Privatae
60 lines
2.3 KiB
Go
60 lines
2.3 KiB
Go
//go:build !windows
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package main
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// The two platform questions the disk map asks, answered for unix: what device
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// is this on, and what counts as a volume root.
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import (
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"io/fs"
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"os"
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"syscall"
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)
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// dmDeviceOf is the device id of a path, and whether the platform would say.
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//
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// The second return is not decoration. A walk that cannot identify a device
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// must NOT fall back to "same device as everything" — that would silently
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// re-enable mount crossing, which is how a walk of / ends up inside an NFS
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// share and takes four minutes. dmScan reads this once per root and skips the
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// crossing test entirely when it is false, so the behaviour is "no boundary
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// check" rather than "a boundary check that always passes".
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func dmDeviceOf(path string) (uint64, bool) {
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fi, err := os.Lstat(path)
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if err != nil {
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return 0, false
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}
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return dmDeviceOfInfo(fi)
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}
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// dmDeviceOfInfo reads the device out of a FileInfo already in hand, so the
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// walk does not pay for a second lstat per directory.
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//
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// The uint64 conversion is load-bearing across the two unixes this ships to:
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// syscall.Stat_t.Dev is uint64 on linux and int32 on darwin, so a typed
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// comparison would not compile on both. Widening both to uint64 compares the
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// same bits either way — the value is only ever tested for equality against
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// another device from the same platform, never interpreted.
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func dmDeviceOfInfo(fi fs.FileInfo) (uint64, bool) {
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st, ok := fi.Sys().(*syscall.Stat_t)
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if !ok || st == nil {
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return 0, false
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}
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return uint64(st.Dev), true
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}
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// dmVolumeRoots on unix is the filesystem root, and there is exactly one.
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//
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// The rejected alternative was reading /proc/self/mountinfo (linux) or running
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// `mount` (darwin) to add every local filesystem as its own root. It is the
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// better answer eventually and it is not the right answer for this unit: it is
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// a fourth platform-specific parser, it needs a filesystem-type allow-list
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// nobody has agreed (is a fuse mount local? is an overlay?), and getting that
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// list wrong walks a network share. What it would buy is already visible
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// without it — dmScan names every boundary it refused to cross, so a user with
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// a second disk SEES that /mnt/data was not entered, which is the difference
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// between a limitation and a lie.
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func dmVolumeRoots() (roots []string, notes []string) {
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return []string{"/"}, nil
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}
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