Cygwin tests are failing because cygwin has a low limit of only 64 fds in
select(). Extend select_wrapper_t to also support using poll(), according to
a FISH_USE_POLL new define. All systems now use poll() except for Mac.
Rename select_wrapper_t to fd_readable_set_t since now it may not wrap
select().
This allows the deep-cmdsub.fish test to pass on Cygwin.
clang-tidy wrongly sees an std::move to a const ref parameter and
believes it to be pointless. The copy constructor however is deleted.
Signed-off-by: Rosen Penev <rosenp@gmail.com>
select_wrapper_t wraps up the annoying bits of using select(): keeping
track of the max fd, passing null for boring parameters, and
constructing the timeout. Introduce a wrapper struct for this and
replace the existing uses of select() with the wrapper.
fd_monitor_t allows observing a collection of fds. It also has its own
fd, which it uses to awaken itself when there are changes. Switch to
using fd_event_signaller_t instead of a pipe; this reduces the number of
file descriptors and is more efficient under Linux.
fds.h will centralize logic around working with file descriptors. In
particular it will be the new home for logic around moving fds to high
unused values, replacing the "avoid conflicts" logic.
In preparation for fixing #7559, add a function poke_item to fd_monitor.
fd_monitor has a list of file descriptors, and invokes a callback when an
fd becomes readable. With this change, we assign each item a unique ID and
return it when the item is added; the ID may then be used to invoke the
callback explicitly.
The idea is that we can stop reading from the pipe associated with the
cmdsub when the job is finished, even if the pipe is still open.
fd_monitor is a new class which can monitor a set of fds, waiting for them
to become readable. When an fd becomes readable, a callback is invoked.
Timeouts are also supported.
This is intended to replace the "bufferfill" threads. Rather than one
thread per bufferfill, we will have a single fd_monitor which can service
multiple bufferfills. This helps today with nested command substitutions,
and will help in the future with concurrent execution.
