libs: add Stat module in Unixext to handle special device IDs #6812
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psafont
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Dec 24, 2025
| Some {major; minor} | ||
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| let encode_st_dev {major; minor} = | ||
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the 0 |^ is added to make ocamlformat behave and keep the first major term parenthesised and aligned. The same has been done in in decode
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last-genius
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Probably worth explaining the motivation for this - the fact that we saw real-world minors > 256 failing to be handled by xapi properly, as it is unable to figure out the format of the backing file, and has to handle the VDI as raw instead.
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These device IDs are handled in an ad-hoc way, and none of the implementations follow the actual linux behaviour. The glibc-provided macros are used for the implementation, with a pure-ocaml implementation that was useful to compare against while implementing, which has been kept in the tests, to detect any behavioural changes. Because Unix.stat returns an `int` instead of an int64, the code does not support all possible values for major supported by glibc. This shouldn't be an issue in Linux since the device value is 32-bit wide. Signed-off-by: Pau Ruiz Safont <pau.safont@vates.tech>
There were 5 ad-hoc implementations, they were all getting wrong results for the minor because only the lower 8 bits were accounted for. Use the unixext one which handles up to 32-bit-wide minors correctly. Signed-off-by: Pau Ruiz Safont <pau.safont@vates.tech>
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Previously, when a failure happened while getting a base vhd, a non-descriptive error was printed. Because there can be many reasons that can cause this code to fail, model the possible errors and print them when needed. Signed-off-by: Pau Ruiz Safont <pau.safont@vates.tech>
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These device IDs are handled in an ad-hoc way, and none of the implementations follow the actual linux behaviour, replace all of them with an in imeplementation that uses the glibc macros to handle these.
Also change vhd_wrapper_tool to print better error messages when there's an error when getting the base VHD.
Reproduction steps:
for i in $(seq 1 512) ; do tap-ctl allocate; done. This forces the minor device ID to be higher than 255 for the local VBD created.The migration succeeds when both hosts in the pool have the changes in this PR running.