[swift driver] BufferedReader uploading 0 bytes instead of actual supplied data

This was observed on a recent (Stein) glance running in a container on RHEL 8.

config extract:
$ 'grep -v ^# /etc/glance/glance-api.conf | grep -e swift'
stores=http,swift
default_store=swift
swift_store_endpoint_type=internalURL
swift_store_create_container_on_put=True
swift_buffer_on_upload = true
swift_upload_buffer_dir = /var/lib/glance
default_swift_reference=ref1
swift_store_config_file=/etc/glance/glance-swift.conf

the file:
$ ls -l overcloud-full.qcow2
-rw-r--r--. 1 stack stack 950599680 Mar 1 13:21 overcloud-full.qcow2

the result:
$ glance image-show 8295d200-0903-48b6-97b8-30b6566a91ee
+------------------+----------------------------------------------------------------------------------+
| Property | Value |
+------------------+----------------------------------------------------------------------------------+
| checksum | d41d8cd98f00b204e9800998ecf8427e |
| container_format | bare |
| created_at | 2019-03-07T06:38:09Z |
| direct_url | swift+config://ref1/glance/8295d200-0903-48b6-97b8-30b6566a91ee |
| disk_format | qcow2 |
| hw_architecture | x86_64 |
| id | 8295d200-0903-48b6-97b8-30b6566a91ee |
| kernel_id | bf673768-3fe1-4ee4-8e76-09f9f0fd7e31 |
| min_disk | 0 |
| min_ram | 0 |
| name | overcloud-full |
| os_hash_algo | sha512 |
| os_hash_value | cf83e1357eefb8bdf1542850d66d8007d620e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0 |
| | ff8318d2877eec2f63b931bd47417a81a538327af927da3e |
| os_hidden | False |
| owner | b837697327f841bc8aebade4a8bd8ba8 |
| protected | False |
| ramdisk_id | 52f3474b-ad53-4e3a-9413-a10369b0ab13 |
| size | 0 |
| status | active |
| tags | [] |
| updated_at | 2019-03-07T06:38:11Z |
| virtual_size | Not available |
| visibility | public |
+------------------+----------------------------------------------------------------------------------+
checksum and os_hash_value match those of an empty file:
$ touch it
$ md5sum it
d41d8cd98f00b204e9800998ecf8427e it
$ sha512sum it
cf83e1357eefb8bdf1542850d66d8007d620e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0ff8318d2877eec2f63b931bd47417a81a538327af927da3e it

log extract:
2019-03-07 01:38:09.792 24 DEBUG glance_store._drivers.swift.store [req-6bd1a2b0-7e04-4773-9213-d1e7fdd375aa 6b6207ebfc464dd1b161979078bd97da b837697327f841bc8aebade4a8bd8ba8 - default default] Adding image object '8295d200-0903-48b6-97b8-30b6566a91ee' to Swift add /usr/lib/python3.6/site-packages/glance_store/_drivers/swift/store.py:941
2019-03-07 01:38:09.792 24 DEBUG glance_store._drivers.swift.store [req-6bd1a2b0-7e04-4773-9213-d1e7fdd375aa 6b6207ebfc464dd1b161979078bd97da b837697327f841bc8aebade4a8bd8ba8 - default default] Cannot determine image size because it is either not provided in the request or chunked-transfer encoding is used. Adding image as a segmented object to Swift. add /usr/lib/python3.6/site-packages/glance_store/_drivers/swift/store.py:964
2019-03-07 01:38:09.793 24 DEBUG glance_store._drivers.swift.buffered [req-6bd1a2b0-7e04-4773-9213-d1e7fdd375aa 6b6207ebfc464dd1b161979078bd97da b837697327f841bc8aebade4a8bd8ba8 - default default] Buffering 204800000 bytes of image segment _buffer /usr/lib/python3.6/site-packages/glance_store/_drivers/swift/buffered.py:139
2019-03-07 01:38:10.321 24 DEBUG glance_store._drivers.swift.store [req-6bd1a2b0-7e04-4773-9213-d1e7fdd375aa 6b6207ebfc464dd1b161979078bd97da b837697327f841bc8aebade4a8bd8ba8 - default default] Not writing zero-length chunk. add /usr/lib/python3.6/site-packages/glance_store/_drivers/swift/store.py:993
2019-03-07 01:38:10.353 24 DEBUG swiftclient [req-6bd1a2b0-7e04-4773-9213-d1e7fdd375aa 6b6207ebfc464dd1b161979078bd97da b837697327f841bc8aebade4a8bd8ba8 - default default] REQ: curl -i http://192.168.24.1:8080/v1/AUTH_82f2841e2ee942359031b62754a033ec/glance/8295d200-0903-48b6-97b8-30b6566a91ee -X PUT -H "ETag: d41d8cd98f00b204e9800998ecf8427e" -H "X-Object-Manifest: glance/8295d200-0903-48b6-97b8-30b6566a91ee-" -H "X-Auth-Token: gAAAAABcgLxRR4MG..." -H "Content-Length: 0" http_log /usr/lib/python3.6/site-packages/swiftclient/client.py:167
2019-03-07 01:38:10.353 24 DEBUG swiftclient [req-6bd1a2b0-7e04-4773-9213-d1e7fdd375aa 6b6207ebfc464dd1b161979078bd97da b837697327f841bc8aebade4a8bd8ba8 - default default] RESP STATUS: 201 Created http_log /usr/lib/python3.6/site-packages/swiftclient/client.py:168
2019-03-07 01:38:10.353 24 DEBUG swiftclient [req-6bd1a2b0-7e04-4773-9213-d1e7fdd375aa 6b6207ebfc464dd1b161979078bd97da b837697327f841bc8aebade4a8bd8ba8 - default default] RESP HEADERS: {'Last-Modified': 'Thu, 07 Mar 2019 06:38:11 GMT', 'Content-Length': '0', 'Etag': 'd41d8cd98f00b204e9800998ecf8427e', 'Content-Type': 'text/html; charset=UTF-8', 'X-Trans-Id': 'tx01a1fc4fb34a421c83bf5-005c80bc52', 'X-Openstack-Request-Id': 'tx01a1fc4fb34a421c83bf5-005c80bc52', 'Date': 'Thu, 07 Mar 2019 06:38:10 GMT'} http_log /usr/lib/python3.6/site-packages/swiftclient/client.py:169