When following a link in the help page for a function or variable to open the source code file, and the file is already open, Emacs moves point in that buffer, but it doesn't push the mark first, so the user loses his previous place. It should push the mark first.

The find-tag command does push the mark, but it's too trigger-happy: it pushes the mark even if the file was _not_ already open. Since the mark ring is for the user's use to keep track of where he was in the buffer, the extraneous push leaves the user later wondering whether that file had already been open for some reason, or whether Emacs was just being sloppy. This affects me when I look at several xref targets in the same files; if Emacs is fixed to avoid the extraneous push, then I can just try to go to the previous mark, and if it says there's no mark set, I immediately know that I looked at no other targets in the current buffer.

The attached patch solves both issues. It pushes the mark for both help xref and find-tag if the buffer is already open, and it avoids pushing otherwise. (Applies to 24.4; not updated for trunk yet.)

I posted an earlier version of this patch in 2012, but it was poorly designed, incomplete, and buggy, and was therefore ignored.

Maybe this updated version will be considered poorly designed too, because it still uses a dynamic variable as an implicit return value. But if not this way, then what other practical solution is there?