MDEV-34994: sql/mysqld: stop accept() loop after the first EAGAIN
Each time a listener socket becomes ready, MariaDB calls accept() ten times (MAX_ACCEPT_RETRY), even if all but the first one return EAGAIN because there are no more connections. This causes unnecessary CPU usage - on our server, the CPU load of that thread, which does nothing but accept(), saturates one CPU core by ~45%. The loop should stop after the first EAGAIN. Perf report: 11.01% mariadbd libc.so.6 [.] accept4 6.42% mariadbd [kernel.kallsyms] [k] finish_task_switch.isra.0 5.50% mariadbd [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore 5.50% mariadbd [kernel.kallsyms] [k] syscall_enter_from_user_mode 4.59% mariadbd [kernel.kallsyms] [k] __fget_light 3.67% mariadbd [kernel.kallsyms] [k] kmem_cache_alloc 2.75% mariadbd [kernel.kallsyms] [k] fput 2.75% mariadbd [kernel.kallsyms] [k] mod_objcg_state 1.83% mariadbd [kernel.kallsyms] [k] __inode_wait_for_writeback 1.83% mariadbd [kernel.kallsyms] [k] __sys_accept4 1.83% mariadbd [kernel.kallsyms] [k] _raw_spin_unlock_irq 1.83% mariadbd [kernel.kallsyms] [k] alloc_inode 1.83% mariadbd [kernel.kallsyms] [k] call_rcu
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