added flag "decoder.seeking"

This flag is used internally; it is set by decoder_seek_where(), and
indicates that the decoder plugin has begun the seek process.  It is
used for the case that the decoder plugin has to read data during the
seek process.  Before this patch, that was impossible, because
decoder_read() would refuse to read data unless dc->command is NONE.
This patch is kind of a dirty workaround, and needs to be redesigned
later.

src/decode.c

@@ -50,6 +50,8 @@ static void decodeStart(void)
 		goto stop_no_close;
 	}
 
+	decoder.seeking = 0;
+
 	dc.state = DECODE_STATE_START;
 	dc.command = DECODE_COMMAND_NONE;
 

src/decoder_api.c

@@ -69,6 +69,8 @@ enum decoder_command decoder_get_command(mpd_unused struct decoder * decoder)
 void decoder_command_finished(mpd_unused struct decoder * decoder)
 {
 	assert(dc.command != DECODE_COMMAND_NONE);
+	assert(dc.command != DECODE_COMMAND_SEEK ||
+	       dc.seekError || decoder->seeking);
 
 	dc.command = DECODE_COMMAND_NONE;
 	notify_signal(&pc.notify);
@@ -78,6 +80,8 @@ double decoder_seek_where(mpd_unused struct decoder * decoder)
 {
 	assert(dc.command == DECODE_COMMAND_SEEK);
 
+	decoder->seeking = 1;
+
 	return dc.seekWhere;
 }
 
@@ -100,7 +104,10 @@ size_t decoder_read(struct decoder *decoder,
 
 	while (1) {
 		/* XXX don't allow decoder==NULL */
-		if (decoder != NULL && dc.command != DECODE_COMMAND_NONE)
+		if (decoder != NULL &&
+		    (dc.command != DECODE_COMMAND_SEEK ||
+		     !decoder->seeking) &&
+		    dc.command != DECODE_COMMAND_NONE)
 			return 0;
 
 		nbytes = readFromInputStream(inStream, buffer, 1, length);

src/decoder_internal.h

@@ -26,6 +26,8 @@ struct decoder {
 	struct decoder_plugin *plugin;
 
 	ConvState conv_state;
+
+	int seeking;
 };
 
 #endif