path-util: add path_extend(), inspired by strextend(), but using path_join()

src/basic/path-util.c

@@ -548,18 +548,22 @@ bool path_equal_filename(const char *a, const char *b) {
         return path_equal(a_basename, b_basename);
 }
 
-char* path_join_internal(const char *first, ...) {
-        char *joined, *q;
+char* path_extend_internal(char **x, ...) {
+        size_t sz, old_sz;
+        char *q, *nx;
         const char *p;
         va_list ap;
         bool slash;
-        size_t sz;
 
         /* Joins all listed strings until the sentinel and places a "/" between them unless the strings end/begin
          * already with one so that it is unnecessary. Note that slashes which are already duplicate won't be
          * removed. The string returned is hence always equal to or longer than the sum of the lengths of each
          * individual string.
          *
+         * The first argument may be an already allocated string that is extended via realloc() if
+         * non-NULL. path_extend() and path_join() are macro wrappers around this function, making use of the
+         * first parameter to distinguish the two operations.
+         *
          * Note: any listed empty string is simply skipped. This can be useful for concatenating strings of which some
          * are optional.
          *
@@ -569,28 +573,39 @@ char* path_join_internal(const char *first, ...) {
          * path_join("foo/", "bar") → "foo/bar"
          * path_join("", "foo", "", "bar", "") → "foo/bar" */
 
-        sz = strlen_ptr(first);
-        va_start(ap, first);
-        while ((p = va_arg(ap, char*)) != POINTER_MAX)
-                if (!isempty(p))
-                        sz += 1 + strlen(p);
+        sz = old_sz = x ? strlen_ptr(*x) : 0;
+        va_start(ap, x);
+        while ((p = va_arg(ap, char*)) != POINTER_MAX) {
+                size_t add;
+
+                if (isempty(p))
+                        continue;
+
+                add = 1 + strlen(p);
+                if (sz > SIZE_MAX - add) /* overflow check */
+                        return NULL;
+
+                sz += add;
+        }
+
         va_end(ap);
 
-        joined = new(char, sz + 1);
-        if (!joined)
+        nx = realloc(x ? *x : NULL, GREEDY_ALLOC_ROUND_UP(sz+1));
+        if (!nx)
                 return NULL;
+        if (x)
+                *x = nx;
 
-        if (!isempty(first)) {
-                q = stpcpy(joined, first);
-                slash = endswith(first, "/");
-        } else {
-                /* Skip empty items */
-                joined[0] = 0;
-                q = joined;
+        if (old_sz > 0)
+                slash = nx[old_sz] == '/';
+        else {
+                nx[old_sz] = 0;
                 slash = true; /* no need to generate a slash anymore */
         }
 
-        va_start(ap, first);
+        q = nx + old_sz;
+
+        va_start(ap, x);
         while ((p = va_arg(ap, char*)) != POINTER_MAX) {
                 if (isempty(p))
                         continue;
@@ -603,7 +618,7 @@ char* path_join_internal(const char *first, ...) {
         }
         va_end(ap);
 
-        return joined;
+        return nx;
 }
 
 static int check_x_access(const char *path, int *ret_fd) {