Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
When asked to upgrade a reader that itself is a downgrade, try to
return the original v2 reader instead, and likewise when downgrading
upgraded v1 readers.
This is desirable because version transformations can result from,
say, entering/leaving a reader concurrency semaphore, and the amount
of such transformations is practically unbounded.
Such short-circuiting is only done if it is safe, that is: the
transforming reader's buffer is empty and its internal range tombstone
tracking state is discardable.
Signed-off-by: Michael Livshin <michael.livshin@scylladb.com>
The transition to v2 will be incremental. To support that, we need
adaptors between v1 and v2 which will be inserted at places which are
boundaries of conversion.
The v1 -> v2 converter needs to accumulate range tombstones, so has unbounded worst case memory footprint.
The v2 -> v1 converter trims range tombstones around clustering rows,
so generates more fragments than necessary.
Because of that, adpators are a temporary solution and we should not
release with them on the produciton code paths.
