[PATCH 1 of 5] util: add filestat class to detect ambiguity of file stat

[FUJIWARA Katsunori]

At Wed, 25 May 2016 21:40:53 +0900,
Yuya Nishihara wrote:
> 
> On Wed, 25 May 2016 01:45:09 +0900, FUJIWARA Katsunori wrote:
> > At Tue, 24 May 2016 21:58:11 +0900,
> > Yuya Nishihara wrote:
> > > > +    def isambig(self, old):
> > > > +        """Examine whether new (= self) stat is ambiguous against old one
> > > > +
> > > > +        "S[N]" below means stat of a file at N-th change:
> > > > +
> > > > +        - S[n-1].ctime  < S[n].ctime: can detect change of a file
> > > > +        - S[n-1].ctime == S[n].ctime
> > > > +          - S[n-1].ctime  < S[n].mtime: means natural advancing (*1)
> > > > +          - S[n-1].ctime == S[n].mtime: is ambiguous (*2)
> > > > +          - S[n-1].ctime  > S[n].mtime: never occurs naturally (don't care)
> > > > +        - S[n-1].ctime  > S[n].ctime: never occurs naturally (don't care)  
> > > 
> > > Does it work well on Windows? I'm not skeptical about it, I just question
> > > because ctime is platform dependent.
> > 
> > According to "Remarks" in the document for BY_HANDLE_FILE_INFORMATION
> > structure, from which os.lstat() of Python gets st_mtime/st_ctime
> > information via GetFileInformationByHandle() API:
> > 
> >   https://msdn.microsoft.com/en-us//library/windows/desktop/aa363788(v=vs.85).aspx
> >     For example, on a Windows FAT file system, create time has a
> >     resolution of 10 milliseconds, write time has a resolution of 2
> >     seconds, and access time has a resolution of 1 day (the access
> >     date).
> > 
> > I confirmed availability of ctime on FAT-formatted USB memory device.
> 
> My concern is ctime is "change" time on Unix, but it is "creation" time on
> Windows. So the characteristics would be very different on Windows. That said,
> I don't have any particular concern in mind.
> 

I updated ExactCacheValidationPlan wiki page with description about
difference of ctime meaning between platforms. Thanks !

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[FUJIWARA Katsunori]                             foozy at lares.dti.ne.jp