When configured in push mode, tsdemux can generate a segment while flushing or with wrong values, when two consecutive seek events are handled in parallel. This can stall the pipeline for very long times.

Here's a stripped/tweaked log from tsdemux that exhibits this issue:

stm0: seek to 95032ms from start

<tsdemux0> seek event, rate: 1.000000 start: 0:01:35.032000000 stop: 99:99:99.999999999
<tsdemux0> Got event flush-start
<tsdemux0> Got event flush-stop
<tsdemux0> soft flush of all streams
<tsdemux0> Got event segment
<tsdemux0> soft flush of all streams
<tsdemux0:audio_0_0045> first PTS 0:01:32.669048481
<tsdemux0:video_0_0044> first PTS 0:01:33.429048481
<tsdemux0> Pushing segment event: 0x73d06ec0, time 99:99:99.999999999, seq-num 1133, GstEventSegment, segment=(GstSegment)"GstSegment, flags=(GstSegmentFlags)GST_SEGMENT_FLAG_NONE, rate=(double)1, applied-rate=(double)1, format=(GstFormat)GST_FORMAT_TIME, base=(guint64)0, offset=(guint64)0, start=(guint64)92669048481, stop=(guint64)18446744073709551615, time=(guint64)92669048481, position=(guint64)92669048481, duration=(guint64)18446744073709551615;";

stm0: seek to 125032ms from start

<tsdemux0> seek event, rate: 1.000000 start: 0:02:05.032000000 stop: 99:99:99.999999999
<tsdemux0> Got event flush-start
<tsdemux0> Pushing segment event: 0x73d06e80, time 99:99:99.999999999, seq-num 1133, GstEventSegment, segment=(GstSegment)"GstSegment, flags=(GstSegmentFlags)GST_SEGMENT_FLAG_NONE, rate=(double)1, applied-rate=(double)1, format=(GstFormat)GST_FORMAT_TIME, base=(guint64)0, offset=(guint64)0, start=(guint64)0, stop=(guint64)18446744073709551615, time=(guint64)0, position=(guint64)0, duration=(guint64)18446744073709551615;";
<tsdemux0:audio_0_0045> first PTS 0:01:34.525048481
<tsdemux0> Got event flush-stop
<tsdemux0> soft flush of all streams
<tsdemux0> Got event segment
<tsdemux0> soft flush of all streams
<tsdemux0:video_0_0044> first PTS 0:02:03.429048481

GStreamer-WARNING **: ../../gitsrc/gst/gstpad.c:4429:gst_pad_push_data:<tsdemux0:video_0_0044> Got data flow before segment event
GStreamer-WARNING **: ../../gitsrc/gst/gstpad.c:4176:gst_pad_chain_data_unchecked:<multiqueue0:sink_0> Got data flow before segment event
...

In this example the second segment is sent too early, and its start value is invalid. The start value should be the lowest PTS in all streams received after the seek event, but here no data was received yet (flush_stop hasn't even been received yet).