"
0c6bbc8 refactored `get_rpc_client_idx()` to select different clients
for statement verbs depending on the current scheduling group.
The goal was to allow statement verbs to be sent on different
connections depending on the current scheduling group. The new
connections use per-connection isolation. For backward compatibility the
already existing connections fall-back to per-handler isolation used
previously. The old statement connection, called the default statement
connection, also used this. `get_rpc_client_idx()` was changed to select
the default statement connection when the current scheduling group is
the statement group, and a non-default connection otherwise.
This inadvertently broke `scheduling_group_for_verb()` which also used
this method to get the scheduling group to be used to isolate a verb at
handle register time. This method needs the default client idx for each
verb, but if verb registering is run under the system group it instead
got the non-default one, resulting in the per-handler isolation not
being set-up for the default statement connection, resulting in default
statement verb handlers running in whatever scheduling group the process
loop of the rpc is running in, which is the system scheduling group.
This caused all sorts of problems, even beyond user queries running in
the system group. Also as of 0c6bbc8 queries on the replicas are
classified based on the scheduling group they are running on, so user
reads also ended up using the system concurrency semaphore.
In particular this caused severe problems with ranges scans, which in
some cases ended up using different semaphores per page resulting in a
crash. This could happen because when the page was read locally the code
would run in the statement scheduling group, but when the request
arrived from a remote coordinator via rpc, it was read in a system
scheduling group. This caused a mismatch between the semaphore the saved
reader was created with and the one the new page was read with. The
result was that in some cases when looking up a paused reader from the
wrong semaphore, a reader belonging to another read was returned,
creating a disconnect between the lifecycle between readers and that of
the slice and range they were referencing.
This series fixes the underlying problem of the scheduling group
influencing the verb handler registration, as well as adding some
additional defenses if this semaphore mismatch ever happens in the
future. Inactive read handles are now unique across all semaphores,
meaning that it is not possible anymore that a handle succeeds in
looking up a reader when used with the wrong semaphore. The range scan
algorithm now also makes sure there is no semaphore mismatch between the
one used for the current page and that of the saved reader from the
previous page.
I manually checked that each individual defense added is already
preventing the crash from happening.
Fixes: #6613Fixes: #6907Fixes: #6908
Tests: unit(dev), manual(run the crash reproducer, observe no crash)
"
* 'query-classification-regressions/v1' of https://github.com/denesb/scylla:
multishard_mutation_query: use cached semaphore
messaging: make verb handler registering independent of current scheduling group
multishard_mutation_query: validate the semaphore of the looked-up reader
reader_concurrency_semaphore: make inactive read handles unique across semaphores
reader_concurrency_semaphore: add name() accessor
reader_concurrency_semaphore: allow passing name to no-limit constructor
3f84d41880