Description of problem:

When you copy underscore names directly under bucket (s3://bucket/_bar) then the s3 copy fails, but when there is a additional folder in the target path, then s3 copy seems to succeed (s3://bucket/foo/_bar).

1. S3 upload (put/sync) works with underscore name as target:
  $ touch _bar60.txt
  $ s3cmd -c essi29-gooditest.cfg put _bar60.txt s3://testix/_bar60.txt
  upload: '_bar60.txt' -> 's3://testix/_bar60.txt'  [1 of 1]
   0 of 0     0% in    0s     0.00 B/s  done
  $ s3cmd -c essi29-gooditest.cfg --recursive ls -l s3://testix/ |grep bar60
  2017-11-16 10:21         0   d41d8cd98f00b204e9800998ecf8427e  STANDARD  s3://testix/_bar60.txt

2. But S3 copy fails if target is underscore object under bucket:
  $ s3cmd -c essi29-gooditest.cfg cp s3://testix/orig/bar/tst/bar/bar.txt s3://testix/_bar61.txt
  WARNING: Key not found s3://testix/orig/bar/tst/bar/bar.txt

3. If you add a subfolder in the path then theS3 copy succeeds to underscore name:
  $ s3cmd -c essi29-gooditest.cfg cp s3://testix/orig/bar/tst/bar/bar.txt s3://testix/case6/_bar61.txt
  remote copy: 's3://testix/orig/bar/tst/bar/bar.txt' -> 's3://testix/case6/_bar61.txt'

Object listing shows that name has changed from _bar61.txt to __bar61.txt, but when copied under folder it’s ok:
  $ s3cmd -c essi29-gooditest.cfg ls --recursive -l s3://testix/ | grep bar61
  2017-11-16 10:13         0   d41d8cd98f00b204e9800998ecf8427e  STANDARD  s3://testix/__bar61.txt
  2017-11-16 10:13         0   d41d8cd98f00b204e9800998ecf8427e  STANDARD  s3://testix/case6/_bar61.txt

4. It can be downloaded and deleted with "__" name
  $ s3cmd -c essi29-gooditest.cfg  rm s3://testix/__bar61.txt
  delete: 's3://testix/__bar61.txt'

So the question is that why it's not possible to copy objects:
  $ s3cmd cp s3://bucket/foo/bar.txt -> s3://bucket/_bar.txt
unless you specify a subfolder into copy:
  $ s3cmd cp s3://bucket/foo/bar.txt -> s3://bucket/subfolder/_bar.txt

And why in the first case the target name gets changed from "_bar.txt to "__bar.txt"  (which has 2x _).

Version-Release number of selected component (if applicable):
Ceph 2.3
package-data:ceph-base-10.2.7-27.el7cp.x86_64	Fri Jul 14 14:55:34 2017	

How reproducible:


Steps to Reproduce:
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Actual results:


Expected results:
it seems resolve on http://tracker.ceph.com/issues/19563 -Ceph - v10.2.10

Additional info: