[leo_object_storage] file:pread/3 could return {ok, Data} which size less than the Number argument

[mocchira]

http://www.erlang.org/doc/man/file.html#pread-3 could return {ok, Data} which size less than the Number argument.

We need to handle this kind of return values.

[yosukehara]

I've implemented a wrapper function of file:pread/3 as follows:

• https://github.com/leo-project/leo_commons/blob/develop/src/leo_file.erl#L216-L273
• Erlang docs - file:pread/3

[mocchira]

@yosukehara LGTM.

[mocchira]

Anyway,
It seems that functions using efile_read|efile_pread in the file module like

• file:read
• file:read_file
• file:pread

could return a object which size less than we expected.

https://github.com/erlang/otp/blob/OTP-17.5.6/erts/emulator/drivers/common/efile_drv.c#L1144
https://github.com/erlang/otp/blob/OTP-17.5.6/erts/emulator/drivers/common/efile_drv.c#L1322
https://github.com/erlang/otp/blob/OTP-17.5.6/erts/emulator/drivers/common/efile_drv.c#L1370

https://github.com/erlang/otp/blob/OTP-17.5.6/erts/emulator/drivers/unix/unix_efile.c#L667
https://github.com/erlang/otp/blob/OTP-17.5.6/erts/emulator/drivers/unix/unix_efile.c#L700