rgw: connection reset/crashed when download large zero object with compression enable

[fangyuxiangGL]

large zero object has a large compression rate,
even 4M compressed data can decompress several GB data.
Handle so much data in single process lead strange issue.

Fixed: http://tracker.ceph.com/issues/20098

Signed-off-by: fang yuxiang fang.yuxiang@eisoo.com

[fangyuxiangGL]

large zero object has a large compression rate, even 4M compressed data can decompress several GB data. Handle so much data in single process lead strange issue.
a) if you upload a zero object with size 4G, then connection reset will happen when you download it (civetweb return error, when send response to client)
b) if you upload a zero object with size 10G, then radosgw will crashed with below info:

/home/fyx/github/ceph/src/common/buffer.cc: 1010: FAILED assert(o+l <= _len)

ceph version 12.0.2-1605-g2f9ee0b (2f9ee0b)
1: (ceph::__ceph_assert_fail(char const*, char const*, int, char const*)+0x110) [0x7f79c8784030]
2: (ceph::buffer::ptr::copy_in(unsigned int, unsigned int, char const*, bool)+0x235) [0x7f79d1e76295]
3: (ceph::buffer::list::rebuild(ceph::buffer::ptr&)+0x4c) [0x7f79d1e76b9c]
4: (ceph::buffer::list::rebuild()+0x8b) [0x7f79d1e7754b]
5: (ceph::buffer::list::c_str()+0x19) [0x7f79d1e775e9]
6: (RGWGetObj_ObjStore_S3::send_response_data(ceph::buffer::list&, long, long)+0x4c2) [0x7f79d26cf5b2]
7: (RGWGetObj::get_data_cb(ceph::buffer::list&, long, long)+0x5b) [0x7f79d25c0f5b]
8: (RGWGetObj_Decompress::handle_data(ceph::buffer::list&, long, long)+0x3b2) [0x7f79d274ec12]
9: (RGWRados::flush_read_list(get_obj_data*)+0xae) [0x7f79d260dc0e]
10: (RGWRados::Object::Read::iterate(long, long, RGWGetDataCB*)+0x345) [0x7f79d2652685]
11: (RGWGetObj::execute()+0xd18) [0x7f79d25e4da8]
12: (rgw_process_authenticated(RGWHandler_REST*, RGWOp*&, RGWRequest*, req_state*, bool)+0x162) [0x7f79d25fb422]
13: (process_request(RGWRados*, RGWREST*, RGWRequest*, std::string const&, rgw::auth::StrategyRegistry const&, RGWRestfulIO*, OpsLogSocket*)+0xb66) [0x7f79d25fc1b6]
14: (RGWCivetWebFrontend::process(mg_connection*)+0x31c) [0x7f79d24e141c]
15: (()+0x1e0c6f) [0x7f79d251ac6f]
16: (()+0x1e25fb) [0x7f79d251c5fb]
17: (()+0x7df5) [0x7f79c80dadf5]
18: (clone()+0x6d) [0x7f79c5d6a1ad]
NOTE: a copy of the executable, or objdump -rdS <executable> is needed to interpret this.

the reason is that the data is so huge in a single process, and lots small decompressed data chunk(4M)
appended to the out buffer. remember that the object has 10G data, but bufferlist->_len is 32bit, so it
overflows, then radosgw crashed.