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scylla-sstable currently has two ways to obtain the schema:
* via a `schema.cql` file.
* load schema definition from memory (only works for system tables).
This meant that for most cases it was necessary to export the schema into a CQL format and write it to a file. This is very flexible. The sstable can be inspected anywhere, it doesn't have to be on the same host where it originates form. Yet in many cases the sstable is inspected on the same host where it originates from. In this cases, the schema is readily available in the schema tables on disk and it is plain annoying to have to export it into a file, just to quickly inspect an sstable file.
This series solves this annoyance by providing a mechanism to load schemas from the on-disk schema tables. Furthermore, an auto-detect mechanism is provided to detect the location of these schema tables based on the path of the sstable, but if that fails, the tool check the usual locations of the scylla data dir, the scylla confguration file and even looks for environment variables that tell the location of these. The old methods are still supported. In fact, if a schema.cql is present in the working directory of the tool, it is preferred over any other method, allowing for an easy force-override.
If the auto-detection magic fails, an error is printed to the console, advising the user to turn on debug level logging to see what went wrong.
A comprehensive test is added which checks all the different schema loading mechanisms. The documentation is also updated to reflect the changes.
This change breaks the backward-compatibility of the command-line API of the tool, as `--system-schema` is now just a flag, the keyspace and table names are supplied separately via the new `--keyspace` and `--table` options. I don't think this will break anybody's workflow as this tools is still lightly used, exactly because of the annoying way the schema has to be provided. Hopefully after this series, this will change.
Example:
```
$ ./build/dev/scylla sstable dump-data /var/lib/scylla/data/ks/tbl2-d55ba230b9a811ed9ae8495671e9e4f8/quarantine/me-1-big-Data.db
{"sstables":{"/var/lib/scylla/data/ks/tbl2-d55ba230b9a811ed9ae8495671e9e4f8/quarantine//me-1-big-Data.db":[{"key":{"token":"-3485513579396041028","raw":"000400000000","value":"0"},"clustering_elements":[{"type":"clustering-row","key":{"raw":"","value":""},"marker":{"timestamp":1677837047297728},"columns":{"v":{"is_live":true,"type":"regular","timestamp":1677837047297728,"value":"0"}}}]}]}}
```
As seen above, subdirectories like qurantine, staging etc are also supported.
Fixes: https://github.com/scylladb/scylladb/issues/10126
Closes #13448
* github.com:scylladb/scylladb:
test/cql-pytest: test_tools.py: add tests for schema loading
test/cql-pytest: add no_autocompaction_context
docs: scylla-sstable.rst: remove accidentally added copy-pasta
docs: scylla-sstable.rst: remove paragraph with schema limitations
docs: scylla-sstable.rst: update schema section
test/cql-pytest: nodetool.py: add flush_keyspace()
tools/scylla-sstable: reform schema loading mechanism
tools/schema_loader: add load_schema_from_schema_tables()
db/schema_tables: expose types schema
(cherry picked from commit 952b455310)
Closes #15386