When using the compositor together with a v4l2src the pipeline will run fine the first time it changes from READY->PLAYING. But if the pipeline is put back to READY and then to PLAYING it will fail to negotiate the caps.

I have encapsulated (using gst_parse_launch) the following pipelines in a simple GTK application which gives me 2 buttons: PLAY and STOP, which respectively set the state to PLAYING and to READY

The following pipeline works all the time:
(I can use the PLAY and STOP buttons as much as I want)

"videotestsrc ! tee name=t1 ! queue ! videoconvert ! compositor name=mux ! videoconvert ! fakesink t1. ! queue ! videoconvert ! gtksink name=sink t1. ! queue ! videoconvert ! mux."

If I then change the videotestsrc to a v4l2src.
It will run fine the first time I click PLAY. But it fails (Internal data flow error) after clicking STOP and then trying PLAY again.

"v4l2src ! tee name=t1 ! queue ! videoconvert ! compositor name=mux ! videoconvert ! fakesink t1. ! queue ! videoconvert ! gtksink name=sink t1. ! queue ! videoconvert ! mux."

At first glance it only shows a Internal data flow error and reason not-negotiated (-4):

Error received from element v4l2src0: Internal data flow error.
Debugging information: /var/lib/jenkins/jobs/qt5022-cesium/workspace/build/tmp/work/bobcat_64-poky-linux/gstreamer1.0/git-r0/git/libs/gst/base/gstbasesrc.c(2943): gst_base_src_loop (): /GstPipeline:pipeline0/GstV4l2Src:v4l2src0:
streaming task paused, reason not-negotiated (-4)
Error received from element queue0: Internal data flow error.
Debugging information: /var/lib/jenkins/jobs/qt5022-cesium/workspace/build/tmp/work/bobcat_64-poky-linux/gstreamer1.0/git-r0/git/plugins/elements/gstqueue.c(968): gst_queue_handle_sink_event (): /GstPipeline:pipeline0/GstQueue:queue0:
streaming task paused, reason not-negotiated (-4)
Error received from element queue2: Internal data flow error.
Debugging information: /var/lib/jenkins/jobs/qt5022-cesium/workspace/build/tmp/work/bobcat_64-poky-linux/gstreamer1.0/git-r0/git/plugins/elements/gstqueue.c(968): gst_queue_handle_sink_event (): /GstPipeline:pipeline0/GstQueue:queue2:
streaming task paused, reason not-negotiated (-4)

Then by setting the debug levels it seems it is something with caps being NULL on the videoaggregator:

0:00:09.029054344 16712 0x7faf40002050 ERROR        videoaggregator gstvideoaggregator.c:856:gst_videoaggregator_pad_sink_setcaps:<mux:sink_0> got input caps video/x-raw, width=(int)320, height=(int)200, framerate=(fraction)100/1, pixel-aspect-ratio=(fraction)1/1, interlace-mode=(string)interleaved, format=(string)RGBx, but current caps are (NULL)
0:00:09.029535582 16712 0x7faf40002050 ERROR        videoaggregator gstvideoaggregator.c:856:gst_videoaggregator_pad_sink_setcaps:<mux:sink_0> got input caps video/x-raw, width=(int)320, height=(int)200, framerate=(fraction)100/1, pixel-aspect-ratio=(fraction)1/1, interlace-mode=(string)interleaved, format=(string)RGBx, but current caps are (NULL)
0:00:09.029782526 16712 0x7faf40002050 WARN                GST_PADS gstpad.c:3989:gst_pad_peer_query:<videoconvert0:src> could not send sticky events
0:00:09.030124577 16712 0x7faf3c001ed0 ERROR        videoaggregator gstvideoaggregator.c:856:gst_videoaggregator_pad_sink_setcaps:<mux:sink_1> got input caps video/x-raw, width=(int)320, height=(int)200, framerate=(fraction)100/1, pixel-aspect-ratio=(fraction)1/1, interlace-mode=(string)interleaved, format=(string)RGBx, but current caps are (NULL)
0:00:09.030581896 16712 0x7faf3c001ed0 ERROR        videoaggregator gstvideoaggregator.c:856:gst_videoaggregator_pad_sink_setcaps:<mux:sink_1> got input caps video/x-raw, width=(int)320, height=(int)200, framerate=(fraction)100/1, pixel-aspect-ratio=(fraction)1/1, interlace-mode=(string)interleaved, format=(string)RGBx, but current caps are (NULL)
0:00:09.030675138 16712 0x7faf3c001ed0 WARN                GST_PADS gstpad.c:3989:gst_pad_peer_query:<videoconvert3:src> could not send sticky events
0:00:09.030266606 16712 0x7faf3c002230 WARN                    v4l2 gstv4l2object.c:3712:gst_v4l2_object_decide_allocation:<v4l2src0> decide allocation
0:00:09.031489911 16712 0x7faf3c002230 WARN          v4l2bufferpool gstv4l2bufferpool.c:749:gst_v4l2_buffer_pool_start:<v4l2src0:pool:src> Uncertain or not enough buffers, enabling copy threshold
0:00:09.056940651 16712 0x7faf40002050 ERROR        videoaggregator gstvideoaggregator.c:856:gst_videoaggregator_pad_sink_setcaps:<mux:sink_0> got input caps video/x-raw, width=(int)320, height=(int)200, framerate=(fraction)100/1, pixel-aspect-ratio=(fraction)1/1, interlace-mode=(string)interleaved, format=(string)RGBx, but current caps are (NULL)
0:00:09.057781339 16712 0x7faf3c001ed0 ERROR        videoaggregator gstvideoaggregator.c:856:gst_videoaggregator_pad_sink_setcaps:<mux:sink_1> got input caps video/x-raw, width=(int)320, height=(int)200, framerate=(fraction)100/1, pixel-aspect-ratio=(fraction)1/1, interlace-mode=(string)interleaved, format=(string)RGBx, but current caps are (NULL)
0:00:09.059067764 16712 0x7faf40002050 ERROR        videoaggregator gstvideoaggregator.c:856:gst_videoaggregator_pad_sink_setcaps:<mux:sink_0> got input caps video/x-raw, width=(int)320, height=(int)200, framerate=(fraction)100/1, pixel-aspect-ratio=(fraction)1/1, interlace-mode=(string)interleaved, format=(string)RGBx, but current caps are (NULL)
0:00:09.059970677 16712 0x7faf3c001ed0 ERROR        videoaggregator gstvideoaggregator.c:856:gst_videoaggregator_pad_sink_setcaps:<mux:sink_1> got input caps video/x-raw, width=(int)320, height=(int)200, framerate=(fraction)100/1, pixel-aspect-ratio=(fraction)1/1, interlace-mode=(string)interleaved, format=(string)RGBx, but current caps are (NULL)
0:00:09.065392758 16712 0x7faf3c002230 WARN                 basesrc gstbasesrc.c:2943:gst_base_src_loop:<v4l2src0> error: Internal data flow error.
0:00:09.065443831 16712 0x7faf3c002230 WARN                 basesrc gstbasesrc.c:2943:gst_base_src_loop:<v4l2src0> error: streaming task paused, reason not-negotiated (-4)
0:00:09.065583363 16712 0x7faf3c002230 WARN                   queue gstqueue.c:968:gst_queue_handle_sink_event:<queue0> error: Internal data flow error.
0:00:09.065619824 16712 0x7faf3c002230 WARN                   queue gstqueue.c:968:gst_queue_handle_sink_event:<queue0> error: streaming task paused, reason not-negotiated (-4)
0:00:09.065687791 16712 0x7faf3c002230 WARN                   queue gstqueue.c:968:gst_queue_handle_sink_event:<queue2> error: Internal data flow error.
0:00:09.065706444 16712 0x7faf3c002230 WARN                   queue gstqueue.c:968:gst_queue_handle_sink_event:<queue2> error: streaming task paused, reason not-negotiated (-4)
Error received from element v4l2src0: Internal data flow error.
Debugging information: /var/lib/jenkins/jobs/qt5022-cesium/workspace/build/tmp/work/bobcat_64-poky-linux/gstreamer1.0/git-r0/git/libs/gst/base/gstbasesrc.c(2943): gst_base_src_loop (): /GstPipeline:pipeline0/GstV4l2Src:v4l2src0:
streaming task paused, reason not-negotiated (-4)
Error received from element queue0: Internal data flow error.
Debugging information: /var/lib/jenkins/jobs/qt5022-cesium/workspace/build/tmp/work/bobcat_64-poky-linux/gstreamer1.0/git-r0/git/plugins/elements/gstqueue.c(968): gst_queue_handle_sink_event (): /GstPipeline:pipeline0/GstQueue:queue0:
streaming task paused, reason not-negotiated (-4)
Error received from element queue2: Internal data flow error.
Debugging information: /var/lib/jenkins/jobs/qt5022-cesium/workspace/build/tmp/work/bobcat_64-poky-linux/gstreamer1.0/git-r0/git/plugins/elements/gstqueue.c(968): gst_queue_handle_sink_event (): /GstPipeline:pipeline0/GstQueue:queue2:
streaming task paused, reason not-negotiated (-4)

Since it works with videotestsrc and it doesnt with v4l2src Im not sure if the issue is in the v4l2src or in the compositor