This is sort of large, but it's annoyingly difficult to
separate out.
The basic idea is to allow us to have a single holder-iterating
block of code, which is associated with the holder, that can be used
for various things, like the snapshot queue background scan, or
for inspect operations.
We invent the concept of a HolderFilter, which is a thing that
can decide what things in a holder it cares about, and a HolderOperator,
which can also process those things selectively.
In the process, we fix up a couple of subtle bugs in the
inspect logic; specifically, the assumption that the mapped flag could
tell you whether a container was modified by the ops log doesn't
work with mmap, so we have a shiny new flag which is used to track
that, internal to the roaring/container code.
All of this leads to the actual *point* of this exercise, which is
making it easier to create an /inspect endpoint which produces almost
the same data we'd have gotten from `pilosa inspect` on a data directory;
the distinction is that it doesn't try to identify the distinction
between data from disk and data from operations since the file was
loaded. Possibly it should, but it doesn't yet.
The snapshot queue is now implemented using the HolderOperator
design, which requires some subtle changes to how it works, but
overall makes it easier to follow the snapshot queue logic,
and also shares that logic with the way Inspect works.
The holder's snapshot queue is now provided by the server, in
a default environment.
The queueless snapshot queue no longer triggers snapshots on
enqueue -- it turns out that breaks badly, because a key
point about enqueueing a snapshot is that it's safe to do it
*during* a transaction on that fragment, and triggering a
snapshot during a transaction actually causes horrible errors
as the ops log ends up being the old file, which we close.
Related to this, we also need to prevent closed fragments from
trying to snapshot, so we track fragment openness when opening
or closing, and bail on trying to snapshot a fragment which is closed.
We also stop using the queueless snapshot queue during tests,
because that's a horrible idea.
We copy a little bit of the partition logic from the cluster code so
we don't have to expose it all, this lets us check whether the node
we're looking at is the one which should be primary for a given shard,
and if not, identify which node would be. This works only when
pointed at a data directory, for now.
The test cases for the holder have to be internal, because pilosa
doesn't export view/fragment, just Index/Field. This means that the
holder test cases can't just use the test/* package, so they duplicate
some of its logic, approximately.
So with the switch to a new linter, we get a lot of new warnings,
and the majority of them are harmless probably, but a few might be
real. Variously just use _ to suppress warnings, or report errors.
There's probably things here that deserve better fixes, but we can
always revisit it.
This commit adds the ability to remove an 'available shard'
from the shard cache. This does not affect shards known to be
available because of local data.
This commit removes the previous `MaxShard` tracking and replaces
it with an `Available Shards` set tracking. This allows sparse shard
tracking without implicitly tracking all shards in between.
