Require a schema and an operation name to be given to each permit when
created. The schema is of the table the read is executed against, and
the operation name, which is some name identifying the operation the
permit is part of. Ideally this should be different for each site the
permit is created at, to be able to discern not only different kind of
reads, but different code paths the read took.
As not all read can be associated with one schema, the schema is allowed
to be null.
The name will be used for debugging purposes, both for coredump
debugging and runtime logging of permit-related diagnostics.
Allow the evictable reader managing the underlying reader to pass its
own permit to it when creating it, making sure they share the same
permit. Note that the two parts can still end up using different
permits, when the underlying reader is kept alive between two pages of a
paged read and thus keeps using the permit received on the previous
page.
Also adjust the `reader_context` in multishard_mutation_query.cc to use
the passed-in permit instead of creating a new one when creating a new
reader.
Currently per-shard reader contexts are cleaned up as soon as the reader
itself is destroyed. This causes two problems:
* Continuations attached to the reader destroy future might rely on
stuff in the context being kept alive -- like the semaphore.
* Shard 0's semaphore is special as it will be used to account buffers
allocated by the multishard reader itself, so it has to be alive until
after all readers are destroyed.
This patch changes this so that contexts are destroyed only when the
lifecycle policy itself is destroyed.
Currently all reader lifecycle policy implementations assume that
`semaphore()` will only be called after at least one call to
`make_reader()`. This assumption will soon not hold, so make sure
`semaphore()` can be called at any time, including before any calls are
made to `make_reader()`.
And use the reader_permit for this instead. This refactoring has
revealed a pre-existing bug in the `test_lifecycle_policy`, which is
also addressed in this patch. The bug is that said policy executes
reader destructions in the background, and these are not waited for. For
some reason, the semaphore -> permit transition pushes these races over
the edge and we start seeing some of these destruction fibers still
being unfinished when test scopes are exited, causing all sorts of
trouble. The solution is to introduce a special gate that tests can use
to wait for all background work to finish, before the test scope is
exited.
Detach test_multishard_combining_reader_as_mutation_source into
individual file.
This particular test runs ~13 minutes. What's left in the origin
completes a bit faster.
The split also requires moving the reader_lifecycle_policy and
the dummy_partitioner into lib/
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
