[Octave-bug-tracker] [bug #59706] Avoid "canonicalize_file_name" on Windows

[Markus Mützel]

Update of bug #59706 (project octave):

              Item Group:         Feature Request => Performance            
                  Status:                    None => In Progress            
                 Summary: Add function that normalizes a file path => Avoid
"canonicalize_file_name" on Windows

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Follow-up Comment #8:

@Rik: I agree with most of your comment.
But `canonicalize_file_name` does more than just normalizing the file name and
checking whether it exists. Among other things it also resolves symbolic
links.
If the result of `canonicalize_file_name` is used as a key to identify a
physical file, that (or some other step this function does) might (or might
not) be relevant.
Gnulib's implementation for `canonicalize_file_name` on Windows is slow. We've
already seen that in the past. So it might be best to avoid it (on Windows)
where possible(!).

I didn't come around to analyzing the call stack of the slow code path here
until now. 
If I understand the call stack in comment #5 correctly, it is in `file_stat`
where `canonicalize_file_name` is called.
I added calling this function with the change here:
https://hg.savannah.gnu.org/hgweb/octave/rev/372f42f122bd

I don't remember now if using that function was strictly necessary or I just
didn't know (or didn't remember) that it was slow on Windows back then.

I'll try whether Octave's file_stat will still work on Windows with a change
similar to this:

diff -r 403a9f626ba2 liboctave/system/file-stat.cc
--- a/liboctave/system/file-stat.cc     Sun Dec 20 18:48:41 2020 +0100
+++ b/liboctave/system/file-stat.cc     Mon Dec 21 09:40:43 2020 +0100
@@ -194,12 +194,7 @@
           std::string full_file_name = sys::file_ops::tilde_expand
(file_name);
 
 #if defined (OCTAVE_USE_WINDOWS_API)
-          std::string canon_file_name
-            = sys::canonicalize_file_name (full_file_name);
-
-          // Might have failed for files with non-ASCII characters in name
-          if (! canon_file_name.empty ())
-            full_file_name = canon_file_name;
+          full_file_name = sys::env::make_absolute (full_file_name);
 
           // Remove trailing slashes
           while (full_file_name.length () > 1


If this works, that would have the additional benefit that it renders the
fallback for non-ASCII strings unnecessary.

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