[data-compaction][leo_object_storage] compaction|diagnose may get slow down when a garbage exist at the end of a avs

[mocchira]

because file:pread always fail if the size of a invalid key|data is gt; the size of a avs file like this.

[E]     storage_0@127.0.0.1     2016-11-17 02:03:53.19699 +0000 1479348233      leo_file:pread/4        284      [{errors,[[{location,1552729},{expected_len,9039025},{actual_len,8933225},{diff,105800}][300/9360],unexpected_len_and_eof}]
[E]     storage_0@127.0.0.1     2016-11-17 02:04:01.807434 +0000        1479348241      leo_file:pread/4284      [{errors,[[{location,2043066},{expected_len,9360062},{actual_len,8442888},{diff,917174}]]},{result
,unexpected_len_and_eof}]
[E]     storage_0@127.0.0.1     2016-11-17 02:04:27.973403 +0000        1479348267      leo_file:pread/4284      [{errors,[[{location,3205264},{expected_len,7916219},{actual_len,7280690},{diff,635529}]]},{result
,unexpected_len_and_eof}]
[E]     storage_0@127.0.0.1     2016-11-17 02:04:34.670135 +0000        1479348274      leo_file:pread/4284      [{errors,[[{location,3393823},{expected_len,9135541},{actual_len,7092131},{diff,2043410}]]},{resul
t,unexpected_len_and_eof}]
[E]     storage_0@127.0.0.1     2016-11-17 02:04:41.831239 +0000        1479348281      leo_file:pread/4284      [{errors,[[{location,3721763},{expected_len,10085277},{actual_len,6764191},{diff,3321086}]]},{resu
lt,unexpected_len_and_eof}]

### ~~~ snip ~~~
### file:pread(s) failed) repeatedly 

[E]     [email protected]     2016-11-17 02:07:10.297342 +0000        1479348430      leo_file:pread/4284      [{errors,[[{location,10485950},{expected_len,128},{actual_len,4},{diff,124}]]},{result,unexpected_
len_and_eof}]
[E]     storage_0@127.0.0.1     2016-11-17 02:07:10.299366 +0000        1479348430      leo_file:pread/4284      [{errors,[[{location,10485951},{expected_len,128},{actual_len,3},{diff,125}]]},{result,unexpected_
len_and_eof}]
[E]     storage_0@127.0.0.1     2016-11-17 02:07:10.302033 +0000        1479348430      leo_file:pread/4284      [{errors,[[{location,10485952},{expected_len,128},{actual_len,2},{diff,126}]]},{result,unexpected_
len_and_eof}]
[E]     storage_0@127.0.0.1     2016-11-17 02:07:10.303306 +0000        1479348430      leo_file:pread/4284      [{errors,[[{location,10485953},{expected_len,128},{actual_len,1},{diff,127}]]},{result,unexpected_
len_and_eof}]
[W]     storage_0@127.0.0.1     2016-11-17 02:07:10.303677 +0000        1479348430      leo_compact_fsm_worker:execute_1/4       1048    [{obj_container_path,"/home/ubuntu/dev/leofs/package/leo_storage/avs/objec
t/0.avs_63646567195"},{error_pos_start,194},{error_pos_end,10485954},{errors,[{invalid_format,unexpected_time_format},{877,"invalid data"},unexpected_len_and_eof,{811,"invalid data"}]}]

In order to avoid this, need to get the size of a avs beforehand and skip pread if the size of a key|data is gt; its file size.

[mocchira]

As file:pread is called with the raw mode,
https://github.com/erlang/otp/blob/OTP-18.3.4.4/erts/preloaded/src/prim_file.erl is called directly then as a result,
https://github.com/erlang/otp/blob/OTP-18.3.4.4/erts/emulator/drivers/unix/unix_efile.c#L706 is called
by a port driver on xunix (our targets).
According to http://pubs.opengroup.org/onlinepubs/7908799/xsh/read.html
(p)read may return less bytes than nbyte if

• the number of bytes left in the file is less than nbyte ( in case of this issue )
• the read() request was interrupted by a signal ( but this is handled by erlang itself so we can ignore )
• the file is a pipe or FIFO or special file and has fewer than nbyte bytes immediately available for reading ( as we handle only regular files so we can ignore )

From these things,
we can regard unexpected_len_and_eof as a sign of close to EOF and
also judge there is no more files if # of bytes returned is less than the AVS header size (128).
so in this case we can stop compaction|diagnose and reduce subsequent file:pread(s) and outputs to error.log as shown above.

[mocchira]

#378 should be rollbacked.