Two patches (patch=28 & patch=72) in Gen3 assembleCoadd HSC-RC2 tract=9615 ran into the following error

"/software/lsstsw/stack_20190330/stack/miniconda3-4.5.12-1172c30/Linux64/meas_algorithms/18.0.0-5-ga38416e7+2/python/lsst/meas/algorithms/detection.py", line 777, in detectFootprints

    psf = self.getPsf(exposure, sigma=sigma)

  File "/software/lsstsw/stack_20190330/stack/miniconda3-4.5.12-1172c30/Linux64/meas_algorithms/18.0.0-5-ga38416e7+2/python/lsst/meas/algorithms/detection.py", line 503, in getPsf

    raise RuntimeError("Unable to determine PSF to use for detection: no sigma provided")

RuntimeError: Unable to determine PSF to use for detection: no sigma provided

Yusra AlSayyad identified that the Gen3 version doesn't have a Selector like how it is in Gen2. For patch=28 (Gen3) or '1,3' (Gen2), visit=27116 isn't selected for coadd and that warp isn't made in Gen2.
From Slack:

I’m 90% sure that the selector in `CoaddDriver` https://github.com/lsst/pipe_drivers/blob/master/python/lsst/pipe/drivers/coaddDriver.py#L268, (retargeted to `from lsst.pipe.tasks.selectImages import PsfWcsSelectImagesTask`) but am just shoving the dataRefs into the right dataId container that the selector likes… hang on

Confirmed. `PsfWcsSelectImagesTask` filters out visit 27116 in `coaddDriver` and the Gen2 version of assembleCoadd if you ask it to use that selector.

 
And attaching her useful snippet from Slack.

One way to reproduce the error in Gen3 (but likely with Oracle permission issue) is 

pipetask -b /project/hchiang2/gen3repos/w_2019_20/repo/butler.yaml -d "tract=9615 and patch=28 and abstract_filter='z'" -i G3M19c_000001_021 -o dummy run --skip-init-writes  -t "assembleCoadd.CompareWarpAssembleCoaddTask"