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Re: mingw and same-inode

From: Eric Blake
Subject: Re: mingw and same-inode
Date: Thu, 24 Sep 2009 06:18:43 -0600
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According to Jim Meyering on 9/24/2009 12:29 AM:
>   bool same = false;
>   ...
>       same = SAME_INODE (source_dir_stats, dest_dir_stats);
>       if (same < 0)
>         same = (identical_basenames
>                 && strcmp (source_basename, dest_basename) == 0);

As written, it wouldn't have worked, anyways.  The idea is that "./file"
and "/path/to/file" can be the same name, so we need more than a strcmp of
the dirnames.  Rather, it needs a strcmp of the canonicalized names.  And
since mingw doesn't have working canonicalize_file_name yet, it probably
even means using getcwd/chdir(dirname)/getcwd/chdir(cwd) twice to
determine the two names to compare.

> Your new test, "same < 0" will never be true when
> compiled on a system with proper "bool" support.

I missed that too.  All the more reason that I reverted the patch.

> Apparently, since your testing did not expose this flaw,
> no tested use of same_name requires the new semantics of SAME_INODE.

Correct - on mingw, I had tested the use of SAME_INODE, but not of
same_name (it made a difference as to whether at_func2 mistakenly assumed
two fds pointed to the same directory).  Fortunately, on every other
platform, SAME_INODE is still 0 or 1 (mingw is the only platform where
st_ino is always 0, and therefore SAME_INODE was returning 1 even when it
should have returned 0, leading to the tri-state proposal in the first
place).  I suppose that for mingw, I could also beef up SAME_INODE to
return 0 instead of -1 if other reliable fields in st_ino, such as st_ctim
or st_size, differ.  (Note that on mingw, st_ctim is mistakenly the file
creation time, corresponding to birthtime in BSD stat, rather than the
last metadata change time as per POSIX, but it still serves as a reliable
indicator of different files).

> If the tri-state test is required in only a few places,
> how about using a new macro, say SAME_INODE_TRISTATE,
> in just those few places?

There's two categories of clients of SAME_INODE - those that want to do
something if the files are provably the same (any where I changed to
SAME_INODE()==1), but which behave correctly if the files are different or
indeterminate; and those that want to do something if the files are
provably different, but which behave correctly if the files are the same
or indeterminate.  If SAME_INODE can be reached on mingw, then a decision
about what to do in the indeterminate case is necessary, even if we can
reduce the indeterminate result.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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