Do not munge contents of local symbolic links

This lets Emacs deal with arbitrary local symlinks without
mishandling their contents (Bug#28156).  For example,
(progn (shell-command "ln -fs '~' 'x'") (rename-file "x" "/tmp/x"))
now consistently creates a symbolic link from '/tmp/x' to '~'.
Formerly, it did that only if the working directory was on the
same filesystem as /tmp; otherwise, it expanded the '~' to
the user's home directory.
* lisp/dired.el (dired-get-filename): Use files--name-absolute-system-p
instead of rolling our own code.
* lisp/files.el (files--name-absolute-system-p): New function.
(file-truename, file-chase-links): Use it to avoid mishandling
symlink contents that begin with ~.
(copy-directory, move-file-to-trash):
Use concat rather than expand-file-name, to avoid mishandling
symlink contents that begin with ~.
* src/fileio.c (Fmake_symbolic_link): Do not expand leading "~" in the
target unless interactive.  Strip leading "/:" if interactive.
(emacs_readlinkat): Do not prepend "/:" to the link target if
it starts with "/" and contains ":" before NUL.
* test/src/fileio-tests.el (try-link): Rename from try-char,
and accept a string instead of a char.  All uses changed.
(fileio-tests--symlink-failure): Also test leading ~, and "/:",
to test the new behavior.

2 files changed, 13 insertions, 6 deletions

diff --git a/doc/emacs/files.texi b/doc/emacs/files.texi
index 9195bc47efe..fa1f9e53165 100644
--- a/doc/emacs/files.texi
+++ b/doc/emacs/files.texi
@@ -1611,8 +1611,12 @@ attempts to open file @var{new} will refer to whatever file is named
 @var{target} at the time the opening is done, or will get an error if
 the name @var{target} is nonexistent at that time.  This command does
 not expand the argument @var{target}, so that it allows you to specify
-a relative name as the target of the link.  On MS-Windows, this
-command works only on MS Windows Vista and later.  On remote systems,
+a relative name as the target of the link.  However, this command
+does expand leading @samp{~} in @var{target} so that you can easily
+specify home directories, and strips leading @samp{/:} so that you can
+specify relative names beginning with literal @samp{~} or @samp{/:}.
+@xref{Quoted File Names}.  On MS-Windows, this command works only on
+MS Windows Vista and later.  When @var{new} is remote,
 it works depending on the system type.
 
 @node Misc File Ops
diff --git a/doc/lispref/files.texi b/doc/lispref/files.texi
index f701d683703..06466c9bba8 100644
--- a/doc/lispref/files.texi
+++ b/doc/lispref/files.texi
@@ -1726,14 +1726,17 @@ default file permissions (see @code{set-default-file-modes} below), if
 SELinux context are not copied over in either case.
 @end deffn
 
-@deffn Command make-symbolic-link filename newname &optional ok-if-already-exists
+@deffn Command make-symbolic-link target newname &optional ok-if-already-exists
 @pindex ln
 @kindex file-already-exists
-This command makes a symbolic link to @var{filename}, named
+This command makes a symbolic link to @var{target}, named
 @var{newname}.  This is like the shell command @samp{ln -s
-@var{filename} @var{newname}}.  The @var{filename} argument
+@var{target} @var{newname}}.  The @var{target} argument
 is treated only as a string; it need not name an existing file.
-If @var{filename} is a relative file name, the resulting symbolic link
+If @var{ok-if-already-exists} is an integer, indicating interactive
+use, then leading @samp{~} is expanded and leading @samp{/:} is
+stripped in the @var{target} string.
+If @var{target} is a relative file name, the resulting symbolic link
 is interpreted relative to the directory containing the symbolic link.
 @xref{Relative File Names}.