ERL-1298: Ram files cannot be open in raw mode

[OTP-Maintainer]

Original reporter: JIRAUSER15707
Affected version: OTP-22.0
Fixed in version: OTP-24.0
Component: kernel
Migrated from: https://bugs.erlang.org/browse/ERL-1298

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When opening a file with both "ram" and "raw" modes I expect to open a ram file that is interfaced through a file descriptor (hence "raw") instead of via a process, but instead of a ram file what is opened is a disk file; that is, the "ram" mode is simply ignored.

For example, the following code opens the file named "hello" on disk instead of ram, and writes " world" to it,
{code:java}
Erlang/OTP 22 [erts-10.7.1] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe]

Eshell V10.7.1  (abort with ^G)
1> {ok, File} = file:open(<<"hello">>, [ram, raw, write, read]).
{ok,{file_descriptor,raw_file_io_list,
                     {file_descriptor,prim_file,
                                      #{handle => #Ref<0.708897726.1743126542.99290>,
                                        owner => <0.80.0>,r_ahead_size => 0,
                                        r_buffer => #Ref<0.708897726.1743126530.99559>}}}}
2> file:position(File, eof).
{ok,36}
3> file:write(File, <<" world">>).
ok
4> {ok, Size} = file:position(File, eof).
{ok,42}
5> file:position(File, bof).
{ok,0}
6> {ok, Bytes} = file:read(File, Size).
{ok," world"}
7> file:close(File).
ok
8>{code}
My expectation was to see Bytes taking the value of "hello world" not " world", and for no disk file to be created.

In the next snippet, observe how the just created file e opened again and grows:

 

 
{code:java}
Erlang/OTP 22 [erts-10.7.1] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe]
Eshell V10.7.1 (abort with ^G)
1> {ok, File} = file:open(<<"hello">>, [ram, raw, write, read]).
{ok,{file_descriptor,raw_file_io_list,
 {file_descriptor,prim_file,
 #{handle => #Ref<0.1917879854.3013476362.109932>,
 owner => <0.80.0>,r_ahead_size => 0,
 r_buffer => #Ref<0.1917879854.3013476354.110173>}}}}
2> file:position(File, eof).
{ok,12}
3> file:write(File, <<" world">>).
ok
4> {ok, Size} = file:position(File, eof).
{ok,18}
5> file:position(File, bof).
{ok,0}
6> {ok, Bytes} = file:read(File, Size).
{ok," world world"}
7> file:close(File).
ok
8>
{code}
 

I am not clear if this is a bug or an undocumented limitation of "file:open" function, so either the codebase must be fixed to support both modes simultaneously or the documentation should take note of this limitation.

Thanks.

 

[OTP-Maintainer]

josevalim said:

"ram" and "raw" modes are exclusive. What is happening here is that, if both are given, "raw" always wins, as you can see here: [https://github.com/erlang/otp/blob/eaab8962c2f75a286f8e36d278613e308ec7e4bc/lib/kernel/src/file.erl#L518]

 

Perhaps a clause should be added to raise if both are given, but that could be seen as a breaking change. If that's the case, documenting if both raw and ram is given, raw "wins".

 

I will be glad to submit a PR either way.

[OTP-Maintainer]

lukas said:

We would gladly accept a PR to master that throws an exception if both ram and raw are given.

[OTP-Maintainer]

josevalim said:

Done here: https://github.com/erlang/otp/pull/2723

[OTP-Maintainer]

josevalim said:

Btw, this can be closed. :)