[rrd-developers] rrd_fetch seek issue

[Daniel.Pocock at barclayscapital.com]

> -----Original Message-----
> From: Tobias Oetiker [mailto:tobi at oetiker.ch] 
> Sent: 27 November 2008 10:57
> To: Pocock, Daniel: IT (LDN)
> Cc: rrd-developers at lists.oetiker.ch
> Subject: Re: [rrd-developers] rrd_fetch seek issue
> 
> Hi Daniel,
> 
> (I have not yet looked at the code).
> 
> I just tried to reproduce the problem here and it did not work.
> 
> I guess you are hitting some sort of a corner case ... can 
> you create a little demo script ...


I encountered the problem in my patched version of trunk, so I decided
to explore it with an unpatched version of 1.2.15, as that is an
unpatched version I already have on a box.

My patched code is quite unforgiving - there is an assert() for bad
seeks.  That's how I discovered the problem.

Testing with 1.2.15, there is also a seek beyond the end of the file,
and NaN data is returned by rrd_fetch.

NOW=`date +%s`
START_T=`expr $NOW - 3600`
TS2=`expr $START_T + 10`
TS3=`expr $START_T + 20`

rrdtool create /tmp/test1.rrd \
    --start ${START_T} \
    --step 10 \
    DS:ds0:COUNTER:900:U:U \
    RRA:AVERAGE:0.8:1:180

$ ls -l /tmp/test1.rrd 
-rw-r--r--  1 daniel daniel 1984 Nov 27 16:28 /tmp/test1.rrd

# notice the file size is 1984 bytes, consistent with 180 rows in the
RRA
# put in a couple of updates with old time stamps:

rrdtool update /tmp/test1.rrd ${TS2}:1000
rrdtool update /tmp/test1.rrd ${TS3}:2000


# Notice that last_update is over 1 hour ago
# There can only be data for 30 minutes (180 rows)
$ rrdtool info /tmp/test1.rrd |grep last_upd
last_update = 1227799751
$ date +%s
1227804688

# Now I inspect the fetch code, notice how the seek is for
# 544 + (569 * 1 * 8) = 5096
# which is beyond the end of the file

$ gdb rrdtool

set args fetch /tmp/test1.rrd AVERAGE -r 10 -s -60s 

Breakpoint 3, rrd_fetch_fn (
    filename=0xfffffe76 <Address 0xfffffe76 out of bounds>,
cf_idx=CF_AVERAGE, 
    start=0xbffb8868, end=0xbffb886c, step=0xbffb8870,
ds_cnt=0xbffb8874, 
    ds_namv=0xbffb8878, data=0xbffb887c) at rrd_fetch.c:346
346         if (start_offset <= 0)
(gdb) next
338         end_offset = (long)(rra_end_time - *end ) / (long)*step; 
(gdb) next
346         if (start_offset <= 0)
(gdb) 
349             rra_pointer =
rrd.rra_ptr[chosen_rra].cur_row+1+start_offset;
(gdb) 
351         if(fseek(in_file,(rra_base 
(gdb) print rra_base
$1 = 544
(gdb) print rra_pointer
$2 = 569
(gdb) print *ds_cnt
$3 = 1
(gdb) next
372         for (i=start_offset;
(gdb) print ftell(in_file)
$4 = 5096
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