tmpfiles: try to handle read-only file systems gracefully

On read-only filesystems trying to create the target will not fail with
EEXIST but with EROFS. Handle EROFS by checking if the target already
exists, and if empty when truncating.
This avoids reporting errors if tmpfiles doesn't actually needs to do
anything.

[zj: revert condition to whitelist rather then blacklisting, and add goto
to avoid stat'ting twice.]

src/tmpfiles/tmpfiles.c

@@ -984,8 +984,12 @@ static int write_one_file(Item *i, const char *path) {
                         return 0;
                 }
 
-                log_error_errno(errno, "Failed to create file %s: %m", path);
-                return -errno;
+                r = -errno;
+                if (!i->argument && errno == EROFS && stat(path, &st) == 0 &&
+                    (i->type == CREATE_FILE || st.st_size == 0))
+                        goto check_mode;
+
+                return log_error_errno(r, "Failed to create file %s: %m", path);
         }
 
         if (i->argument) {
@@ -1012,6 +1016,7 @@ static int write_one_file(Item *i, const char *path) {
         if (stat(path, &st) < 0)
                 return log_error_errno(errno, "stat(%s) failed: %m", path);
 
+ check_mode:
         if (!S_ISREG(st.st_mode)) {
                 log_error("%s is not a file.", path);
                 return -EEXIST;
@@ -1154,6 +1159,10 @@ static int create_item(Item *i) {
 
                 log_debug("Copying tree \"%s\" to \"%s\".", resolved, i->path);
                 r = copy_tree(resolved, i->path, false);
+
+                if (r == -EROFS && stat(i->path, &st) == 0)
+                        r = -EEXIST;
+
                 if (r < 0) {
                         struct stat a, b;