libmount: allow to convert /dev/loopN to backing filename

Signed-off-by: Karel Zak <kzak@redhat.com>

libmount/samples/mount.c

@@ -118,13 +118,13 @@ static void print_all(struct libmnt_context *cxt, char *pattern, int show_label)
 		const char *type = mnt_fs_get_fstype(fs);
 		const char *src = mnt_fs_get_source(fs);
 		const char *optstr = mnt_fs_get_options(fs);
+		char *xsrc;
 
 		if (type && pattern && !mnt_match_fstype(type, pattern))
 			continue;
 
-		/* TODO: print loop backing file instead of device name */
-
-		printf ("%s on %s", src ? : "none", mnt_fs_get_target(fs));
+		xsrc = mnt_pretty_path(src, cache);
+		printf ("%s on %s", xsrc, mnt_fs_get_target(fs));
 		if (type)
 			printf (" type %s", type);
 		if (optstr)
@@ -135,6 +135,7 @@ static void print_all(struct libmnt_context *cxt, char *pattern, int show_label)
 				printf (" [%s]", lb);
 		}
 		fputc('\n', stdout);
+		free(xsrc);
 	}
 
 	mnt_free_cache(cache);

libmount/src/cache.c

@@ -27,6 +27,7 @@
 
 #include "canonicalize.h"
 #include "mountP.h"
+#include "loopdev.h"
 
 /*
  * Canonicalized (resolved) paths & tags cache
@@ -447,6 +448,10 @@ char *mnt_get_fstype(const char *devname, int *ambi, struct libmnt_cache *cache)
  * @path: "native" path
  * @cache: cache for results or NULL
  *
+ * Converts path:
+ *	- to the absolute path
+ *	- /dev/dm-N to /dev/mapper/<name>
+ *
  * Returns: absolute path or NULL in case of error. The result has to be
  * deallocated by free() if @cache is NULL.
  */
@@ -489,6 +494,52 @@ error:
 	return NULL;
 }
 
+/**
+ * mnt_pretty_path:
+ * @path: any path
+ * @cache: NULL or pointer to the cache
+ *
+ * Converts path:
+ *	- to the absolute path
+ *	- /dev/dm-N to /dev/mapper/<name>
+ *	- /dev/loopN to the loop backing filename
+ *	- empty path (NULL) to 'none'
+ *
+ * Returns: new allocated string with path, result has to be always deallocated
+ *          by free().
+ */
+char *mnt_pretty_path(const char *path, struct libmnt_cache *cache)
+{
+	char *pretty = mnt_resolve_path(path, cache);
+
+	if (!pretty)
+		return strdup("none");
+
+	/* users assume backing file name rather than /dev/loopN in
+	 * output if the device has been initialized by mount(8).
+	 */
+	if (strncmp(pretty, "/dev/loop", 9) == 0) {
+		struct loopdev_cxt lc;
+
+		loopcxt_init(&lc, 0);
+		loopcxt_set_device(&lc, pretty);
+
+		if (loopcxt_is_autoclear(&lc)) {
+			char *tmp = loopcxt_get_backing_file(&lc);
+			if (tmp) {
+				if (!cache)
+					free(pretty);	/* not cached, deallocate */
+				return tmp;		/* return backing file */
+			}
+		}
+		loopcxt_deinit(&lc);
+
+	}
+
+	/* don't return pointer to the cache, allocate a new string */
+	return cache ? strdup(pretty) : pretty;
+}
+
 /**
  * mnt_resolve_tag:
  * @token: tag name

libmount/src/libmount.h.in

@@ -148,6 +148,8 @@ extern char *mnt_resolve_tag(const char *token, const char *value,
 			     struct libmnt_cache *cache);
 extern char *mnt_resolve_spec(const char *spec, struct libmnt_cache *cache);
 
+extern char *mnt_pretty_path(const char *path, struct libmnt_cache *cache);
+
 /* optstr.c */
 extern int mnt_optstr_next_option(char **optstr, char **name, size_t *namesz,
 				char **value, size_t *valuesz);

libmount/src/libmount.sym

@@ -157,6 +157,7 @@ global:
 	mnt_optstr_remove_option;
 	mnt_optstr_set_option;
 	mnt_parse_version_string;
+	mnt_pretty_path;
 	mnt_reset_context;
 	mnt_reset_fs;
 	mnt_reset_iter;