Allow setting user for all modules and capabilities per module

[corneliusclaussen]

When setting system_user="myuser" property under settings in a config file all modules are started as this user and its default gid.

After starting all modules the manager also switches to this user.

For each module, special Linux capabilities can be specified with e.g. capabilities: "cap_chown+ep"
in the module section.

The manager will need to start as root or sufficient capabilities to do that. If no user/capabilities are specified no change happens.

[corneliusclaussen]

one thing that we also need to fix: if one child cannot be forked (e.g. capabilities config wrong), manager exits but the already forked children are not killed

[a-w50]

that is, what PR_SET_PDEATHSIG in the SubProcess class is for, but somehow, this doesn't always work, as it seems

[hikinggrass]

that is, what PR_SET_PDEATHSIG in the SubProcess class is for, but somehow, this doesn't always work, as it seems

I think that's because that flag is cleared when executing setuid, so we will have to implement a workaround along these lines:
https://stackoverflow.com/a/42498370

[a-w50]

that is, what PR_SET_PDEATHSIG in the SubProcess class is for, but somehow, this doesn't always work, as it seems

I think that's because that flag is cleared when executing setuid, so we will have to implement a workaround along these lines: https://stackoverflow.com/a/42498370

Good find, I wasn't aware of that yet.

[golovasteek]

Hi in this PR quite nice refactoring is done, and all system-specific functions are encapsulated in a dedicated file.

Do you think system_linux.cpp would be more appropriate name for it, since the functionality there is rather linux specific and do not work unix-wide (macos, FreeBSD).

Do you think it makes sense to have one header system.hpp that declares all the system-dependent functions. And set of implementations e.g. here would be system_linux.cpp later system_macos.cpp could be added.

[corneliusclaussen]

PR_SET_PDEATHSIG is now fixed, it is set after the user change. Also support to switch back to root user in manager for restarting of modules when admin panel/controller is used was added. If admin panel is disabled (which should be done on real deployments) manager does not have the caps to switch back from unprivileged user.