improve setgroups error message and inline documentation #129
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Problem: the code for setting supplemental groups under Linux seems fragile at first glance. It was carefully designed, but the code comments have not aged well. Also, when the runtime check fails, the warning message is not very helpful. Improve the warning message. Improve the code comments.
Problem: on failure, the runtime test of SYS_setgroups replaces diod's supplementary groups with the test ones. Pass size=0 to getgroups() when testing the group count.
Problem: the diod server checks at runtime whether or not its supplemenatary group switching strategy works with the kernel hosting the server, but we also have a unit test that does the same on the build system. It's a little pointless to test the build system. Drop that test.
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The diod server, when mounted in
access=usermode, uses a thread pool to service the requests from different users. Setting the user's credentials in each thread is tricky. The scheme for setting supplemental groups in this mode is fragile, but includes a runtime test that ensures its tenuous assumption still holds. When the test fails, the error message is not super helpful. Furthermore, when auditing this code, I found it to look a little scary at first and it took me a while to understand it. The comments were outdated and insufficient.So this improves both the error message and the comments, plus tidies up some related stuff.