dissect: Don't count RPMB and boot partitions

Filter-out RPMB partitions and boot partitions from MMC devices when
counting partitions enumerated by the kernel. Also factor out the now
duplicated code into a separate function.

This complement the previous fixes to the problem reported in
systemd/systemd#5806

https://phabricator.endlessm.com/T21744

src/shared/dissect-image.c

@@ -108,6 +108,14 @@ int probe_filesystem(const char *node, char **ret_fstype) {
 #endif
 }
 
+/* Detect RPMB and Boot partitions, which are not listed by blkid.
+ * See https://github.com/systemd/systemd/issues/5806. */
+static bool device_is_mmc_special_partition(struct udev_device *d) {
+        const char *sysname = udev_device_get_sysname(d);
+        return (sysname && startswith(sysname, "mmcblk") &&
+                (endswith(sysname, "rpmb") || endswith(sysname, "boot0") || endswith(sysname, "boot1")));
+}
+
 int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectImageFlags flags, DissectedImage **ret) {
 
 #if HAVE_BLKID
@@ -277,8 +285,17 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
                 /* Count the partitions enumerated by the kernel */
                 n = 0;
                 first = udev_enumerate_get_list_entry(e);
-                udev_list_entry_foreach(item, first)
+                udev_list_entry_foreach(item, first) {
+                        _cleanup_udev_device_unref_ struct udev_device *q;
+
+                        q = udev_device_new_from_syspath(udev, udev_list_entry_get_name(item));
+                        if (!q)
+                                return -errno;
+
+                        if (device_is_mmc_special_partition(q))
+                                continue;
                         n++;
+                }
 
                 /* Count the partitions enumerated by blkid */
                 z = blkid_partlist_numof_partitions(pl);
@@ -337,7 +354,7 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
                 _cleanup_udev_device_unref_ struct udev_device *q;
                 unsigned long long pflags;
                 blkid_partition pp;
-                const char *node, *sysname;
+                const char *node;
                 dev_t qn;
                 int nr;
 
@@ -352,11 +369,7 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
                 if (st.st_rdev == qn)
                         continue;
 
-                /* Filter out weird MMC RPMB partitions, which cannot reasonably be read, see
-                 * https://github.com/systemd/systemd/issues/5806 */
-                sysname = udev_device_get_sysname(q);
-                if (sysname && startswith(sysname, "mmcblk") &&
-                        (endswith(sysname, "rpmb") || endswith(sysname, "boot0" ) || endswith(sysname, "boot1")))
+                if (device_is_mmc_special_partition(q))
                         continue;
 
                 node = udev_device_get_devnode(q);