path-util: add path_extract_directory(), to match path_extract_filename()

These two together are a lot like dirname() + basename() but have the
benefit that they return clear errors when one passes a special case
path to them where the extraction doesn't make sense, i.e. "", "/",
"foo", "foo/" and so on.

Sooner or later we should probably port all our uses of
dirname()/basename() over to this, to catch these special cases more
safely.

src/basic/path-util.c

@@ -846,6 +846,48 @@ int path_extract_filename(const char *p, char **ret) {
         return 0;
 }
 
+int path_extract_directory(const char *p, char **ret) {
+        _cleanup_free_ char *a = NULL;
+        const char *c;
+
+        /* The inverse of path_extract_filename(), i.e. returns the directory path prefix. Returns:
+         *
+         * -EINVAL        → if the passed in path is not a valid path
+         * -EDESTADDRREQ  → if no directory was specified in the passed in path, i.e. only a filename was passed
+         * -EADDRNOTAVAIL → if the passed in parameter had no filename but did have a directory, i.e. the root dir itself was specified
+         * -ENOMEM        → no memory (surprise!)
+         *
+         * This function guarantees to return a fully valid path, i.e. one that passes path_is_valid().
+         */
+
+        if (!path_is_valid(p))
+                return -EINVAL;
+
+        /* Special case the root dir, because otherwise for an input of "///" last_path_component() returns
+         * the pointer to the last slash only, which might be seen as a valid path below. */
+        if (path_equal(p, "/"))
+                return -EADDRNOTAVAIL;
+
+        c = last_path_component(p);
+
+        /* Delete trailing slashes, but keep one */
+        while (c > p+1 && c[-1] == '/')
+                c--;
+
+        if (p == c) /* No path whatsoever? Then return a recognizable error */
+                return -EDESTADDRREQ;
+
+        a = strndup(p, c - p);
+        if (!a)
+                return -ENOMEM;
+
+        if (!path_is_valid(a))
+                return -EINVAL;
+
+        *ret = TAKE_PTR(a);
+        return 0;
+}
+
 bool filename_is_valid(const char *p) {
         const char *e;
 

src/basic/path-util.h

@@ -147,6 +147,7 @@ int fsck_exists(const char *fstype);
 char* dirname_malloc(const char *path);
 const char *last_path_component(const char *path);
 int path_extract_filename(const char *p, char **ret);
+int path_extract_directory(const char *p, char **ret);
 
 bool filename_is_valid(const char *p) _pure_;
 bool path_is_valid(const char *p) _pure_;