Do not treat ~nosuchuser as an absolute file name

Derived from Ken Brown’s patch (Bug#36502#97).
* doc/lispref/files.texi (Relative File Names):
* etc/NEWS: Document this.
* src/fileio.c (user_homedir): New function.
(Fexpand_file_name, file_name_absolute_p): Use it.
(search_embedded_absfilename): Simplify via file_name_absolute_p.
* test/src/fileio-tests.el (fileio-tests--no-such-user): New test.

1 files changed, 9 insertions, 1 deletions

diff --git a/doc/lispref/files.texi b/doc/lispref/files.texi
index 0519f787dce..0ea8a4f0a19 100644
--- a/doc/lispref/files.texi
+++ b/doc/lispref/files.texi
@@ -2154,7 +2154,11 @@ backslash, or with a drive specification @samp{@var{x}:/}, where
 
 @defun file-name-absolute-p filename
 This function returns @code{t} if file @var{filename} is an absolute
-file name or begins with @samp{~}, @code{nil} otherwise.
+file name, @code{nil} otherwise.  A file name is considered to be
+absolute if its first component is @samp{~}, or is @samp{~@var{user}}
+where @var{user} is a valid login name.  In the following examples,
+assume that there is a user named @samp{rms} but no user named
+@samp{nosuchuser}.
 
 @example
 @group
@@ -2162,6 +2166,10 @@ file name or begins with @samp{~}, @code{nil} otherwise.
      @result{} t
 @end group
 @group
+(file-name-absolute-p "~nosuchuser/foo")
+     @result{} nil
+@end group
+@group
 (file-name-absolute-p "rms/foo")
      @result{} nil
 @end group