reaper: new module, uses a signalfd to wait() on child processes

Use a signalfd to listen for SIGCHLD signals.

When we receive a SIGCHLD over the signalfd, reap all dead children by
looping over all registered child PIDs and call waitpid(WNOHANG) on
them.

main.c

@@ -21,6 +21,7 @@
 
 #include "config.h"
 #include "fdm.h"
+#include "reaper.h"
 #include "render.h"
 #include "server.h"
 #include "shm.h"
@@ -353,6 +354,7 @@ main(int argc, char *const *argv)
     conf.hold_at_exit = hold;
 
     struct fdm *fdm = NULL;
+    struct reaper *reaper = NULL;
     struct wayland *wayl = NULL;
     struct renderer *renderer = NULL;
     struct terminal *term = NULL;
@@ -378,6 +380,9 @@ main(int argc, char *const *argv)
     if ((fdm = fdm_init()) == NULL)
         goto out;
 
+    if ((reaper = reaper_init(fdm)) == NULL)
+        goto out;
+
     if ((wayl = wayl_init(&conf, fdm)) == NULL)
         goto out;
 
@@ -429,6 +434,7 @@ out:
     shm_fini();
     render_destroy(renderer);
     wayl_destroy(wayl);
+    reaper_destroy(reaper);
     fdm_destroy(fdm);
 
     config_free(conf);

meson.build

@@ -114,6 +114,7 @@ executable(
   'misc.c', 'misc.h',
   'osc.c', 'osc.h',
   'quirks.c', 'quirks.h',
+  'reaper.c', 'reaper.h',
   'render.c', 'render.h',
   'search.c', 'search.h',
   'selection.c', 'selection.h',

reaper.c

@@ -0,0 +1,142 @@
+#include "reaper.h"
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/epoll.h>
+#include <sys/wait.h>
+#include <sys/signalfd.h>
+
+#define LOG_MODULE "reaper"
+#define LOG_ENABLE_DBG 1
+#include "log.h"
+#include "tllist.h"
+
+struct reaper {
+    struct fdm *fdm;
+    int fd;
+    tll(pid_t) children;
+};
+
+static bool fdm_reap(struct fdm *fdm, int fd, int events, void *data);
+
+struct reaper *
+reaper_init(struct fdm *fdm)
+{
+    sigset_t mask;
+    sigemptyset(&mask);
+    sigaddset(&mask, SIGCHLD);
+
+    /* Block normal signal handling - we're using a signalfd instead */
+    if (sigprocmask(SIG_BLOCK, &mask, NULL) < 0) {
+        LOG_ERRNO("failed to block SIGCHLD");
+        return NULL;
+    }
+
+    int fd = signalfd(-1, &mask, SFD_NONBLOCK | SFD_CLOEXEC);
+    if (fd < 0) {
+        LOG_ERRNO("failed to create signal FD");
+        sigprocmask(SIG_UNBLOCK, &mask, NULL);
+        return NULL;
+    }
+
+    struct reaper *reaper = malloc(sizeof(*reaper));
+    *reaper = (struct reaper){
+        .fdm = fdm,
+        .fd = fd,
+        .children = tll_init(),
+    };
+
+    if (!fdm_add(fdm, fd, EPOLLIN, &fdm_reap, reaper)){
+        LOG_ERR("failed to register with the FDM");
+        goto err;
+    }
+
+    return reaper;
+
+err:
+    tll_free(reaper->children);
+    close(fd);
+    free(reaper);
+    return NULL;
+}
+
+void
+reaper_destroy(struct reaper *reaper)
+{
+    if (reaper == NULL)
+        return;
+
+    fdm_del(reaper->fdm, reaper->fd);
+    tll_free(reaper->children);
+    free(reaper);
+
+    sigset_t mask;
+    sigemptyset(&mask);
+    sigaddset(&mask, SIGCHLD);
+    sigprocmask(SIG_UNBLOCK, &mask, NULL);
+}
+
+void
+reaper_add(struct reaper *reaper, pid_t pid)
+{
+    LOG_DBG("adding pid=%d", pid);
+    tll_push_back(reaper->children, pid);
+}
+
+static bool
+fdm_reap(struct fdm *fdm, int fd, int events, void *data)
+{
+    struct reaper *reaper = data;
+
+    bool pollin = events & EPOLLIN;
+    bool hup = events & EPOLLHUP;
+
+    if (hup && !pollin)
+        return false;
+
+    assert(pollin);
+
+    struct signalfd_siginfo info;
+    ssize_t amount = read(reaper->fd, &info, sizeof(info));
+
+    if (amount < 0) {
+        LOG_ERRNO("failed to read");
+        return false;
+    }
+
+    assert((size_t)amount >= sizeof(info));
+
+    if (info.ssi_signo != SIGCHLD) {
+        LOG_WARN("got non-SIGCHLD signal: %d", info.ssi_signo);
+        return true;
+    }
+
+    tll_foreach(reaper->children, it) {
+        /* Don't use wait() since we don't want to accidentaly reap
+         * the PTS slave */
+
+        pid_t pid = it->item;
+
+        int result;
+        int res = waitpid(pid, &result, WNOHANG);
+
+        if (res <= 0) {
+            if (res < 0)
+                LOG_ERRNO("waitpid failed for pid=%d", pid);
+            continue;
+        }
+
+        else if (WIFEXITED(result))
+            LOG_DBG("pid=%d: exited with status=%d", pid, WEXITSTATUS(result));
+        else if (WIFSIGNALED(result))
+            LOG_DBG("pid=%d: killed by signal=%d", pid, WTERMSIG(result));
+        else
+            LOG_DBG("pid=%d: died of unknown resason", pid);
+        tll_remove(reaper->children, it);
+    }
+
+    if (hup)
+        return false;
+
+    return true;
+}

reaper.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <sys/wait.h>
+
+#include "fdm.h"
+
+struct reaper;
+
+struct reaper *reaper_init(struct fdm *fdm);
+void reaper_destroy(struct reaper *reaper);
+
+void reaper_add(struct reaper *reaper, pid_t pid);