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    Refactor Blob Service API · 593bbccd
    Stephen J Day authored
    
    
    This PR refactors the blob service API to be oriented around blob descriptors.
    Identified by digests, blobs become an abstract entity that can be read and
    written using a descriptor as a handle. This allows blobs to take many forms,
    such as a ReadSeekCloser or a simple byte buffer, allowing blob oriented
    operations to better integrate with blob agnostic APIs (such as the `io`
    package). The error definitions are now better organized to reflect conditions
    that can only be seen when interacting with the blob API.
    
    The main benefit of this is to separate the much smaller metadata from large
    file storage. Many benefits also follow from this. Reading and writing has
    been separated into discrete services. Backend implementation is also
    simplified, by reducing the amount of metadata that needs to be picked up to
    simply serve a read. This also improves cacheability.
    
    "Opening" a blob simply consists of an access check (Stat) and a path
    calculation. Caching is greatly simplified and we've made the mapping of
    provisional to canonical hashes a first-class concept. BlobDescriptorService
    and BlobProvider can be combined in different ways to achieve varying effects.
    
    Recommend Review Approach
    -------------------------
    
    This is a very large patch. While apologies are in order, we are getting a
    considerable amount of refactoring. Most changes follow from the changes to
    the root package (distribution), so start there. From there, the main changes
    are in storage. Looking at (*repository).Blobs will help to understand the how
    the linkedBlobStore is wired. One can explore the internals within and also
    branch out into understanding the changes to the caching layer. Following the
    descriptions below will also help to guide you.
    
    To reduce the chances for regressions, it was critical that major changes to
    unit tests were avoided. Where possible, they are left untouched and where
    not, the spirit is hopefully captured. Pay particular attention to where
    behavior may have changed.
    
    Storage
    -------
    
    The primary changes to the `storage` package, other than the interface
    updates, were to merge the layerstore and blobstore. Blob access is now
    layered even further. The first layer, blobStore, exposes a global
    `BlobStatter` and `BlobProvider`. Operations here provide a fast path for most
    read operations that don't take access control into account. The
    `linkedBlobStore` layers on top of the `blobStore`, providing repository-
    scoped blob link management in the backend. The `linkedBlobStore` implements
    the full `BlobStore` suite, providing access-controlled, repository-local blob
    writers. The abstraction between the two is slightly broken in that
    `linkedBlobStore` is the only channel under which one can write into the global
    blob store. The `linkedBlobStore` also provides flexibility in that it can act
    over different link sets depending on configuration. This allows us to use the
    same code for signature links, manifest links and blob links.  Eventually, we
    will fully consolidate this storage.
    
    The improved cache flow comes from the `linkedBlobStatter` component
    of `linkedBlobStore`. Using a `cachedBlobStatter`, these combine together to
    provide a simple cache hierarchy that should streamline access checks on read
    and write operations, or at least provide a single path to optimize. The
    metrics have been changed in a slightly incompatible way since the former
    operations, Fetch and Exists, are no longer relevant.
    
    The fileWriter and fileReader have been slightly modified to support the rest
    of the changes. The most interesting is the removal of the `Stat` call from
    `newFileReader`. This was the source of unnecessary round trips that were only
    present to look up the size of the resulting reader. Now, one must simply pass
    in the size, requiring the caller to decide whether or not the `Stat` call is
    appropriate. In several cases, it turned out the caller already had the size
    already. The `WriterAt` implementation has been removed from `fileWriter`,
    since it is no longer required for `BlobWriter`, reducing the number of paths
    which writes may take.
    
    Cache
    -----
    
    Unfortunately, the `cache` package required a near full rewrite. It was pretty
    mechanical in that the cache is oriented around the `BlobDescriptorService`
    slightly modified to include the ability to set the values for individual
    digests. While the implementation is oriented towards caching, it can act as a
    primary store. Provisions are in place to have repository local metadata, in
    addition to global metadata. Fallback is implemented as a part of the storage
    package to maintain this flexibility.
    
    One unfortunate side-effect is that caching is now repository-scoped, rather
    than global. This should have little effect on performance but may increase
    memory usage.
    
    Handlers
    --------
    
    The `handlers` package has been updated to leverage the new API. For the most
    part, the changes are superficial or mechanical based on the API changes. This
    did expose a bug in the handling of provisional vs canonical digests that was
    fixed in the unit tests.
    
    Configuration
    -------------
    
    One user-facing change has been made to the configuration and is updated in
    the associated documentation. The `layerinfo` cache parameter has been
    deprecated by the `blobdescriptor` cache parameter. Both are equivalent and
    configuration files should be backward compatible.
    
    Notifications
    -------------
    
    Changes the `notification` package are simply to support the interface
    changes.
    
    Context
    -------
    
    A small change has been made to the tracing log-level. Traces have been moved
    from "info" to "debug" level to reduce output when not needed.
    
    Signed-off-by: default avatarStephen J Day <stephen.day@docker.com>
    593bbccd