I'm reading data from the network and I'd like to write it to a file whenever I get them. The writes are concurrent and non sequential (think P2P file sharing). In C, I would get a file descriptor to the file(for the duration of the program) then use lseek, followed by write and eventually close the fd. These operations could be protected by a mutex in a multithreaded setting (especially, lseek and write should be atomic).
I don't really see how to get that behavior in Async. My initial idea is to have something like this.
let write fd s pos =
let posl = Int64.of_int pos in
Async_unix.Unix_syscalls.lseek fd ~mode:`Set posl
>>| fun _ ->
let wr = Writer.create t.fd in
let len = String.length s in
Writer.write wr s ~pos:0 ~len
Then, the writes are scheduled asynchronously when data is received.
My solution isn't correct. For one thing, this write task need to be atomic but it is not the case, since two lseek can be executed before the first Writer.write. Even if I can schedule the write sequentially it wouldn't help since Writer.write doesn't return a Deferred.t. Any idea?
BTW, this is a follow-up to a previous answered question.