I introduced this issue with the recent rate limiting changes. If we
generated sufficient events to fill the pipe buffer and trigger an
EAGAIN on the write side of the pipe, we'd end up in a state where epoll
would continually wake us up to deal with it, but because we gated
reading from the pipe on having entries in the queue we could decide
that there was nothing to do and leave the pipe unread.
This commit adjusts things so that we always try to read some data from
it.
This is OK because we're using the pipe to knock the main thread out of
a sleep rather than as the definitive count of events.
I'm seeing occasional 100% cpu usage on my linux system and I'm
not sure if its just because I'm running a stale binary.
I added this (commented out in this commit, but live on my local
system) debug print to help understand it.
The `SpawnQueue::run_impl` would loop until it had exhausted
all queued items. This prevents returning to the main loop
and resulted in the UI hanging while eg: `yes` was running,
and could also block accepting keyboard input, which is
pretty bad.
In addition, the queue implementation could fill up a pipe
and block the write side while it held a lock, which in
turn would prevent the read side from making room for the
write to succeed!
This commit changes the behavior on linux to change the wakeup
behavior of the queue from having a 1:1 relationship between
enqueue:wakeup to n:m where n and m are both >= 1. This is
sufficient to wake a sleeping gui thread. The gui thread
can then pop and process a single item at a time, interleaved
with dispatching the gui events.
The result is a bit more responsive, however, there is no
backpressure from the gui to the read side, so if the read
side is eating 2MB/s of data and the GUI side is processing
less than this, then an interrupt signal may still take a
few seconds to take effect.
I have mixed feelings about adding backpressure, because
I'm not sure that it is worth actually rendering all of
the parsed output text when there is a lot of it.
I need to follow up and verify these changes on macOS
and Windows too.
Refs: https://github.com/wez/wezterm/issues/65
