Detect "empty" socket credentials on Linux
Linux uses struct ucred to pass over socket credentials. Historically this has always worked in recievemsg, if SO_PASSCRED was set on the socket, even if the remote side didn't pass any credits. But this change broke that: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=16e5726269611b71c930054ffe9b858c1cea88eb;hp=a9e9fd7182332d0cf5f3e601df3e71dd431b70d7 However, it doesn't actually fail getting the credentials, it just returns an "empty" one, as initialized by cred_to_ucred() at: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=net/core/sock.c;h=b29ab61b029cf7f76fda992ecfcb8dcaa06b0483;#l756 So, we detect this and fail the credentials reading. This actually happened in real life with gdbus acting as a server, as gdbus expected an ucred but libdbus didn't send one.
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