gpt-auto-generator: Ignore zero sized block devices

[yaneti]

e.g usb connected card readers with no card in them

Otherwise I get a little bit of ugliness on startup here with Fedora 37 like:

systemd-gpt-auto-generator[504]: Failed to dissect: Permission denied
(sd-executor)[497]: /usr/lib/systemd/system-generators/systemd-gpt-auto-generator failed with exit status 1.

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[keszybz]

read_one_line_file() returns the number of characters read. If the file is empty, this will be 0. The conditional, as written, cannot be true.

Did this get tested at all?

[poettering]

i don't get this.

we dissect the root disk from which the root fs is mounted. A zero-sized block device would not be backing your root fs, hence this PR looks wrong to me.

What made you think this as anything to do with an usb card reader?

Can you please provide logs of the issue you saw?

[yaneti]

I think I know how I got this wrong:

• systemd-251 in f37 does give off the errors mentioned above so I simply assumed it has something to do with the empty card reader
• then produced the wrong patch but applied it to systemd-253 which I then tested on f37 to convince myself if fixes the issue, while apparently the issue was already fixed somewhere between 251 and 253

Sorry for the noise