Freebsd: Fixed 'write' syscall to cause 'ESPIPE' on the pipe fd.

[wisonye]

If you use posix.pipe() to create a bidirectional pipe that allows you to write to one end and read from the other end, you will find that the write syscall will cause espipe error!

Here is the minimal example to prove that:

import "core:sys/freebsd"
import "core:sys/posix"

WriteSysCall :: freebsd.write

pipe_fds: [2]PosixFd,

// ... Run the following code in a function:

//
// Sample bytes
//
event_bytes := [?]u8{0x0A, 0x0B, 0x0C, 0x0D};

//
// Write the event bytes to the `WRITE_PIPE`
//
count, error := freebsd.write(
    cast(Fd)self.pipe_fds[1],
    event_bytes[:]
)

Then the error will be espipe!!!

But if you use posix.write, it works expected:

 count := posix.write(
     self.pipe_fds[1],
     &event_bytes[0],
     len(event_bytes)
 )

So, if you look into $ODIN_ROOT/core/sys/freebsd/syscalls.odin, you will see that the write procedure actually call SYS_pwrite syscall which is wrong!

Since pwrite requires an offset, which in turn triggers a lseek/seek call, it will fail when the file descriptor belongs to a pipe.

If you run man 2 write, you should see the following description about that:

The pwrite() and pwritev() system calls may also return the following
errors:

[EINVAL]           The offset value was negative.

[ESPIPE]           The file descriptor is associated with a pipe, socket,
                or FIFO.