual bool IsSeekingAllowed(void) { return false; }
 
   protected:
     virtual int safe_read(void *data, uint sz);

mythtv/libs/libmythtv/tv_play.cpp

@@ -5456,6 +5456,12 @@ bool TV::DoPlayerSeek(PlayerContext *ctx, float time)
         return false;
     }
 
+    if (!ctx->buffer->IsSeekingAllowed())
+    {
+        ctx->UnlockDeletePlayer(__FILE__, __LINE__);
+        return false;
+    }
+
     if (ctx == GetPlayer(ctx, 0))
         PauseAudioUntilBuffered(ctx);