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Downgrading docker 17.xx -> 1.12 should not be a delayed timebomb, but an immediate explosion #2196
We're implementing a mechanism to allow selecting an older version of Docker (more news on that soon(tm)).
This is all well and good, but there's a caveat...
The following sequence of events leads to a timebomb:
This is especially bad since significant time could pass between 2. and 3., leading this to be a timebomb.
Since the only way to get into this situation is by doing something inherently broken (downgrading docker without removing its data), the failure is expected.
The best solution is simply to ensure that the initial launch, step 3., fails noisily as soon as it recognizes that it would create a second graphdriver.
Letting 1.12 use overlay2 if it's already present is an alternate solution to this specific instance of the downgrade problem.