During `(*Graph).Register, there was no protection on adding new layers
concurrently. In some cases, this resulted in corruption of a layer by creating
the directory but not the underlying data. This manifested in several different
IO errors reported in the client. This attempts to fix this by adding a mutex
by Image ID to protect the Register operation.
We do not completely understand the root cause of this corruption other than
the result is somehow tied to this particular function. This fix has been
confirmed to address the issue through testing.
Unfortunately, this fix does not address existing corruption. The user will
have to remove and re-pull the corrupt layer to stop the error from happening
in the future. This change only ensures that the layer will not become corrupt.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
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