rajvarun77 opened a new pull request, #3348:
URL: https://github.com/apache/brpc/pull/3348
Generic `Controller`/`Socket` plumbing that lets a protocol pin a
pooled/short connection across multiple RPCs (connection affinity). This is the
shared substrate extracted from the MySQL client PR #3330 so it can be reviewed
independently; #3330 is now stacked on top of this branch and contains only the
MySQL-specific code.
### Controller
- `BindSockAction {NONE,RESERVE,USE}` encoded in two `Controller::_flags`
bits (`FLAGS_BIND_SOCK_RESERVE`/`USE`) via
`set_bind_sock_action()`/`bind_sock_action()` — no dedicated member added to
the hot `Controller`.
- `_bind_sock` holds a socket reserved by a previous RPC. `EndRPC`
propagates the action to the current `Call`; `Call::OnComplete` reserves the
socket (on success) instead of returning it to the pool / failing it;
`IssueRPC` reuses the reserved socket when the action is `USE`.
- `_session_data`: opaque per-RPC slot a protocol codec may use to carry
typed state between serialize/pack/parse (not owned by `Controller`).
### Socket
- `_fd_version` (relaxed atomic, bumped on every `ResetFileDescriptor`) +
`fd_version()` accessor — an ABA guard so an affinity holder can detect that a
reserved fd was reconnected underneath it. Atomic because it is written on the
reconnect path (no socket lock held) and read from other threads.
- `Socket::Write` accepts an empty buffer when `WriteOptions::auth_flags` is
set, for server-greets-first handshakes that send their first bytes from the
connection-phase handler rather than from `data`.
### Backup/retry fix
Also fixes an uninitialized read this machinery would otherwise introduce:
the `Call(Call*)` copy constructor used for backup requests and retries did not
initialize `bind_sock_action`, so `OnComplete` could read indeterminate bits
and (when they matched `RESERVE`/`USE`) divert the backup call's socket away
from the pool-return path, hanging the RPC (manifested as
`ChannelTest.backup_request` timing out). Initialized to `BIND_SOCK_NONE`,
matching `Call::Reset()`; a backup/retry never inherits affinity.
No protocol uses these primitives in this PR; the first consumer is MySQL
(#3330).
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