[pacman-dev] updpkgsums read/write collision

[Dave Reisner]

On Sun, Dec 14, 2014 at 05:13:11PM +0100, David Macek wrote:
> Hi all.
> 
> I have noticed a problem with updpkgsums on Windows when it's run on a subst'd drive (created with `subst` or using an entry in `DOS Devices` registry key). The error message I see is following:
> 
> /usr/bin/updpkgsums: line 85: PKGBUILD: Permission denied
> 
> The PKGBUILD is then gone. 

I don't really understand this. You're saying that the PKGBUILD itself
isn't subst'd, but the underlying device that it resides on? That sounds
an awful lot like a deficiency in the underlying implementation (or some
problem with cygwin's handling of such drives).

> I don't fully understand what's going on on lines 78-97 with the input/output redirection and deleting

The redirection on line 97 keeps an open FD to the original file,
allowing us to delete it without actually dropping the file's link count
to 0. We can then modify the stream through awk, and write back to the
original file. Doing this directly, i.e.

  awk PROGRAM file >file

...doesn't work, because you're truncating the file before reading it.

> but I assume this can be fixed by
> 1) either enhancing the routine for dereferencing symlinks (lines 54-62) to recognize subst'd drives,

Not having used Windows as a day-to-day OS in last 5-6 years, I have no
idea how to do this, nor can I easily test it. Patches welcome for such
an approach.

> 2) or by modifying the script to first rename the original $buildfile, and delete it after finishing the new $buildfile.

So, the implemenation is the way it is right now because I didn't want
to play games with temporary files (and the naming problems that come
along with it). I already conceded to making a tempdir for the purposes
of keeping the build directory clean, so I suppose I can be persuaded to
use a tempfile for the newly rewritten PKGBUILD.

Does this patch fix your problem?

https://paste.xinu.at/oKQMN/

> For the time being, I worked around the problem by modifying the script to first read $buildfile into a variable, then delete it and then pass the variable contents to awk to write to $buildfile.
> 
> My question is following: can the script be changed in order to avoid this issue?
> 
> Ref: https://github.com/Alexpux/MSYS2-packages/issues/143