pipe(): input to, and output from, a single process

I am not sure if `pipe()` works for this, but if it turns out that it
does not, then you can use the processx package, e.g.:

p <- processx::process$new("sed", c("-l", "s/a/x/g"), stdin = "|", stdout = "|")
p$write_input("foobar\n")
p$read_output()

[1] "foobxr\n"

The `-l` sed flag is to make sed line-buffered, otherwise it is will
not produce output until there is enough.

`$write_input()` and `$read_output()` are not easy to program, in particular:
* `$write_input()` returns the chunk of data that it hasn't managed to
write into the pipe. You need to call `$write_input() again, with this
data next, usually.
* `$read_output()` returns an empty string if there is no data to
read, so typically you want to call `p$poll()` first, to make sure
that there is something to read.
* `$read_output()` might not read whole lines, so maybe
`$read_output_lines()` is better for you.
* Close the stdin of the process if you want to quit cleanly:
`close(p$get_input_connection())`.
* There is currently no way to poll the input side of the pipe. :(

HTH, Gabor

hi.  i'd like to instantiate sed(1), send it some input, and retrieve
its output, all via pipes (rather than an intermediate file).

my sense from pipe and looking at the sources (sys-unix.c) is that is
not possible.  is that true?  are there any thoughts of providing such a
facility?

cheers, Greg

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