Help with `:file` blocking GenServer process

Okay so while investigating this, i discovered something.

This code will hang forever no matter what. It doesn’t matter how much data
is written to the pipe, the data will never be read by Erlang/Elixir.

{:ok, file} = :file.open(path_to_fifo, [:read, :write, :binary])
{:ok, "hello world"} = :file.read(file, 11)
# in a different tab to `File.write(path_to_fifo, "hello world")`

This code however works fine. I suspect this is because the :raw option is a NIF now. (just a guess?)

# notice the `:raw` option.
{:ok, file} = :file.open(path_to_fifo, [:read, :write, :binary, :raw]) 
{:ok, "hello world"} = :file.read(file, 11)
# in a different tab to `File.write(path_to_fifo, "hello world")`

The only problem with opening in :raw mode is that i can’t use Task to call read/2 because the Task is not an owning process. I get the following error:

** (ErlangError) Erlang error: :not_on_controlling_process
        :prim_file.get_fd_data/1
        :prim_file.read/2
        (elixir) lib/task/supervised.ex:89: Task.Supervised.do_apply/2
        (elixir) lib/task/supervised.ex:38: Task.Supervised.reply/5
        (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

I guess this isn’t really an Elixir issue, but an Erlang one but any help would be appreciated.

Doesn’t that need to be used with Task.await to receive the result? I also think tasks use receive blocks that wouldn’t work in a genserver process.

From the Task docs:

Oh, sorry then.

if i manually change my code to do a :file.read(fd, 0) the behaviour works. I recieve the result in handle_info, so i at least am fairly certain Task is not the problem here.

The solution is to use ports, which will also give asynchrony. See previous discussion here: Elixir vs Unix named pipe

I think it was recently announced that erlang supports pipes, but I cant recall if in port or file. Or I may be completely misremebering it. Anyway, a port should do nowadays.

Sorry, i should have stated this in the original post, but i have tried this as well and it did not work either.

Interactive Elixir (1.7.3) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> System.cmd("mkfifo", ["/tmp/fifo", "-m", "0666"])          
{"", 0}
iex(2)> port = :erlang.open_port({:spawn, '/tmp/fifo'}, [:binary]) 
#Port<0.7>
iex(3)> sh: /tmp/fifo: Permission denied
                                        sh: line 0: exec: /tmp/fifo: cannot execute: Permission denied

and in another terminal, writing to the FIFO just hangs until i send a ctrl+c

echo "hello world" -e > "/tmp/fifo"                       Thu 24 Jan 2019 10:28:34 AM PST
^CAn error occurred while redirecting file '/tmp/fifo'
open: Interrupted system call

So this worked:

~/OSS/elixir[jv-basic-releases %]$ mkfifo test.pipe
~/OSS/elixir[jv-basic-releases %]$ erl
Erlang/OTP 21 [erts-10.0] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Eshell V10.0  (abort with ^G)
1> Fifo = open_port("test.pipe", [eof]),
1> receive
1> Msg -> io:format("Got ~p", [Msg])
1> end.
Got {#Port<0.5>,{data,"foo\n"}}ok

After typing the receive, I ran echo "foo" > test.pipe in another terminal. Could it be something related to permissions as the error message says?

ah ha! i was trying to use spawn which was not needed. Will try this method out and report back.

I can confirm that this does in fact allow reading and writing from a named pipe. However it does add the requirement of buffering now, since you are not telling the port how much data you want to read. Either way thanks for the help!