Normally bundles configurations are installed asynchronously after a bundle is saved to the JCR during package installation (vault packages). This is due to the backgroundThread that the OsgiInstallerImpl is using. Since the introduction of the pauseInstallation signal node via
SLING-3747, the installation of bundles and configurations is even forced to after the completed vault package installation. This behaviour also means that the package dependency can be declared, but it is not effective on "contained bundle level" but only on JCR content level. The following is not possible today:
Complete Package with sub packages:
- Package A (containing Bundle A)
- Package B (containing Bundle B) with dependency to Bundle A
If it is just a OSGi package dependency from Bundle B to Bundle A the installation order does not matter (the OSGi framework will sort it out). However if the content of Package B has a dependency to bundle A (think custom oak restrictions  or install hooks referenced) there will be an error.
To allow for those cases and to avoid manual steps (not providing a complete package often means exactly that), it would be good to have a means to install contained bundles of a package synchronously. This can be achieved by a fairly simple install hook that "pre-installs" the contained InstallableResources with the correct digest to the OsgiInstaller. The subsequent call of sling-org-apache-sling-installer-provider-jcr will just update the already installed resource again with the same digest (which will not cause any action in the system).
Obviously this mechanism should only be used where needed and only for configurations and bundles that are safe to install (bundles/configs that will never restart any of the base services). Using the install hook makes the mechanism "opt-in".