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Determine the size of a block device off Linux - #413
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Stat reports zero for a device, so every non-Linux platform returned zero for one. `desync make` on a block device produced an index for an empty blob rather than failing. Linux reads the size with an ioctl. Seeking to the end is what's portable across the BSDs and macOS, and reports the same value the ioctl does where both are available. A device with no size to report, a character device such as /dev/zero, is now an error rather than a silent zero.
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GetFileSizehad aTODOwhere the device case should be, so on every platform but Linux it fell through toinfo.Size(), which is zero for a device.desync makeagainst a block device produced an index for an empty blob and reported no problem.Linux reads the size with
BLKGETSIZE64. Seeking to the end is the portable equivalent across the BSDs and macOS, and it reports the same value the ioctl does. A device with no size to report — a character device like/dev/zero— now returns an error rather than the zero, which is the same outcome Linux already produces there.Verification. The shipped function was compiled standalone and run against a real block device, since Linux builds the ioctl file instead:
8589934592 is exactly what
BLKGETSIZE64returns for that device, againstStat().Size()of 0.The tests assert behaviour both implementations share, so they cover the ioctl path on Linux and the new path on the four BSDs in CI, including that
/dev/zerois an error on each. What no test covers is a real disk on a BSD or macOS, which needs a device the runner can open; the equivalence above and lseek being POSIX are what that rests on.