testsuite: Fix mkdir_p corner cases

- Fix infinite loop when path is relative
 - Fix not considering EEXIST as a success
 - General refactor to mkdir_p so it never calls mkdir for an existing
   dir (given no one creates it from outside)

testsuite/mkdir.c

@@ -23,47 +23,60 @@
 #include "mkdir.h"
 #include "testsuite.h"
 
+static inline int is_dir(const char *path)
+{
+	struct stat st;
+
+	if (stat(path, &st) >= 0)
+		return S_ISDIR(st.st_mode);
+
+	return -errno;
+}
+
 TS_EXPORT int mkdir_p(const char *path, mode_t mode)
 {
 	char *start = strdupa(path);
 	int len = strlen(path);
 	char *end = start + len;
-	struct stat st;
 
 	/*
 	 * scan backwards, replacing '/' with '\0' while the component doesn't
 	 * exist
 	 */
 	for (;;) {
-		if (stat(start, &st) >= 0) {
-			if (S_ISDIR(st.st_mode))
-				break;
+		int r = is_dir(start);
+		if (r > 0) {
+			end += strlen(end);
+
+			if (end == start + len)
+				return 0;
+
+			/* end != start, since it would be caught on the first
+			 * iteration */
+			*end = '/';
+			break;
+		} else if (r == 0)
 			return -ENOTDIR;
-		}
 
-		/* Find the next component, backwards, discarding extra '/'*/
-		for (; end != start && *end != '/'; end--)
-			;
-
-		for (; end != start - 1 && *end == '/'; end--)
-			;
-
-		end++;
 		if (end == start)
 			break;
 
 		*end = '\0';
+
+		/* Find the next component, backwards, discarding extra '/'*/
+		while (end > start && *end != '/')
+			end--;
+
+		while (end > start && *(end - 1) == '/')
+			end--;
 	}
 
-	if (end == start + len)
-		return 0;
-
 	for (; end < start + len;) {
-		*end = '/';
-		end += strlen(end);
-
-		if (mkdir(start, mode) < 0)
+		if (mkdir(start, mode) < 0 && errno != EEXIST)
 			return -errno;
+
+		end += strlen(end);
+		*end = '/';
 	}
 
 	return 0;