memcached源码阅读笔记(二)
来源：互联网 发布：华为算法工程师 编辑：程序博客网 时间：2024/05/16 10:08
之前的文章,主要分析过main()函数.那本文从分析主线程的执行流程开始,分析一下memcached的核心函数drive_machine()函数.\
在main()函数中,会调用server_sockets()函数初始化监听的连接.在server_sockets()函数中,会调用server_socket()函数.server_socket()函数中,对于UDP会调用dispacth_conn_new()函数;对于TCP,会调用conn_new()函数.对于UDP,是直接将连接轮询到子线程中处理,主线程不做任何处理,对于TCP,会设置listen的fd的event回调函数为event_handler()函数,该函数中调用的是核心函数drive_machine()函数，该函数内部实现了一个connection的状态机。
drive_machine()函数的源代码分析：
static void drive_machine(conn *c) { bool stop = false; int sfd, flags = 1; socklen_t addrlen; struct sockaddr_storage addr; int nreqs = settings.reqs_per_event; int res; const char *str; assert(c != NULL); while (!stop) { switch(c->state) { case conn_listening: addrlen = sizeof(addr); if ((sfd = accept(c->sfd, (struct sockaddr *)&addr, &addrlen)) == -1) { if (errno == EAGAIN || errno == EWOULDBLOCK) { /* these are transient, so don't log anything */ stop = true; } else if (errno == EMFILE) { if (settings.verbose > 0) fprintf(stderr, "Too many open connections\n"); accept_new_conns(false); stop = true; } else { perror("accept()"); stop = true; } break; } if ((flags = fcntl(sfd, F_GETFL, 0)) < 0 || fcntl(sfd, F_SETFL, flags | O_NONBLOCK) < 0) { perror("setting O_NONBLOCK"); close(sfd); break; } if (settings.maxconns_fast && stats.curr_conns + stats.reserved_fds >= settings.maxconns - 1) { str = "ERROR Too many open connections\r\n"; res = write(sfd, str, strlen(str)); close(sfd); STATS_LOCK(); stats.rejected_conns++; STATS_UNLOCK(); } else { dispatch_conn_new(sfd, conn_new_cmd, EV_READ | EV_PERSIST, DATA_BUFFER_SIZE, tcp_transport); } stop = true; break; case conn_waiting: if (!update_event(c, EV_READ | EV_PERSIST)) { if (settings.verbose > 0) fprintf(stderr, "Couldn't update event\n"); conn_set_state(c, conn_closing); break; } conn_set_state(c, conn_read); stop = true; break; case conn_read: res = IS_UDP(c->transport) ? try_read_udp(c) : try_read_network(c); switch (res) { case READ_NO_DATA_RECEIVED: conn_set_state(c, conn_waiting); break; case READ_DATA_RECEIVED: conn_set_state(c, conn_parse_cmd); break; case READ_ERROR: conn_set_state(c, conn_closing); break; case READ_MEMORY_ERROR: /* Failed to allocate more memory */ /* State already set by try_read_network */ break; } break; case conn_parse_cmd : if (try_read_command(c) == 0) { /* wee need more data! */ conn_set_state(c, conn_waiting); } break; case conn_new_cmd: /* Only process nreqs at a time to avoid starving other connections */ --nreqs; if (nreqs >= 0) { reset_cmd_handler(c); } else { pthread_mutex_lock(&c->thread->stats.mutex); c->thread->stats.conn_yields++; pthread_mutex_unlock(&c->thread->stats.mutex); if (c->rbytes > 0) { /* We have already read in data into the input buffer, so libevent will most likely not signal read events on the socket (unless more data is available. As a hack we should just put in a request to write data, because that should be possible ;-) */ if (!update_event(c, EV_WRITE | EV_PERSIST)) { if (settings.verbose > 0) fprintf(stderr, "Couldn't update event\n"); conn_set_state(c, conn_closing); } } stop = true; } break; case conn_nread: if (c->rlbytes == 0) { complete_nread(c); break; } /* first check if we have leftovers in the conn_read buffer */ if (c->rbytes > 0) { int tocopy = c->rbytes > c->rlbytes ? c->rlbytes : c->rbytes; if (c->ritem != c->rcurr) { memmove(c->ritem, c->rcurr, tocopy); } c->ritem += tocopy; c->rlbytes -= tocopy; c->rcurr += tocopy; c->rbytes -= tocopy; if (c->rlbytes == 0) { break; } } /* now try reading from the socket */ res = read(c->sfd, c->ritem, c->rlbytes); if (res > 0) { pthread_mutex_lock(&c->thread->stats.mutex); c->thread->stats.bytes_read += res; pthread_mutex_unlock(&c->thread->stats.mutex); if (c->rcurr == c->ritem) { c->rcurr += res; } c->ritem += res; c->rlbytes -= res; break; } if (res == 0) { /* end of stream */ conn_set_state(c, conn_closing); break; } if (res == -1 && (errno == EAGAIN || errno == EWOULDBLOCK)) { if (!update_event(c, EV_READ | EV_PERSIST)) { if (settings.verbose > 0) fprintf(stderr, "Couldn't update event\n"); conn_set_state(c, conn_closing); break; } stop = true; break; } /* otherwise we have a real error, on which we close the connection */ if (settings.verbose > 0) { fprintf(stderr, "Failed to read, and not due to blocking:\n" "errno: %d %s \n" "rcurr=%lx ritem=%lx rbuf=%lx rlbytes=%d rsize=%d\n", errno, strerror(errno), (long)c->rcurr, (long)c->ritem, (long)c->rbuf, (int)c->rlbytes, (int)c->rsize); } conn_set_state(c, conn_closing); break; case conn_swallow: /* we are reading sbytes and throwing them away */ if (c->sbytes == 0) { conn_set_state(c, conn_new_cmd); break; } /* first check if we have leftovers in the conn_read buffer */ if (c->rbytes > 0) { int tocopy = c->rbytes > c->sbytes ? c->sbytes : c->rbytes; c->sbytes -= tocopy; c->rcurr += tocopy; c->rbytes -= tocopy; break; } /* now try reading from the socket */ res = read(c->sfd, c->rbuf, c->rsize > c->sbytes ? c->sbytes : c->rsize); if (res > 0) { pthread_mutex_lock(&c->thread->stats.mutex); c->thread->stats.bytes_read += res; pthread_mutex_unlock(&c->thread->stats.mutex); c->sbytes -= res; break; } if (res == 0) { /* end of stream */ conn_set_state(c, conn_closing); break; } if (res == -1 && (errno == EAGAIN || errno == EWOULDBLOCK)) { if (!update_event(c, EV_READ | EV_PERSIST)) { if (settings.verbose > 0) fprintf(stderr, "Couldn't update event\n"); conn_set_state(c, conn_closing); break; } stop = true; break; } /* otherwise we have a real error, on which we close the connection */ if (settings.verbose > 0) fprintf(stderr, "Failed to read, and not due to blocking\n"); conn_set_state(c, conn_closing); break; case conn_write: /* * We want to write out a simple response. If we haven't already, * assemble it into a msgbuf list (this will be a single-entry * list for TCP or a two-entry list for UDP). */ if (c->iovused == 0 || (IS_UDP(c->transport) && c->iovused == 1)) { if (add_iov(c, c->wcurr, c->wbytes) != 0) { if (settings.verbose > 0) fprintf(stderr, "Couldn't build response\n"); conn_set_state(c, conn_closing); break; } } /* fall through... */ case conn_mwrite: if (IS_UDP(c->transport) && c->msgcurr == 0 && build_udp_headers(c) != 0) { if (settings.verbose > 0) fprintf(stderr, "Failed to build UDP headers\n"); conn_set_state(c, conn_closing); break; } switch (transmit(c)) { case TRANSMIT_COMPLETE: if (c->state == conn_mwrite) { while (c->ileft > 0) { item *it = *(c->icurr); assert((it->it_flags & ITEM_SLABBED) == 0); item_remove(it); c->icurr++; c->ileft--; } while (c->suffixleft > 0) { char *suffix = *(c->suffixcurr); cache_free(c->thread->suffix_cache, suffix); c->suffixcurr++; c->suffixleft--; } /* XXX: I don't know why this wasn't the general case */ if(c->protocol == binary_prot) { conn_set_state(c, c->write_and_go); } else { conn_set_state(c, conn_new_cmd); } } else if (c->state == conn_write) { if (c->write_and_free) { free(c->write_and_free); c->write_and_free = 0; } conn_set_state(c, c->write_and_go); } else { if (settings.verbose > 0) fprintf(stderr, "Unexpected state %d\n", c->state); conn_set_state(c, conn_closing); } break; case TRANSMIT_INCOMPLETE: case TRANSMIT_HARD_ERROR: break; /* Continue in state machine. */ case TRANSMIT_SOFT_ERROR: stop = true; break; } break; case conn_closing: if (IS_UDP(c->transport)) conn_cleanup(c); else conn_close(c); stop = true; break; case conn_max_state: assert(false); break; } } return;}
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