Network based PR service

The network based PR service is a missing building block in the current poky build system to enable the "basichash" BB_SIGNATURE_HANDLER for shared state. By having the network based PR service and the "basichash" signature handler, the poky users can modify the recipes without worrying about bumping the PR value manually in the recipes. The build system will automatically detect the change, rebuild only the necessary pieces of work, and have the changes reflected in the 'package revision' field value of the final package feed. (See for the details about shared state and BB_SIGNATURE_HANDLER).
The basic scenario of using the network based PR service is that the client asks the server for a value (i.e. PRAUTO) by including the task checksum as part of the search index. The server returns the stored value if the search index is found in its backend database. If the search index is notfound, the server bumps the value, stores the new index into the database, and returns the incremented value to the client. The client then includes the PRAUTO as part of the 'package revision' field value in the created package feed. The service is network based so a central database can be used to provide multiple build clients with the same value for the same index.
The base patches enabling the above scenario is in poky master branch now(disabled by default though). To use the PR service, the users should first start the service daemon by running the following commands:
 source ./oe-init-build-env
 bitbake-prserv --start
Then the users should make sure the PRHOST_ADDR and PRHOST_PORT are set correctly in conf/local.conf to match the configurations of the machine on which the service daemon is running, and then "bitbake" whatever they want to. (Please also make sure the BB_SIGNATURE_HANDLER is set to "basichash" instead of "basic").
By leaving the PRHOST_ADDR and PRHOST_PORT unset, the PR service mechanism is disabled and the poky runs exactly the same as it is now: the user needs to bump the PR manually in the recipe when changes are made .
There are still some pending tasks, however, to make the PR service mechanism adapted to various usage model and scenarios:
----1. Automatically launching service daemon (Source: Richard Purdie)
For many users, they may want to run the service daemon and the poky build system on the same machine most of the time. So they may not want to bother typing "bitbake-prserv --start". In this localhost case, it is desired to have poky launch the service daemon automatically at the start and stop it when the build finishes. Also it might need to run multiple daemons on the same machine at the same time.
Status: Done. To start launching local service automatically, set PRSERV_HOST to "localhost" and PRSERV_PORT to "0". See Bug #1126 for details.
----2. Use different PR service for different recipes (Source: Frans Meulenbroeks)
Some users want to use different PR service or disable the PR service per recipe basis. We need to support something like:
 PRSERV_HOST_pn-myprivaterecipe = "someotherhost"
 PRSERV_HOST_pn-myprivaterecipe = ""
Status: Done. See Bug #1126 for details.
----3. Support proxy (Source: Koen Kooi)
Since the PR service is a network based XMLRPC service, it would be nice to support the http proxy for the intranet build clients to contact the outside PR service.
----4. Write permission control (Source: Koen Kooi)
Some usage scenario might only allow certain authorized clients to have the write permission to bump the value in the PR service database, and other unauthorized clients can only "read" the value back from the database. The question is what if an unauthorized client who doesn't have the write permission asks for a value which is not stored in the database. Several ways are brought up to handle this kind of situation. One is to generate a warning on the client side and an error in the server log. Another way is to return a specific string as the PRAUTO(i.e. "custom") to indicate this situation. 
----5. database sync-up (Source: Richard Purdie)
If the user uses local PR services, we need to way to dump the local databases to the central server, so other users may use the central server to generate the same package feeds as those using the local servers.
Status: Done. See Bug #1556 for details.