Comment 2 for bug 1044466

I suppose that's the same problem as explained in http://stackoverflow.com/a/12189425/94687 and http://lists.gnu.org/archive/html/bug-bash/2012-06/msg00027.html :

"Coproc file descriptors are not available to subshells. They're implemented using pipes, and leaving pipe file descriptors open in subshells causes processes to hang and not terminate properly, which results in very hard-to-track-down-and-reproduce bugs."

Although my redirection is outside of () (or {}), the file descriptor used must be available in the forked subshell, but it not might be the way bash treats fds.