mirror of
https://github.com/scylladb/scylladb.git
synced 2026-04-24 18:40:38 +00:00
3bfd47da4b
The connection's `cpu_concurrency_t` struct tracks the state of a connection
to manage the admission of new requests and prevent CPU overload during
connection storms. When a connection holds units (allowed only 0 or 1), it is
considered to be in the "CPU state" and contributes to the concurrency limits
used when accepting new connections.
The bug stems from the fact that `counted_data_source_impl::get` and
`counted_data_sink_impl::put` calls can interleave during execution. This
occurs because of `should_parallelize` and `_ready_to_respond`, the latter being
a future chain that can run in the background while requests are being read.
Consequently, while reading request (N), the system may concurrently be
writing the response for request (N-1) on the same connection.
This interleaving allows `return_all()` to be called twice before the
subsequent `consume_units()` is invoked. While the second `return_all()` call
correctly returns 0 units, the matching `consume_units()` call would
mistakenly take an extra unit from the semaphore. Over time, a connection
blocked on a read operation could end up holding an unreturned semaphore
unit. If this pattern repeats across multiple connections, the semaphore
units are eventually depleted, preventing the server from accepting any
new connections.
The fix ensures that we always consume the exact number of units that were
previously returned. With this change, interleaved operations behave as
follows:
get() return_all — returns 1 unit
put() return_all — returns 0 units
get() consume_units — takes back 1 unit
put() consume_units — takes back 0 units
Logically, the networking phase ends when the first network operation
concludes. But more importantly, when a network operation
starts, we no longer hold any units.
Other solutions are possible but the chosen one seems to be the
simplest and safest to backport.
Fixes SCYLLADB-485
Backport: all supported affected versions, bug introduced with initial feature implementation in: ed3e4f33fd
Closes scylladb/scylladb#28530
* github.com:scylladb/scylladb:
test: auth_cluster: add test for hanged AUTHENTICATING connections
transport: fix connection code to consume only initially taken semaphore units