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content: allow reset via Truncate

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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>
This commit is contained in:
Stephen J Day 2017-02-27 19:02:16 -08:00
parent be20bb1eb8
commit d99756a8a2
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GPG key ID: 67B3DED84EDC823F
7 changed files with 64 additions and 13 deletions
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10
api/services/content/content.pb.go
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@ -186,8 +186,14 @@ type WriteRequest struct {
// with the commit action message.
Expected github_com_opencontainers_go_digest.Digest `protobuf:"bytes,4,opt,name=expected,proto3,customtype=github.com/opencontainers/go-digest.Digest" json:"expected"`
// Offset specifies the number of bytes from the start at which to begin
// the write. If zero or less, the write will be from the start. This uses
// standard zero-indexed semantics.
// the write. For most implementations, this means from the start of the
// file. This uses standard, zero-indexed semantics.
//
// If the action is write, the remote may remove all previously written
// data up to the offset. Implementations may support arbitrary offsets but
// MUST support reseting this value to zero with with a write. If an
// implementation does not support a write at a particular offset, an
// OutOfRange error must be returned.
Offset int64 `protobuf:"varint,5,opt,name=offset,proto3" json:"offset,omitempty"`
// Data is the actual bytes to be written.
//