[erlang-questions] Maybe potential incompatibility of file_info record

[Björn Gustavsson]

Thanks for digging deeper.

The old efile_driver has been rewritten and
replaced with a NIF. I am not sure whether
this change is an oversight/bug or a deliberate
change. We will look into it before the release
of OTP 21 and either fix the bug or add a
note about the potential incompatibility.

/Björn



On Thu, Jun 7, 2018 at 8:52 AM, Shunichi Shinohara <shino.shun@REDACTED> wrote:
> I looked into the source code and I think the difference between 20
> and 21.0-rc2 is identified.
>
> # The subject of this thread is not suitable,
> # but please let me report to not leave just a wrong information.
>
> First, the difference between OTP 20.3.4 and 21.0-rc2 is as follows.
> (on macOS, OTPs are installed from git by kerl)
>
> Setup file a and b, a is owned by root user.
> % rm -f a && rm -f b && sudo touch a && sudo chmod 755 a && touch b
>
> Let's start OTP 21.0-rc2 and read file_info of a and write it to b,
> it fails:
> % ~/local/otp/OTP-21.0-rc2/bin/erl
> Erlang/OTP 21 [RELEASE CANDIDATE 2] [erts-10.0] [source] [64-bit]
> [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe] [sharing-preserving]
>
> Eshell V10.0  (abort with ^G)
> 1> {ok, Fi} = file:read_file_info("a").
> {ok,#file_info{size = 0,type = regular,access = read,
>                atime = {{2018,6,7},{15,32,41}},
>                mtime = {{2018,6,7},{15,32,41}},
>                ctime = {{2018,6,7},{15,32,41}},
>                mode = 33261,links = 1,major_device = 16777220,
>                minor_device = 0,inode = 17846256,uid = 0,gid = 20}}
> 2> file:write_file_info("b", Fi).
> {error,eperm}
>
> Then, start OTP 20.3.4 and replay the same step, it succeeds:
> % ~/local/otp/OTP-20.3.4/bin/erl
> Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:8:8] [ds:8:8:10]
> [async-threads:10] [hipe] [kernel-poll:false]
>
> Eshell V9.3  (abort with ^G)
> 1> {ok, F} = file:read_file_info("a").
> {ok,#file_info{size = 0,type = regular,access = read,
>                atime = {{2018,6,7},{15,32,41}},
>                mtime = {{2018,6,7},{15,32,41}},
>                ctime = {{2018,6,7},{15,32,41}},
>                mode = 33261,links = 1,major_device = 16777220,
>                minor_device = 0,inode = 17846256,uid = 0,gid = 20}}
> 2> file:write_file_info("b", F).
> ok
>
> The file_info records are the same (sorry again...), but the results differ.
>
> Looking into unix_efile.c on maint-20 branch:
>     https://github.com/erlang/otp/blob/maint-20/erts/emulator/drivers/unix/unix_efile.c#L572
> it ignores eperm error and proceeds forward:
>     if (chown(name, pInfo->uid, pInfo->gid) && errno != EPERM) {
>
> I'm not sure the difference is judged as incompatibility or not,
> it's nice if this note helps someone (including future me).
>
> Thanks,
> Shino
>
> 2018-06-07 15:07 GMT+09:00 Shunichi Shinohara <shino.shun@REDACTED>:
>> I'm sorry I was wrong. Thanks for correct me, Bjorn.
>> I have to dig the root cause down ;)
>>
>> Shino
>>
>> 2018-06-07 15:04 GMT+09:00 Björn Gustavsson <bjorn@REDACTED>:
>>> On Thu, Jun 7, 2018 at 7:39 AM, Shunichi Shinohara <shino.shun@REDACTED> wrote:
>>> [...]
>>>> It seems that some fields are added to file_info record between 20 and 21-rc2,
>>>
>>> "Seems"?
>>>
>>> No fields have been added for a very long time. The only
>>> modifications done in recent releases are updates
>>> to the types of the existing fields. The last such change
>>> was made at the end of 2015.
>>>
>>> /Bjorn
>>>
>>> --
>>> Björn Gustavsson, Erlang/OTP, Ericsson AB



-- 
Björn Gustavsson, Erlang/OTP, Ericsson AB