mountpoint-util: use mount_option_supported() to detect if 'discard' is support for an fs

src/basic/mountpoint-util.c

@@ -461,6 +461,15 @@ bool fstype_is_ro(const char *fstype) {
 }
 
 bool fstype_can_discard(const char *fstype) {
+        int r;
+
+        assert(fstype);
+
+        /* On new kernels we can just ask the kernel */
+        r = mount_option_supported(fstype, "discard", NULL);
+        if (r >= 0)
+                return r;
+
         return STR_IN_SET(fstype,
                           "btrfs",
                           "f2fs",

src/test/test-mountpoint-util.c

@@ -349,6 +349,12 @@ TEST(mount_option_supported) {
         assert_se(r >= 0 || (r < 0 && ERRNO_IS_PRIVILEGE(r)));
 }
 
+TEST(fstype_can_discard) {
+        assert_se(fstype_can_discard("ext4"));
+        assert_se(!fstype_can_discard("squashfs"));
+        assert_se(!fstype_can_discard("iso9660"));
+}
+
 static int intro(void) {
         /* let's move into our own mount namespace with all propagation from the host turned off, so
          * that /proc/self/mountinfo is static and constant for the whole time our test runs. */