The tree.Nodes will be replaced by an iterator to loads and serializes
tree node ondemand. Thus, the processing moves from StreamTrees into the
callback. Schedule them onto the workers used by StreamTrees for proper
load distribution.
The new method combines both step into a single wrapper function. Thus
it ensures that both are always called in pairs. As an additional
benefit this slightly reduces the boilerplate to upload blobs.
Drop the `packs` map from the internal state of the checker. Instead the
Packs(...) method now calls a filter callback that can select the
packs intended for checking.
Those methods now only allow modifying snapshots. Internal data types
used by the repository are now read-only. The repository-internal code
can bypass the restrictions by wrapping the repository in an
`internalRepository` type.
The restriction itself is implemented by using a new datatype
WriteableFileType in the SaveUnpacked and RemoveUnpacked methods. This
statically ensures that code cannot bypass the access restrictions.
The test changes are somewhat noisy as some of them modify repository
internals and therefore require some way to bypass the access
restrictions. This works by capturing an `internalRepository` or
`Backend` when creating the Repository using a test helper function.
RemoveUnpacked will eventually block removal of all filetypes other than
snapshots. However, getting there requires a major refactor to provide
some components with privileged access.
This ensures that the pack header is actually read completely.
Previously, for a truncated file it was possible to only read a part of
the header, as backend.Load(...) is not guaranteed to return as many
bytes as requested by the length parameter.
Due to the interface of streamPack, we cannot guarantee that operations
progress fast enough that the underlying connections remains open. This
introduces partial failures which massively complicate the error
handling.
Switch to a simpler approach that retrieves the pack in chunks of 32MB.
If a blob is larger than this limit, then it is downloaded separately.
To avoid multiple copies in memory, an auxiliary interface
`discardReader` is introduced that allows directly accessing the
downloaded byte slices, while still supporting the streaming used by the
`check` command.
To only stream the content of a pack file once, check used StreamPack
with a custom pack load function. This combination was always brittle
and complicates using StreamPack everywhere else. Now that StreamPack
internally uses PackBlobIterator use that primitive instead, which is a
much better fit for what the check command requires.
