The incomplete result of mapper GetSubTree/Paths

[Lei Yu]

This email is to describe an issue in mapper that the
GetSubTree/GetSubTreePaths could return incomplete results when it's
doing introspect.

Steps to reproduce the issue:
1. Configure phosphor-logging to get max 1000 entries. (with
-Derror_info_cap=1000 meson option)
2. Create 1000 logging entries.
3. Call GetSubTreePaths and make sure it gets the correct 1000 entries:
    # busctl call "xyz.openbmc_project.ObjectMapper"
"/xyz/openbmc_project/object_mapper"
"xyz.openbmc_project.ObjectMapper" GetSubTreePaths sias
/xyz/openbmc_project/logging/entry 0 1
xyz.openbmc_project.Logging.Entry | awk '{print $2;}'
    1000
4. Restart logging service
    # systemctl restart xyz.openbmc_project.Logging.service
5. After the service is restarted, call GetSubTreePaths for multiple
times in the short time (e.g. within 10 seconds)
    # busctl call "xyz.openbmc_project.ObjectMapper"
"/xyz/openbmc_project/object_mapper"
"xyz.openbmc_project.ObjectMapper" GetSubTreePaths sias
/xyz/openbmc_project/loggiz.openbmc_project.Logging.Entry | awk
'{print $2;}'
    47
    # busctl call "xyz.openbmc_project.ObjectMapper"
"/xyz/openbmc_project/object_mapper"
"xyz.openbmc_project.ObjectMapper" GetSubTreePaths sias
/xyz/openbmc_project/loggiz.openbmc_project.Logging.Entry | awk
'{print $2;}'
    375
    # busctl call "xyz.openbmc_project.ObjectMapper"
"/xyz/openbmc_project/object_mapper"
"xyz.openbmc_project.ObjectMapper" GetSubTreePaths sias
/xyz/openbmc_project/loggiz.openbmc_project.Logging.Entry | awk
'{print $2;}'
    851
    # busctl call "xyz.openbmc_project.ObjectMapper"
"/xyz/openbmc_project/object_mapper"
"xyz.openbmc_project.ObjectMapper" GetSubTreePaths sias
/xyz/openbmc_project/loggiz.openbmc_project.Logging.Entry | awk
'{print $2;}'
    1000

We can see that the result of GetSubTreePaths is increasing until it gets 1000.
This actually happens when mapper is doing introspect of the logging
service, and getting more and more objects.

The above "incomplete" behavior will impact the logic that depends on
the result of GetSubTreePaths.
E.g. in ipmid, the "cached SEL" feature depends on the reliable result
of GetSubTreePath, to get the number of current logging entries. If
it's not correct, ipmid will not know the "missed" entries.

The question is, should we make sure mapper returns the "stable"
result in the above case?
When it's doing introspect of a service (e.g. nameOwnerChanged), it
could throw if the service is not fully introspected, and only return
the "correct" result after the service is fully introspected.

If mapper could not guarantee the stable result, the service calling
mapper will have to add more complex logic to make sure it gets the
"full and correct" result.

-- 
BRs,
Lei YU