Calling WebAssembly UDFs requires wasmtime instance. Creating such an instance is expensive,
but these instances can be reused for subsequent calls of the same UDF on various inputs.
This patch introduces a way of reusing wasmtime instances: a wasm instance cache.
The cache stores a wasmtime instance for each UDF and scheduling group. The instances are
evicted using LRU strategy and their size is based on the size of their wasm memories.
The instances stored in the cache are also dropped when the UDF is dropped itself. For that reason,
the first patch modifies the current implementation of UDF dropping, so that the instance dropping may be added
later. The patch also removes the need of compiling the UDF again when dropping it.
The second patch contains the implementation and use of the new cache. The cache is implemented
in `lang/wasm_instance_cache.hh` and the main ways of using it are the `run_script` methods from `wasm.hh`
The third patch adds tests to `test_wasm.py` that check the correctness and performance of the new
cache. The tests confirm the instance reuse, size limits, instance eviction after timeout and after dropping the UDF.
Closes#10306
* github.com:scylladb/scylladb:
wasm: test instances reuse
wasm: reuse UDF instances
schema_tables: simplify merge_functions and avoid extra compilation
268e4abe77