Add Lwt_unix.pread and pwrite

[chambart]

I just happened to need those function at the same time as #767. Hence, here it is.

I had to document that pwrite and pread leaves the current file position undefined because unix pread and windows ReadFile don't agree on that. This doesn't seem to be a serious problem. I don't expect anybody to be using both pread and read/write on the same fd.

Thanks for maintaining a functional test suite for Windows I would have been too lazy to bother starting a VM to test that !

Otherwise, I don't know whether the test suite triggers both the job and standard version of the functions. If it doesn't, how should I do this ?

[aantron]

Thanks!

Otherwise, I don't know whether the test suite triggers both the job and standard version of the functions. If it doesn't, how should I do this ?

There probably isn't a legitimate way to trigger the non-blocking version of these functions. To my knowledge, on both Unix and Windows, all fds that allow positional I/O also do not support non-blocking I/O. The only exception to that may be asynchronous HANDLEs on Windows, but we don't open HANDLEs in asynchronous mode.

You can still force the non-blocking code to run by calling Lwt_unix.set_blocking ~set_flags:false fd false in additional test cases for this. The non-blocking code added in this PR will then run, but the underlying pread (etc.) calls will still block, and it would be a bug in a user's program if it did this.

You can check whether the code is running by either inserting prints or running make coverage and looking at the branches.

cc @tomjridge

[tomjridge]

Thanks you very much @chambart and @aantron !

[aantron]

Thanks @chambart! I agreed with making the functions separate, rather than using an optional argument.