Werk #8124: Avoid configuration changes to be become active without activate changes

The Check_MK keepalive mode, which is used for the Check_MK helpers and for the RRD creation helper, now uses a new method for reading the configuration from main.mk, the other *.mk files and the autochecks. This new method has two advantages:

• Changes done to *.mk files can no longer become active unless they are activated. This even holds for core or server restarts and for crashed check helpers
• Check helpers and RRD helper do not any longer need to restart themselves after a configuration change but simply reload the changed configuration. This has a muss smaller CPU footprint when activating changes.
• Autochecks files are now only read for the host that is currently being delt with. This makes the internal check table much smaller and speeds configuration reloads.

The gross result is that configuration changes are now really automic and also faster.

Implementation details

Whenever you create the configuration for the core (options -B, -O, -R or -U or Activate Changes via WATO) then the complete configuration (the result from parsing main.mk and friends) is written to the file var/check_mk/core/config.mk. The core then triggers the check helpers for configuration reload. The helpers simply re-read that file and are immediately up-to-date. At the same time copies (hard links) of the autochecks files in var/check_mk/autochecks are being created in var/check_mk/core/autochecks. These files are used during the monitoring.

Note: The Discovery check (formerly known as Inventory check) does not use the activated configuration but still that one that is modified via WATO. That way after doing a service discovery the discovery check will immediately be happy - without a core restart. This is just as it used to be.

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