TLS socket writes in 16kb chunks with intervening epoll/setsockopt syscalls

I noticed that krpc has the following syscall pattern:


 rpc reactor-231 23122 [002] 35488410.994309: syscalls:sys_enter_epoll_wait: epfd: 0x00000007, events: 0x02137520, maxevents: 0x00000040, timeout: 0x00000050
 rpc reactor-231 23122 [002] 35488410.994310: syscalls:sys_exit_epoll_wait: 0x1
 rpc reactor-231 23122 [002] 35488410.994313: syscalls:sys_enter_setsockopt: fd: 0x00000011, level: 0x00000006, optname: 0x00000003, optval: 0x7fc80910175c, optlen: 0x00000004
 rpc reactor-231 23122 [002] 35488410.994314: syscalls:sys_exit_setsockopt: 0x0
 rpc reactor-231 23122 [002] 35488410.994351: syscalls:sys_enter_write: fd: 0x00000011, buf: 0x7fc7e8059e93, count: 0x0000401d
 rpc reactor-231 23122 [002] 35488410.994370: syscalls:sys_exit_write: 0x401d
 rpc reactor-231 23122 [002] 35488410.994372: syscalls:sys_enter_setsockopt: fd: 0x00000011, level: 0x00000006, optname: 0x00000003, optval: 0x7fc80910175c, optlen: 0x00000004
 rpc reactor-231 23122 [002] 35488410.994378: syscalls:sys_exit_setsockopt: 0x0


This block of syscalls repeats in a pretty tight loop – epoll_wait, CORK, write, UNCORK. The writes are always 0x401d bytes (just more than 16kb). I found the following in the ssl_write manpage:

SSL_write() will only return with success, when the complete contents of buf of length num has been written. This default behaviour can be changed with the SSL_MODE_ENABLE_PARTIAL_WRITE option of ssl_ctx_set_mode(3). When this flag is set, SSL_write() will also return with success, when a partial write has been successfully completed. In this case the SSL_write() operation is considered completed. The bytes are sent and a new SSL_write() operation with a new buffer (with the already sent bytes removed) must be started. A partial write is performed with the size of a message block, which is 16kB for SSLv3/TLSv1.

Seems likely we should be looping the writes before uncorking – either until we run into a temporary socket error or run out of stuff to write.