test-loop-block: let's explicitly flush buffer cache on whole block device

Let's explicitly flush the kernel's buffer cache on the whole block
device once we ran "mkfs". This is necessary, because partition and
whole block devices maintain separate buffer caches, and thus writing
to one will not be visible on the other if cached there already, until
the latter's cache is explicitly flushed.

This is preparation for later adding support for probing file sytems
also if we have no open partition block devices, and hence want to use
the whole block device instead.

src/test/test-loop-block.c

@@ -4,6 +4,8 @@
 #include <linux/loop.h>
 #include <pthread.h>
 #include <sys/file.h>
+#include <sys/ioctl.h>
+#include <sys/mount.h>
 
 #include "alloc-util.h"
 #include "capability-util.h"
@@ -256,6 +258,13 @@ static int run(int argc, char *argv[]) {
 
         dissected = dissected_image_unref(dissected);
 
+        /* We created the file systems now via the per-partition block devices. But the dissection code might
+         * probe them via the whole block device. These block devices have separate buffer caches though,
+         * hence what was written via the partition device might not appear on the whole block device
+         * yet. Let's hence explicitly flush the whole block device, so that the read-back definitely
+         * works. */
+        assert_se(ioctl(loop->fd, BLKFLSBUF, 0) >= 0);
+
         /* Try to read once, without pinning or adding partitions, i.e. by only accessing the whole block
          * device. */
         assert_se(dissect_loop_device(loop, NULL, NULL, 0, &dissected) >= 0);