Session C measured it live: a drive whose device vanished raised the GENERIC
storage_disconnected while its return raised the SPECIFIC
backup_target_restored -- an alarm and an all-clear an operator cannot pair.
backup_target_absent never fired at all.
The mechanism is not what the Session-C audit first said, and the difference
decides the fix. RoleForStorage returns RoleSystem whenever backingDevice == ""
(internal/storage/role.go:180-181). When the device goes, exactMountDevice
fails, BackingDevice becomes "", the target row's role flips to system and it
loses its guest path -- but keeps its MountPath. The union loop skips any drive
whose MountPath is already seen, so the registry row is DEDUPED AWAY ENTIRELY.
/disks carries no row with that guest path, so isTarget[guestPath] is a MISSING
KEY, not a false. Setting BackupTarget on the union row -- the obvious fix --
could not have worked, because that row is not emitted when the alarm is needed.
The audit is corrected in the same push.
Fix: on the Observe row only, carry the guest path when the row IS the backup
target and its role flipped because the device vanished.
Three gates, verified not assumed:
- t.BackingDevice == "" restricts it to the vanished-device flip; a genuinely
system-BACKED storage has a real device and is excluded, so a dir storage at
/mnt/<name> on the root disk cannot acquire a guest path.
- Case B, the common fresh-box shape, is safe twice over: its target is the
builtin local on /var/lib/vz and StablePathForRaw returns "" for anything not
exactly /mnt/<name>, so nothing is set even before the gates apply.
- It cannot make the gate read an absent drive as PRESENT. BoundUnderParent is
assigned at exactly two sites, both inside guest-path blocks a system-role row
never enters, so it stays false and planDriveGates computes false || false.
Pinned by TestAbsentTargetRowDoesNotRegisterPresence -- getting this backwards
would have silenced the alarm the fix exists to raise.
The :213-214 boundary stands: no system or backup mount gains a guest path.
Tests +5, asserting the emitted /disks JSON through a faithful copy of the
controller's driveTargetByPath, because the failure class is "the value is on
the wrong row". Red-proof: removing the block fails with "isTarget[...] is a
MISSING KEY"; reverted byte-identical.
Filed not closed: the two-row shape that produced this survives.
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