Commit 7a7357dae1 ("LCOW: Implemented support for docker cp + build")
changed `container.BaseFS` from being a string (that could be empty but
can't lead to nil pointer dereference) to containerfs.ContainerFS,
which could be be `nil` and so nil dereference is at least theoretically
possible, which leads to panic (i.e. engine crashes).
Such a panic can be avoided by carefully analysing the source code in all
the places that dereference a variable, to make the variable can't be nil.
Practically, this analisys are impossible as code is constantly
evolving.
Still, we need to avoid panics and crashes. A good way to do so is to
explicitly check that a variable is non-nil, returning an error
otherwise. Even in case such a check looks absolutely redundant,
further changes to the code might make it useful, and having an
extra check is not a big price to pay to avoid a panic.
This commit adds such checks for all the places where it is not obvious
that container.BaseFS is not nil (which in this case means we do not
call daemon.Mount() a few lines earlier).
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
d6ea46ceda