bug#28156: Emacs quietly munges symlink contents

[Michael Albinus <michael.albinus@gmx.de>]

Paul Eggert <eggert@cs.ucla.edu> writes:

> On 08/22/2017 12:28 AM, Michael Albinus wrote:
>> If you want to have an absolute filename as LINKNAME, you typically call
>>
>> (make-symbolic-link (expand-file-name target) (expand-file-name linkname))
>>
>> In case of remote file names, (expand-file-name linkname) will always
>> return something like "/method:user@host:/path/to/linkname". I doubt,
>> that a user wants to see this literal string as symbolic link.
>
> There seems to be some confusion here, as Bug#28156 does not propose
> any change to how make-symbolic-link's 2nd argument works,
> (expand-file-name linkname) in this case. If the 2nd argument
> specifies a remote name, as in your example, then Tramp will take over
> and do whatever it wants with both arguments, so Bug#28156 is not
> proposing any change there (although perhaps some changes would be
> helpful for consistency, that's a different matter).

Sorry, I have confused the 1st and 2nd argument of make-symbolic-link.
Tramp's docstring is wrong, because it does the same error, for decades.
Will fix it.

However, it is Tramp's (almost undocumented) make-symbolic-link
implementation to handle TARGET as well, and to strip the remote part of
this. I wouldn't be surprised if existing packages will be broken if we
change this.

Coming to your examples, I believe the existing implementation gives you
almost all what is needed, with quoting. Using "~" in TARGET:

--8<---------------cut here---------------start------------->8---
M-x make-symbolic-link RET /:~eggert RET /tmp/foo RET

# ls -l /tmp/foo
lrwxrwxrwx 1 albinus albinus 21 Aug 24 13:19 /tmp/foo -> /home/albinus/~eggert
--8<---------------cut here---------------end--------------->8---

It is questionable whether "/:~eggert" shall be expanded to
"/home/albinus/~eggert". I agree with you, that it shouldn't, the target
shall be just  "~eggert".

Using ":" in TARGET would look like:

--8<---------------cut here---------------start------------->8---
M-x make-symbolic-link RET /:/x:y RET /tmp/foo RET

# ls -l /tmp/foo
lrwxrwxrwx 1 albinus albinus 4 Aug 24 13:23 /tmp/foo -> /x:y
--8<---------------cut here---------------end--------------->8---

For a TARGET which looks like a remote file name, I propose to keep the
current behaviour (stripping the remote part). It shall be equivalent to
just using the local file name of TARGET. So we would have:

--8<---------------cut here---------------start------------->8---
M-x make-symbolic-link RET /sudo::/:~eggert RET /sudo::/tmp/bar RET

# ls -l /tmp/bar
lrwxrwxrwx 1 root root 7 Aug 24 13:26 /tmp/bar -> ~eggert

;; This example does not work yet.
M-x make-symbolic-link RET /:~eggert RET /sudo::/tmp/bar RET

# ls -l /tmp/bar
lrwxrwxrwx 1 root root 7 Aug 24 13:26 /tmp/bar -> ~eggert
--8<---------------cut here---------------end--------------->8---



--8<---------------cut here---------------start------------->8---
M-x make-symbolic-link RET /sudo::/:/x:y RET /sudo::/tmp/bar RET

# ls -l /tmp/foo
lrwxrwxrwx 1 root root 4 Aug 24 13:30 /tmp/bar -> /x:y

;; This example does not work yet.
M-x make-symbolic-link RET /:/x:y RET /sudo::/tmp/bar RET

# ls -l /tmp/foo
lrwxrwxrwx 1 root root 4 Aug 24 13:30 /tmp/bar -> /x:y
--8<---------------cut here---------------end--------------->8---

By this, you don't need to handle "~" special in make-symbolic-link, as
your patch proposes. After all, it is just proper file name quoting, and
fixing the bug mentioned above.

Best regards, Michael.