How to redirect an output file descriptor of a subshell to an input file descriptor in the parent shell?

Question

(In BASH) I want a subshell to use a non-STDOUT non-STDERR file descriptor to pass some data back to the parent shell. How can I do that? Eventually I would love to save the data into some variable of the parent shell.

(
  # The following two lines show the behavior of the subshell.
  # We cannot change them.
  echo "This should go to STDOUT"
  echo "This is the data I want to pass to the parent shell" >&3
)
#...
data_from_subshell=... # Somehow assign the value of &3 of the
                       # subshell to this variable

EDIT: The subshell runs a black-box program that writes to STDOUT and &3.

Answer 1

BEWARE, BASHISM AHEAD (there are posix shells that are significantly faster than bash, e.g. ash or dash, that don't have process substitution).

You can do a handle dance to move original standard output to a new descriptor to make standard output available for piping (from the top of my head):

exec 3>&1 # creates 3 as alias for 1
run_in_subshell() { # just shortcut for the two cases below
    echo "This goes to STDOUT" >&3
    echo "And this goes to THE OTHER FUNCTION"
}

Now you should be able to write:

while read line; do
    process $line
done < <(run_in_subshell)

but the <() construct is a bashism. You can replace it with pipeline

run_in_subshell | while read line; do
    process $line
done

except than the second command also runs in subshell, because all commands in pipeline do.