fish: edit: write to the real file name

When saving, resolve the path of the file being edited and use that as
real target to write to. Otherwise, if the file name is a symlink then
it will be replaced by a regular file with the new content, leaving the
old file untouched.

Extend test-edit.sh to check for this situation.

fish/file-edit.c

@@ -216,11 +216,24 @@ do_download (guestfs_h *g, const char *filename, char **tempfile)
 }
 
 static int
-do_upload (guestfs_h *g, const char *filename, const char *tempfile,
+do_upload (guestfs_h *g, const char *fn, const char *tempfile,
            const char *backup_extension)
 {
+  CLEANUP_FREE char *filename = NULL;
   CLEANUP_FREE char *newname = NULL;
 
+  /* Resolve the file name and write to the actual target, since
+   * that is the file it was opened earlier; otherwise, if it is
+   * a symlink it will be overwritten by a regular file with the
+   * new content.
+   *
+   * Theoretically realpath should work, but just check again
+   * to be safe.
+   */
+  filename = guestfs_realpath (g, fn);
+  if (filename == NULL)
+    return -1;
+
   /* Upload to a new file in the same directory, so if it fails we
    * don't end up with a partially written file.  Give the new file
    * a completely random name so we have only a tiny chance of

fish/test-edit.sh

@@ -43,6 +43,12 @@ cat /file.txt
 stat /file.txt | grep mode:
 stat /file.txt | grep uid:
 stat /file.txt | grep gid:
+echo ==========
+write /file-2.txt "symlink test\n"
+ln-s /file-2.txt /symlink-2.txt
+edit /symlink-2.txt
+is-symlink /symlink-2.txt
+cat /symlink-2.txt
 EOF
 )
 
@@ -51,7 +57,11 @@ second line of text
 
 mode: 33152
 uid: 10
-gid: 11" ]; then
+gid: 11
+==========
+true
+symlink test
+second line of text" ]; then
     echo "$0: error: output of guestfish after edit command did not match expected output"
     echo "$output"
     exit 1