[enricolorenzoni59]

Summary

The macOS epoll shim (compat/apple/epoll_shim.c) does not translate the epoll interest mask correctly:

• with EPOLLIN | EPOLLOUT in one call, the second EV_SET overwrites the first, so only the write filter is submitted;
• EPOLL_CTL_MOD only ever calls EV_ADD, so a filter that is no longer requested is left registered.

Impact

macOS only, no Linux code path is touched.

flushClient() adds EPOLLOUT with EPOLL_CTL_MOD when a client's send queue cannot be flushed and removes it again once the queue drains. On kqueue the removal never happened, so the stale EVFILT_WRITE keeps reporting the now writable socket: epoll_wait() returns immediately on every loop with an empty send queue, and the network thread spins at 100% until that client disconnects.

Repro

Small program built against the shim:

#include "epoll_shim.h"
#include <stdio.h>
#include <sys/socket.h>
#include <unistd.h>

int main(void) {
    int pair[2], ep = epoll_create1(0);
    struct epoll_event ev = { 0 }, out[4];
    socketpair(AF_UNIX, SOCK_STREAM, 0, pair);
    ev.events = EPOLLIN | EPOLLOUT;
    ev.data.ptr = &pair;
    epoll_ctl(ep, EPOLL_CTL_ADD, pair[0], &ev);
    int initial = epoll_wait(ep, out, 4, 10);
    ev.events = EPOLLIN;
    epoll_ctl(ep, EPOLL_CTL_MOD, pair[0], &ev);
    int idle = epoll_wait(ep, out, 4, 10);
    write(pair[1], "x", 1);
    int n = epoll_wait(ep, out, 4, 10), readable = 0;
    for (int i = 0; i < n; i++)
        readable += !!(out[i].events & EPOLLIN);
    printf("initial=%d idle_after_MOD=%d readable=%d\n", initial, idle, readable);
    return idle == 0 && readable == 1 ? 0 : 1;
}

Baseline on macOS: initial=1 idle_after_MOD=1 readable=1, exit 1. Native Linux epoll with the same sequence gives idle_after_MOD=0; with this patch the shim matches Linux.

Fix

Build the full change set for both directions and submit it in one kevent() call with EV_RECEIPT, so every change reports its own result instead of the call stopping at the first error. MOD adds/removes each direction to match the requested mask; DEL deletes both and returns ENOENT only when neither was registered.

Verification

• repro above, plus MOD from IN to IN|OUT and back, MOD to OUT, DEL of a read-only registration, double DEL;
• local macOS A/B: 4.2 MB fed to a raw output port with one slow sink and one fast sink, then the slow sink drained. Old shim: 1.000 s of process CPU in the second after the drain. Patched shim: 0.000 s. The slow sink received the same 473,680 bytes in both runs, the fast sink stream stayed valid, no epoll_ctl errors;
• build with -W -Wall -Werror;
• make test (cprtests, crctests) passes.

EPOLL_CTL_ADD on an already registered fd and EPOLL_CTL_MOD on an unregistered one keep their previous best-effort behavior (no EEXIST/ENOENT tracking). readsb does not rely on either.