pub fn write_line_to_real_stderr(line: &str)Expand description
Write line plus a newline to the parent’s real stderr. Used by
the recursive-run output multiplexer, which pipes child stderr
through aube and re-emits each line with a <package>: prefix —
eprintln! writes to fd 2, which SilentStderrGuard has redirected
to /dev/null under --silent, so child stderr would otherwise be
silently swallowed in --silent --parallel mode. Routes through the
saved real-stderr fd when silent mode is active, fd 2 otherwise.
write_all of a pre-built <line>\n buffer issues a single short
write to the kernel; on TTYs and pipes the kernel’s PIPE_BUF
(= 4096+ on every supported unix) atomicity keeps lines from
concurrent pump tasks intact without explicit locking. The dup
happens per line so we don’t share a long-lived File handle that
would need its own lock — a duplicate write syscall pair is
cheaper than an Arc<Mutex<File>> and correct under concurrency.