Removed THREAD_MAX and the barrier and pfd fields from udev_monitor

We can fix THREAD_MAX at one since we don't gain much by having multiple
threads. All the threads did were translate inotify events. We also
no longer need pfd and the barrier fields of udev_monitor since we don't
need to signal the single thread or guard against any races.

udev_monitor.c

@@ -9,13 +9,12 @@
 #include <pthread.h>
 #include <sys/stat.h>
 #include <sys/socket.h>
+#include <sys/eventfd.h>
 #include <sys/inotify.h>
 
 #include "udev.h"
 #include "udev_list.h"
 
-#define THREADS_MAX 5
-
 #ifndef UDEV_MONITOR_DIR
 #define UDEV_MONITOR_DIR "/tmp/.libudev-zero"
 #endif
@@ -23,12 +22,11 @@
 struct udev_monitor {
     struct udev_list_entry subsystem_match;
     struct udev_list_entry devtype_match;
-    pthread_t thread[THREADS_MAX];
-    pthread_barrier_t barrier;
+    pthread_t thread;
     struct udev *udev;
     int refcount;
+    int signal_fd;
     int sfd[2];
-    int pfd[2];
     int ifd;
 };
 
@@ -123,7 +121,7 @@ static void *handle_event(void *ptr)
     poll_fds[0].fd = udev_monitor->ifd;
     poll_fds[0].events = POLLIN;
 
-    poll_fds[1].fd = udev_monitor->pfd[0];
+    poll_fds[1].fd = udev_monitor->signal_fd;
     poll_fds[1].events = POLLIN;
 
     while (1) {
@@ -134,9 +132,8 @@ static void *handle_event(void *ptr)
             break;
         }
 
+        // exit on explicit signal
         if (poll_fds[1].revents & POLLIN) {
-            read(udev_monitor->pfd[0], data, 1);
-            write(udev_monitor->pfd[1], "1", 1);
             break;
         }
 
@@ -146,6 +143,11 @@ static void *handle_event(void *ptr)
             continue;
         }
 
+        // exit on ifd error or close
+        if (!(poll_fds[0].revents & POLLIN) || len == 0) {
+            break;
+        }
+
         for (i = 0; i < len; i += sizeof(struct inotify_event) + event->len) {
             event = (struct inotify_event *)&data[i];
 
@@ -158,28 +160,16 @@ static void *handle_event(void *ptr)
         }
     }
 
-    pthread_barrier_wait(&udev_monitor->barrier);
     return NULL;
 }
 
 int udev_monitor_enable_receiving(struct udev_monitor *udev_monitor)
 {
-    pthread_attr_t attr;
-    int i;
-
     if (!udev_monitor) {
         return -1;
     }
 
-    pthread_attr_init(&attr);
-    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
-    pthread_barrier_init(&udev_monitor->barrier, NULL, THREADS_MAX + 1);
-
-    for (i = 0; i < THREADS_MAX; i++) {
-        pthread_create(&udev_monitor->thread[i], &attr, handle_event, udev_monitor);
-    }
-
-    pthread_attr_destroy(&attr);
+    pthread_create(&udev_monitor->thread, NULL, handle_event, udev_monitor);
     return 0;
 }
 
@@ -232,7 +222,6 @@ struct udev_monitor *udev_monitor_new_from_netlink(struct udev *udev, const char
 {
     struct udev_monitor *udev_monitor;
     struct stat st;
-    int i;
 
     if (!udev || !name) {
         return NULL;
@@ -257,41 +246,35 @@ struct udev_monitor *udev_monitor_new_from_netlink(struct udev *udev, const char
         return NULL;
     }
 
+    udev_monitor->signal_fd = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
+    if (udev_monitor->signal_fd == -1) {
+        goto error_free;
+    }
+
     udev_monitor->ifd = inotify_init1(IN_CLOEXEC | IN_NONBLOCK);
 
     if (udev_monitor->ifd == -1) {
-        goto error_free;
+        goto error_signal_fd;
     }
 
     if (inotify_add_watch(udev_monitor->ifd, UDEV_MONITOR_DIR, IN_CLOSE_WRITE | IN_EXCL_UNLINK) == -1) {
         goto error_ifd;
     }
 
-    if (pipe(udev_monitor->pfd) == -1) {
-        goto error_ifd;
-    }
-
-    for (i = 0; i < 2; i++) {
-        fcntl(udev_monitor->pfd[i], F_SETFD, FD_CLOEXEC);
-        fcntl(udev_monitor->pfd[i], F_SETFL, O_NONBLOCK);
-    }
-
     if (socketpair(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0, udev_monitor->sfd) == -1) {
-        goto error_pfd;
+        goto error_ifd;
     }
 
     udev_monitor->refcount = 1;
     udev_monitor->udev = udev;
     return udev_monitor;
 
-error_pfd:
-    for (i = 0; i < 2; i++) {
-        close(udev_monitor->pfd[i]);
-    }
-
 error_ifd:
     close(udev_monitor->ifd);
 
+error_signal_fd:
+    close(udev_monitor->signal_fd);
+
 error_free:
     free(udev_monitor);
     return NULL;
@@ -319,18 +302,19 @@ struct udev_monitor *udev_monitor_unref(struct udev_monitor *udev_monitor)
         return NULL;
     }
 
-    write(udev_monitor->pfd[1], "1", 1);
-    pthread_barrier_wait(&udev_monitor->barrier);
-    pthread_barrier_destroy(&udev_monitor->barrier);
+    // Wake up the event thread
+    eventfd_write(udev_monitor->signal_fd, 1);
+    // waiting for event thread to end before freeing udev_monitor
+    pthread_join(udev_monitor->thread, NULL);
 
     udev_list_entry_free_all(&udev_monitor->devtype_match);
     udev_list_entry_free_all(&udev_monitor->subsystem_match);
 
     for (i = 0; i < 2; i++) {
-        close(udev_monitor->pfd[i]);
         close(udev_monitor->sfd[i]);
     }
 
+    close(udev_monitor->signal_fd);
     close(udev_monitor->ifd);
     free(udev_monitor);
     return NULL;