errors on releasing mounts with typed execerror refs

[tonistiigi]

With the new exec errors that return snapshot state reference count goes off.

First issue:

from busybox
workdir /src
run --mount=target=.  ls -l && fail

In daemon logs see many msg="releasing already released reference".

Second issue:

from busybox
workdir /src
run --mount=type=cache,target=/cache ls -l && fail

At least sometimes fails with error: failed to solve: rpc error: code = Unknown desc = error committing j1t2azk8c5bokbz3t55jq60ry: invalid mutable ref: invalid: executor failed running [/bin/sh -c ls -l && fail]: exit code: 127 . I debugged that it appears because mutable is already released and ref count is 0.

@hinshun Tried to debug this a bit but quite confusing. Not exactly sure who is the owner of refs in certain points. I think in long term using the wrapped error chain for snapshot refs is not stable enough and causes errors like this or df3a9ca . I think we need to return a special extra return value from vtx.Exec to make clear that everyone calling and passing these objects need to understand who owns and releases these objects. Would be good to find a smaller fix for the v0.8 patch release though.

Not sure if this is related to #1956 . The message is the same but to me, the error is not persistent. This might have an extra effect in combination with prune.

[kshcherban]

Seems like a dup of #1956