Significant problem with current expiration code - fix included

[Brad Fitzpatrick]

Ah:

+ * We keep the current time of day in a global variable that's updated by a
+ * timer event. This saves us a bunch of time() system calls (we really only
+ * need to get the time once a second, whereas there can be tens of thousands
+ * of requests a second) and allows us to use server-start-relative timestamps
+ * rather than absolute UNIX timestamps, a space savings on systems where
+ * sizeof(time_t) > sizeof(unsigned int).


So I think some comments in memcached.h, function names and variable
names should be updated.  As-is, the code doesn't quite read like
reality.



On Sun, 3 Sep 2006, Brad Fitzpatrick wrote:

> Hm, but realtime() should be a no-op on a unixtime value.  And since it
> converts a relative time TO an absolute unixtime value, this should be no
> harm:
>
>    abs_time = realtime(realtime(realtime(realtime(5))))
>
> The first one takes 5 to the unixtime, then the rest do nothing.
>
> At least this is always how it was in 1.1.x, which I just verified.  It
> seems the facebook changes made the behavior of realtime() change.  Off to
> read svn logs to figure out why....
>
>
> On Fri, 1 Sep 2006, Neulinger, Nathan wrote:
>
> > I started diagnosing a weird issue with current facebook branch, and
> > tracked down the expiration appeared to not be working in a predictable
> > manner.
> >
> > Turns out that realtime(expire) is being called twice. This results in
> > the following behavior:
> >
> > At Start+100 seconds, an expiration time of 15 seconds:
> > 	100 + 15 = 115, 100 + 115 = 215
> >
> > At Start+1000 seconds, ...
> > 	1000 + 15 = 1015, 1000 + 1015 = 2015...
> >
> > So basically, unless the value is set almost immediately after memcached
> > startup, it's expiration time will be huge compared to what was actually
> > requested.
> >
> > This should fix it... Looks like overlapping changes in 287 combined
> > with 288 broke it.
> >
> > Index: memcached.c
> > ===================================================================
> > --- memcached.c (revision 318)
> > +++ memcached.c (working copy)
> > @@ -814,7 +814,7 @@
> >          }
> >
> >          expire = realtime(expire);
> > -        it = item_alloc(key, flags, realtime(expire), len+2);
> > +        it = item_alloc(key, flags, expire, len+2);
> >
> >          if (it == 0) {
> >              if (! item_size_ok(key, flags, len + 2))
> >
> >
> >
> >
> >
> > ------------------------------------------------------------
> > Nathan Neulinger                       EMail:  nneul at umr.edu
> > University of Missouri - Rolla         Phone: (573) 341-6679
> > UMR Information Technology             Fax: (573) 341-4216
> >
> >
>
>