content: allow reset via Truncate

To make restarting after failed pull less racy, we define `Truncate(size
int64) error` on `content.Writer` for the zero offset. Truncating a
writer will dump any existing data and digest state and start from the
beginning. All subsequent writes will start from the zero offset.

For the service, we support this by defining the behavior for a write
that changes the offset. To keep this narrow, we only support writes out
of order at the offset 0, which causes the writer to dump existing data
and reset the local hash.

This makes restarting failed pulls much smoother when there was a
previously encountered error and the source doesn't support arbitrary
seeks or reads at arbitrary offsets. By allowing this to be done while
holding the write lock on a ref, we can restart the full download
without causing a race condition.

Once we implement seeking on the `io.Reader` returned by the fetcher,
this will be less useful, but it is good to ensure that our protocol
properly supports this use case for when streaming is the only option.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

content/content.go

@@ -44,6 +44,7 @@ type Writer interface {
 	Status() (Status, error)
 	Digest() digest.Digest
 	Commit(size int64, expected digest.Digest) error
+	Truncate(size int64) error
 }
 
 type Ingester interface {