spurrious whiteout rejection failures (error not propagated)

[cyphar]

It appears that the whiteout rejection code (which is implemented through CloseWithError) can fail spurriously. I've noticed this failure in the past but it appears that it's happening much more often in our CI (causing it to fail):

not ok 73 umoci repack [whiteout]
# (in test file test/repack.bats, line 172)
#   `[ "$status" -ne 0 ]' failed
# oci-image-tool: reference "latest": OK
# /tmp/umoci-integration/umoci-integration-tmpdir.IAQ0Qfmf/image: OK
# Validation succeeded
# df  (status=0)
# Filesystem     1K-blocks     Used Available Use% Mounted on
# overlay         71088948 14596812  56475752  21% /
# tmpfs              65536        0     65536   0% /dev
# tmpfs            4084220        0   4084220   0% /sys/fs/cgroup
# /dev/sda1       71088948 14596812  56475752  21% /etc/hosts
# shm                65536        0     65536   0% /dev/shm
# tmpfs            4084220        0   4084220   0% /proc/acpi
# tmpfs            4084220        0   4084220   0% /proc/scsi
# tmpfs            4084220        0   4084220   0% /sys/firmware
# du -h -d 2 /tmp/umoci-integration/umoci-integration-tmpdir.agv5xRAj (status=0)
# 4.0K	/tmp/umoci-integration/umoci-integration-tmpdir.agv5xRAj
# umoci -test.coverprofile=/tmp/umoci-coverage.cuRihw/umoci.cov.59NMYn __DEVEL--i-heard-you-like-tests unpack --image /tmp/umoci-integration/umoci-integration-tmpdir.IAQ0Qfmf/image:latest /tmp/umoci-integration/umoci-integration-tmpdir.agv5xRAj/umoci-integration-tmpdir.B10VXimo (status=0)
# PASS
# coverage: 24.9% of statements in ./...
# Bundle validation succeeded.
# umoci -test.coverprofile=/tmp/umoci-coverage.cuRihw/umoci.cov.7M7ITq __DEVEL--i-heard-you-like-tests repack --image /tmp/umoci-integration/umoci-integration-tmpdir.IAQ0Qfmf/image:latest-new /tmp/umoci-integration/umoci-integration-tmpdir.agv5xRAj/umoci-integration-tmpdir.B10VXimo (status=0)
# PASS
# coverage: 18.8% of statements in ./...
# oci-image-tool: reference "latest": OK
# oci-image-tool: reference "latest-new": OK
# /tmp/umoci-integration/umoci-integration-tmpdir.IAQ0Qfmf/image: OK
# Validation succeeded
# umoci -test.coverprofile=/tmp/umoci-coverage.cuRihw/umoci.cov.f2j0qE __DEVEL--i-heard-you-like-tests unpack --image /tmp/umoci-integration/umoci-integration-tmpdir.IAQ0Qfmf/image:latest-new /tmp/umoci-integration/umoci-integration-tmpdir.agv5xRAj/umoci-integration-tmpdir.ffYJ1M7n (status=0)
# PASS
# coverage: 24.1% of statements in ./...
# Bundle validation succeeded.
# gomtree -K sha256digest -p /tmp/umoci-integration/umoci-integration-tmpdir.agv5xRAj/umoci-integration-tmpdir.B10VXimo/rootfs -f /tmp/umoci-integration/umoci-integration-tmpdir.agv5xRAj/umoci-integration-tmpdir.ffYJ1M7n/sha256_2a1083ca750ab3ef7090ff5d05915b630f9b6f4fbb2bf9305e6eb5f965b11614.mtree (status=0)
# 
# umoci -test.coverprofile=/tmp/umoci-coverage.cuRihw/umoci.cov.56yPQm __DEVEL--i-heard-you-like-tests stat --image /tmp/umoci-integration/umoci-integration-tmpdir.IAQ0Qfmf/image:latest-new --json (status=0)
# {"history":[{"layer":{"mediaType":"application/vnd.oci.image.layer.v1.tar+gzip","digest":"sha256:376057ac6fa17f65688c56977118e2e764b27c348b3f70637fa829cd2a12b200","size":50391294},"diff_id":"sha256:8c02234b86056c009036ff0c31efb9a726412392d9872dacf95103767ac3b101","created":"2020-05-15T06:28:12.60008518Z","created_by":"/bin/sh -c #(nop) ADD file:fb54c709daa205bf9d04eb3d90ba068db4c34dfe3b6ec0d7691f677286120903 in / "},{"layer":null,"diff_id":"","created":"2020-05-15T06:28:13.142776633Z","created_by":"/bin/sh -c #(nop)  CMD [\"bash\"]","empty_layer":true},{"layer":{"mediaType":"application/vnd.oci.image.layer.v1.tar+gzip","digest":"sha256:623029273a2b8571d91fe12d0a602a3591d6b5c2c92269aca560b4bb00030dfe","size":234},"diff_id":"sha256:f58f13f1a93fd64e89ba4494db48852ea9eca5581508e76e7fa940b8622ed8c4","created":"2020-06-05T21:53:27.884107403Z","created_by":"umoci repack"}]}
# PASS
# coverage: 9.2% of statements in ./...
# umoci -test.coverprofile=/tmp/umoci-coverage.cuRihw/umoci.cov.iCcaOy __DEVEL--i-heard-you-like-tests repack --image /tmp/umoci-integration/umoci-integration-tmpdir.IAQ0Qfmf/image:latest-new2 /tmp/umoci-integration/umoci-integration-tmpdir.agv5xRAj/umoci-integration-tmpdir.ffYJ1M7n (status=1)
#    • generate layer: could not add file 'whiteout_test/.wh. THIS IS A TEST ': invalid path has whiteout prefix ".wh.": whiteout_test/.wh. THIS IS A TEST 
#    ⨯ add diff layer: add layer: put layer blob: copy to temporary blob: compressing layer: generate layer: generate layer file: invalid path has whiteout prefix ".wh.": whiteout_test/.wh. THIS IS A TEST 
# oci-image-tool: reference "latest": OK
# oci-image-tool: reference "latest-new": OK
# /tmp/umoci-integration/umoci-integration-tmpdir.IAQ0Qfmf/image: OK
# Validation succeeded
# umoci -test.coverprofile=/tmp/umoci-coverage.cuRihw/umoci.cov.7edOFs __DEVEL--i-heard-you-like-tests repack --image /tmp/umoci-integration/umoci-integration-tmpdir.IAQ0Qfmf/image:latest-new3 /tmp/umoci-integration/umoci-integration-tmpdir.agv5xRAj/umoci-integration-tmpdir.ffYJ1M7n (status=0)
#    • generate layer: could not add file '.wh.another_whiteout': invalid path has whiteout prefix ".wh.": .wh.another_whiteout/
# PASS
# coverage: 18.4% of statements in ./...

Note that the second invocation of umoci (where the test failure occurs), only the logged warning is printed and there isn't a

#    ⨯ add diff layer: add layer: put layer blob: copy to temporary blob: compressing layer: generate layer: generate layer file: invalid path has whiteout prefix ".wh.": whiteout_test/.wh. THIS IS A TEST 

line afterwards, indicating that the error never made it to the top-level main function.