Help: how to obtain a correct file path from a possible absolute or relative path considering remote environment

[Fan Yang]

Hi all,

Recently I am trying to write a package but get stuck on handling remote
file path correctly.  I wonder if it is OK to ask here.

What I want seems simple: I want to `find-file' a file based on a path
name obtained from the output of a command line tool.  However,

1. This command line tool may output relative or absolute path.
2. I want my package work in both local and remote environment.  That
   is, the command line tool may be run on a remote machine based on the
   user's current `default-directory' using `process-file'.  Thus I need
   to somehow "transform" the output path into a path with correct TRAMP
   prefix.

Here is an example.  Suppose,

- I am at buffer "/scp:fan@vm00:/home/fan/dir1/dir2/a.c".
- The output of the command line tool can be
  - a relative path, "b.c", or
  - an absolute path, "/home/fan/dir1/dir2/b.c".

Then I want to use `find-file' to open the "b.c" file.

Attempt 1:
,----
| (find-file (concat default-directory path))
| ;; for a relative path, it produces "/scp:fan@vm00:/home/fan/dir1/dir2/b.c", 
correct.
| ;; for an absolute path, it produces 
"/scp:fan@vm00:/home/fan/dir1/dir2//home/fan/dir1/dir2/virtio_pm_balloon.c", 
wrong.
`----

Attempt 2:
,----
| (find-file (concat (file-remote-p default-directory) path))
| ;; for a relative path, it produces "/scp:fan@vm00:b.c, wrong.
| ;; for an absolute path, it produces "/scp:fan@vm00:/home/fan/dir1/dir2/b.c", 
correct.
`----

Attempt 3:
,----
| (if (file-name-absolute-p path)
|     (find-file (concat (file-remote-p default-directory) path))
|   (find-file (concat default-directory path)))
`----

Attempt 3 seems correct, but here `file-name-absolute-p' uses the local
rule to judge a remote path.  Is it still correct if the remote machine
is Windows, the local machine is Linux, and the output path is like
"E:/xxx/xxx"?  This is an absolute path for the remote machine, but
`file-name-absolute-p' on the local machine will treat it incorrectly.

How do I "transform" the path correctly, and elegantly?  Thanks in
advance for any help you are able to provide.

Fan