Some comments after playing with stream tags and metadata this afternoon.
(1) Although the discussion of stream tag insertion hints that this should be done within the scheduler's call to work() it could be more clear that doing it in any other context can result in race conditions. (I did think I saw it stated more clearly somewhere, but can't find that now, so maybe this point has been addressed.) (2) In the current implementation it's further necessary that tags be added to an output in monotonic non-decreasing offset order. file_meta_sink does not sort the return value from get_tags_in_range(), and emits all data up to the timestamp of the next tag, so a subsequent tag with an earlier offset is dropped from the archive. (I note that tagged_file_sink() does sort the tags it receives in one case, but not in others.) I don't see this requirement on ordered generation documented. In some cases, it may be inconvenient to do this, e.g. when a block's analysis discovers after-the-fact that something interesting can be associated with a past sample. Similarly, a user might want a block to associate a tag with sample that not yet arrived, to notify a downstream block that will need to process the event. A simple solution for the infrastructure is to require that tags only be generated from within work(), with offsets corresponding to samples generated in that call to work(), and in non-decreasing offset order (though this last requirement could be handled by add_item_tag()). The developer must then handle the too-late/too-early tag associations through some other mechanism, such as carrying the effective offset as part of the tag value. (3) Qt GUI Range with widget Counter + Slider invokes callbacks twice, even if the value itself was set exactly once through the counter text entry. If the callback records the change by queuing a stream tag for addition to the output, multiple tags with the same offset/key/value will be generated. There are ugly solutions to this but it's probably sufficient to note somewhere that it can happen. It's really not specific to tags, but is clearly visible in that case. (4) The in-memory stream of tags can produce multiple settings of the same key at the same offset. However, when stored to a file only the last setting of the key is recorded. I believe this last behavior is incorrect and that it's a mistake to use a map instead of a multimap or simple list for the metadata record of stream tags associated with a sample. One argument is that it's critical that a stream archive of a processing session faithfully record the contents of the stream so that re-running the application using playback reproduces that stream and thus the original behavior (absent non-determinism due to asynchrony). This faithful reproduction is what would allow a maintainer to diagnose an operational failure caused by a block with a runtime failure when the same tag is processed twice at the same offset. This is true even if the same key is set to the same value at the same sample offset multiple times, which some might otherwise want to argue is redundant. A corollary argument is that the sample number at which an event like a tuner configuration change occurs usually can't be exactly associated with a sample; the best estimate is likely to be the index of the first sample generated by the next call to work. But depending on processing speed an application might change an attribute of a data source multiple times before work was invoked. The effect of those intermediate changes may be visible in the signal, and to lose the fact they occurred by discarding all but the last change affects both reproducibility and interpretation of the signal itself. (5) All stream tags are placed in the extras block, and when a segment is completed file_meta_sink will generate a new header. The new header contains copies of the unique tags, but updates their offsets to be the start of the new segment. This is incorrect as the original stream did not have those tags associated with those samples, so re-playing will introduce a behavioral difference. For example, a tag that is meant to be associated with the start of a packet will be duplicated at an offset that is probably not the start of a packet. Solutions include (a) leave the original offset setting for tags in the extras section when they're reproduced in a new segment, even though that offset is not present in the segment; (b) treat stream tags as ephemeral and do not persist them in the extras section when generating a new segment; (c) extend the add_item_tag API to record whether the tag is ephemeral or persistent. Offhand I can see no argument supporting persisting a tag and updating its offset, and only rare cases where it's appropriate to replicate outdated information in a new segment, so (b) seems to be the right move. All the above is based on my understanding and expectations of how stream tags are/should be used. If my understanding is mistaken, please let me know. Peter _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
