Minor file checking optimization: set_dir_cache_file.

[Ralf Wildenhues]

This patch has a bit nonobvious consequences.  The
dir_has_case_matching_file function in FileUtils.pm maintains a
directory entry cache, and when we install an aux file, automake
simply forgets the cache for that directory.  This optimization
simply updates the cache with the knowledge we have: new file,
no other changes.

The change can help to uncover code paths where files are changed
behind automake's back (for example from another thread), which I
consider a Good Thing.

FWIW, this is really a minor optimization with no visible speed
benefits.

Pushed to master.

Cheers,
Ralf

        Minor file checking optimization: set_dir_cache_file.
        * lib/Automake/FileUtils.pm (set_dir_cache_file): New function.
        * automake.in (require_file_internal): Instead of resetting the
        cache when a file has been installed, simply correct the cache
        using set_dir_cache_file.

diff --git a/automake.in b/automake.in
index fea0906..fabeb48 100755
--- a/automake.in
+++ b/automake.in
@@ -7387,7 +7387,7 @@ sub require_file_internal ($$$@)
                          $suppress = 0;
                          $trailer = "\n    error while copying";
                        }
-                     reset_dir_cache ($dir);
+                     set_dir_cache_file ($dir, $file);
                    }
 
                  if (! maybe_push_required_file (dirname ($fullfile),
diff --git a/lib/Automake/FileUtils.pm b/lib/Automake/FileUtils.pm
index 2200be0..facde3f 100644
--- a/lib/Automake/FileUtils.pm
+++ b/lib/Automake/FileUtils.pm
@@ -47,7 +47,9 @@ use vars qw (@ISA @EXPORT);
 @EXPORT = qw (&open_quote &contents
              &find_file &mtime
              &update_file &up_to_date_p
-             &xsystem &xsystem_hint &xqx &dir_has_case_matching_file 
&reset_dir_cache);
+             &xsystem &xsystem_hint &xqx
+             &dir_has_case_matching_file &reset_dir_cache
+             &set_dir_cache_file);
 
 
 =item C<open_quote ($file_name)>
@@ -416,6 +418,19 @@ sub reset_dir_cache ($)
   delete $_directory_cache{$_[0]};
 }
 
+=item C<set_dir_cache_file ($dirname, $file_name)>
+
+State that C<$dirname> contains C<$file_name> now.
+
+=cut
+
+sub set_dir_cache_file ($$)
+{
+  my ($dirname, $file_name) = @_;
+  $_directory_cache{$dirname}{$file_name} = 1
+    if exists $_directory_cache{$dirname};
+}
+
 1; # for require
 
 ### Setup "GNU" style for perl-mode and cperl-mode.