Fix use-after-free in example/poll.c

As noticed by valgrind in issue #907 example/poll.c
triggers a use-after-free

==85200== Thread 2:
==85200== Invalid read of size 4
==85200==    at 0x485E54A: send_notify_iov (fuse_lowlevel.c:2267)
==85200==    by 0x485E54A: fuse_lowlevel_notify_poll (fuse_lowlevel.c:2289)
==85200==    by 0x1096F2: fsel_producer (poll.c:245)
==85200==    by 0x4897EA6: start_thread (pthread_create.c:477)
==85200==    by 0x49ADA6E: clone (clone.S:95)
==85200==  Address 0x5291d68 is 392 bytes inside a block of size 920 free'd
==85200==    at 0x48399AB: free (vg_replace_malloc.c:538)
==85200==    by 0x485A12C: fuse_destroy (fuse.c:5103)
==85200==    by 0x486220F: fuse_main_real (helper.c:389)
==85200==    by 0x1091D6: main (poll.c:288)
==85200==  Block was alloc'd at
==85200==    at 0x483AB65: calloc (vg_replace_malloc.c:760)
==85200==    by 0x485BAA0: fuse_session_new (fuse_lowlevel.c:3036)
==85200==    by 0x4859AF2: fuse_new@@FUSE_3.1 (fuse.c:4966)
==85200==    by 0x4862129: fuse_main_real (helper.c:345)
==85200==    by 0x1091D6: main (poll.c:288)

Issue is that the "fsel_producer" thread is still active after
fuse_destroy - it gets destructed too late.

1 files changed, 16 insertions, 6 deletions

diff --git a/example/poll.c b/example/poll.c
index f9430a9..fd53ec0 100644
--- a/example/poll.c
+++ b/example/poll.c
@@ -33,6 +33,7 @@
 #include <time.h>
 #include <pthread.h>
 #include <poll.h>
+#include <stdbool.h>
 
 /*
  * fsel_open_mask is used to limit the number of opens to 1 per file.
@@ -51,6 +52,9 @@ static pthread_mutex_t fsel_mutex;	/* protects notify_mask and cnt array */
 static unsigned fsel_poll_notify_mask;	/* poll notification scheduled? */
 static struct fuse_pollhandle *fsel_poll_handle[FSEL_FILES]; /* poll notify handles */
 static unsigned fsel_cnt[FSEL_FILES];	/* nbytes stored in each file */
+static _Atomic bool fsel_stop = false;
+static pthread_t fsel_producer_thread;
+
 
 static int fsel_path_index(const char *path)
 {
@@ -61,6 +65,15 @@ static int fsel_path_index(const char *path)
 	return ch <= '9' ? ch - '0' : ch - 'A' + 10;
 }
 
+static void fsel_destroy(void *private_data)
+{
+	(void)private_data;
+
+	fsel_stop = true;
+
+	pthread_join(fsel_producer_thread, NULL);
+}
+
 static int fsel_getattr(const char *path, struct stat *stbuf,
 			struct fuse_file_info *fi)
 {
@@ -205,6 +218,7 @@ static int fsel_poll(const char *path, struct fuse_file_info *fi,
 }
 
 static const struct fuse_operations fsel_oper = {
+	.destroy        = fsel_destroy,
 	.getattr	= fsel_getattr,
 	.readdir	= fsel_readdir,
 	.open		= fsel_open,
@@ -220,7 +234,7 @@ static void *fsel_producer(void *data)
 
 	(void) data;
 
-	while (1) {
+	while (!fsel_stop) {
 		int i, t;
 
 		pthread_mutex_lock(&fsel_mutex);
@@ -263,7 +277,6 @@ static void *fsel_producer(void *data)
 
 int main(int argc, char *argv[])
 {
-	pthread_t producer;
 	pthread_attr_t attr;
 	int ret;
 
@@ -279,7 +292,7 @@ int main(int argc, char *argv[])
 		return 1;
 	}
 
-	errno = pthread_create(&producer, &attr, fsel_producer, NULL);
+	errno = pthread_create(&fsel_producer_thread, &attr, fsel_producer, NULL);
 	if (errno) {
 		perror("pthread_create");
 		return 1;
@@ -287,8 +300,5 @@ int main(int argc, char *argv[])
 
 	ret = fuse_main(argc, argv, &fsel_oper, NULL);
 
-	pthread_cancel(producer);
-	pthread_join(producer, NULL);
-
 	return ret;
 }