Re: 'withfile' function implementation

[David Boyce]

First, I think there may be a bug. Did you intend that appending empty
data would always append a newline, such that the file grows by one
newline every time? It's consistent with the new documentation but not
with what you wrote above ("If the file does exist and it's opened for
append then the modtime on the file will NOT be updated."). I think it
makes more sense to append a trailing newline only when new data was
added, and the attached patch implements this (along with the rest)
and documents it.

On Tue, Jan 31, 2012 at 1:16 AM, Paul Smith <address@hidden> wrote:
> The problem, as I alluded to before, is that there's no way to get the
> exact semantics of updating a file without actually updating the file.
>
> In order to change the modtime on a file without changing the file you
> have to use utime()...

Yes, this is something you and I discussed a while ago in a different
context. What I'm thinking of doesn't involve utime and is illustrated
by the attached patch. In the special case where $(file) is appending
an empty string to an existing file, it reads one byte and writes it
back. This gives timestamp semantics identical to an actual write,
because it is an actual write. And it's safely atomic because the file
has the same contents whether the write succeeds or fails.

A proof-of-concept patch is attached along with a demo makefile.

-David Boyce

Makefile
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func_file-dsb.diff
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