blockdev-util: split out partition device node generation from dissect-util.c

src/shared/blockdev-util.c

@@ -904,3 +904,46 @@ int blockdev_get_root(int level, dev_t *ret) {
 
         return 1;
 }
+
+int partition_node_of(const char *node, unsigned nr, char **ret) {
+        int r;
+
+        assert(node);
+        assert(nr > 0);
+        assert(ret);
+
+        /* Given a device node path to a block device returns the device node path to the partition block
+         * device of the specified partition */
+
+        _cleanup_free_ char *fn = NULL;
+        r = path_extract_filename(node, &fn);
+        if (r < 0)
+                return r;
+        if (r == O_DIRECTORY)
+                return -EISDIR;
+
+        _cleanup_free_ char *dn = NULL;
+        r = path_extract_directory(node, &dn);
+        if (r < 0 && r != -EDESTADDRREQ) /* allow if only filename is specified */
+                return r;
+
+        size_t l = strlen(fn);
+        assert(l > 0); /* underflow check for the subtraction below */
+
+        bool need_p = ascii_isdigit(fn[l-1]); /* Last char a digit? */
+
+        _cleanup_free_ char *subnode = NULL;
+        if (asprintf(&subnode, "%s%s%u", fn, need_p ? "p" : "", nr) < 0)
+                return -ENOMEM;
+
+        if (dn) {
+                _cleanup_free_ char *j = path_join(dn, subnode);
+                if (!j)
+                        return -ENOMEM;
+
+                *ret = TAKE_PTR(j);
+        } else
+                *ret = TAKE_PTR(subnode);
+
+        return 0;
+}

src/shared/blockdev-util.h

@@ -57,3 +57,5 @@ int blockdev_get_sector_size(int fd, uint32_t *ret);
 int blockdev_get_device_size(int fd, uint64_t *ret);
 
 int blockdev_get_root(int level, dev_t *ret);
+
+int partition_node_of(const char *node, unsigned nr, char **ret);

src/shared/dissect-image.c

@@ -627,14 +627,9 @@ static int make_partition_devname(
                         if (!s)
                                 return -ENOMEM;
                 } else {
-                        size_t l = strlen(whole_devname);
-                        if (l < 1) /* underflow check for the subtraction below */
-                                return -EINVAL;
-
-                        bool need_p = ascii_isdigit(whole_devname[l-1]); /* Last char a digit? */
-
-                        if (asprintf(&s, "%s%s%i", whole_devname, need_p ? "p" : "", nr) < 0)
-                                return -ENOMEM;
+                        r = partition_node_of(whole_devname, nr, &s);
+                        if (r < 0)
+                                return r;
                 }
         } else {
                 if (nr < 0) /* whole disk? */