Dear Bob, I've analyzed your code, and the issue is on your end: you simply didn't set the timeout correctly. When using an external event loop, it is mandatory that you ask MHD for the timeout using MHD_get_timeout() and use that with select/poll/epoll. Then, you must call MHD_run() once whenever epoll() returns (including timeouts!).
In your case, MHD would have given you a timeout of 0, but you used infinity instead, with predictable results... I've attached a corrected version of the code. Happy hacking! Christian On 03/02/2018 08:26 PM, Robert D Kocisko wrote: > First, thanks for your amazing work on MHD! > > This question is a near duplicate of the 2014 message thread from Tom > Cornell entitled "Trouble getting a response sent from a separate worker > thread (with external select)". However, I am not using separate worker > threads--everything is in one thread and so I don't think the > recommendations found in that thread apply to my scenario. > > Basically after receiving a request from an HTTP client, I want to be > able to do some asynchronous 'work' which is really just waiting on > another process such as a database engine to calculate and return the > result which, when complete, I will forward back to the client. This is > all done in one thread using epoll, so I don't want any blocking and I > don't want any busy loops. MHD's external epoll support combined with > suspend/resume fits into this architecture perfectly, but there's a > problem: after resuming the connection and queueing the data, the > headers are sent to the client immediately, but the body of the response > does not get sent until another client request arrives. > > Anyway, to make this all concrete, I've put together a small working > example (below) which shows the problem. This is built against the > latest dev rev (7f1dbb2) on elementaryOS (which is Ubuntu 16.04). Every > time a request comes in it suspends the connection and starts a 1 second > timer which, when it expires, resumes the connection. When the > connection is resumed the response is queued (simply echos the request > url). I realize this example leaks timer fds and doesn't clean up > properly but it successfully demonstrates the problem. > > I have experimented with calling MHD_run() twice after > MHD_resume_connection() rather than the once required by the docs, and > that does seem to work, but that seems extremely hacky and I'm not sure > if twice is enough (why twice and not three times?). I've skimmed the > source code looking for obvious answers but none are readily apparent to me. > > At this point I'm pretty sure that this is a bug with MHD but am I > missing something? > > Thanks! > Bob Kocisko > > ------------------------- > > #include "platform.h" > #include <microhttpd.h> > #include <sys/epoll.h> > #include <sys/timerfd.h> > > #define TIMEOUT_INFINITE -1 > > struct Request { > struct MHD_Connection *connection; > int timerfd; > }; > > int epfd; > struct epoll_event evt; > > static int > ahc_echo (void *cls, > struct MHD_Connection *connection, > const char *url, > const char *method, > const char *version, > const char *upload_data, size_t *upload_data_size, void **ptr) > { > struct MHD_Response *response; > int ret; > struct Request* req; > struct itimerspec ts; > (void)url; /* Unused. Silent compiler warning. */ > (void)version; /* Unused. Silent compiler warning. */ > (void)upload_data; /* Unused. Silent compiler warning. */ > (void)upload_data_size; /* Unused. Silent compiler warning. */ > > req = *ptr; > if (!req) > { > > req = malloc(sizeof(struct Request)); > req->connection = connection; > req->timerfd = 0; > *ptr = req; > return MHD_YES; > } > > if (req->timerfd) > { > // send response (echo request url) > response = MHD_create_response_from_buffer (strlen (url), > (void *) url, > MHD_RESPMEM_MUST_COPY); > ret = MHD_queue_response (connection, MHD_HTTP_OK, response); > MHD_destroy_response (response); > return ret; > } > else > { > // create timer and suspend connection > req->timerfd = timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK); > if (-1 == req->timerfd) > { > printf("timerfd_create: %s", strerror(errno)); > return MHD_NO; > } > evt.events = EPOLLIN; > evt.data.ptr = req; > if (-1 == epoll_ctl(epfd, EPOLL_CTL_ADD, req->timerfd, &evt)) > { > printf("epoll_ctl: %s", strerror(errno)); > return MHD_NO; > } > ts.it_value.tv_sec = 1; > ts.it_value.tv_nsec = 0; > ts.it_interval.tv_sec = 0; > ts.it_interval.tv_nsec = 0; > if (-1 == timerfd_settime(req->timerfd, 0, &ts, NULL)) > { > printf("timerfd_settime: %s", strerror(errno)); > return MHD_NO; > } > > MHD_suspend_connection(connection); > return MHD_YES; > } > } > > static int > connection_done(struct MHD_Connection *connection, > void **con_cls, > enum MHD_RequestTerminationCode toe) > { > free(*con_cls); > } > > int > main (int argc, char *const *argv) > { > struct MHD_Daemon *d; > const union MHD_DaemonInfo * info; > int current_event_count; > struct epoll_event events_list[1]; > struct Request *req; > uint64_t timer_expirations; > > if (argc != 2) > { > printf ("%s PORT\n", argv[0]); > return 1; > } > d = MHD_start_daemon (MHD_USE_EPOLL | MHD_ALLOW_SUSPEND_RESUME, > atoi (argv[1]), > NULL, NULL, &ahc_echo, NULL, > MHD_OPTION_NOTIFY_COMPLETED, &connection_done, NULL, > MHD_OPTION_END); > if (d == NULL) > return 1; > > info = MHD_get_daemon_info(d, MHD_DAEMON_INFO_EPOLL_FD); > if (info == NULL) > return 1; > > epfd = epoll_create1(EPOLL_CLOEXEC); > if (-1 == epfd) > return 1; > > evt.events = EPOLLIN; > evt.data.ptr = NULL; > if (-1 == epoll_ctl(epfd, EPOLL_CTL_ADD, info->epoll_fd, &evt)) > return 1; > > while (1) > { > current_event_count = epoll_wait(epfd, events_list, 1, > TIMEOUT_INFINITE); > > if (1 == current_event_count) > { > if (events_list[0].data.ptr) > { > // A timer has timed out > req = events_list[0].data.ptr; > // read from the fd so the system knows we heard the notice > if (-1 == read(req->timerfd, &timer_expirations, > sizeof(timer_expirations))) > { > return 1; > } > // Now resume the connection > MHD_resume_connection(req->connection); > if (!MHD_run(d)) > return 1; > } > else > { > // MHD is ready > if (!MHD_run(d)) > return 1; > } > } > else if (0 == current_event_count) > { > // no events: continue > } > else > { > // error > return 1; > } > } > > return 0; > } > >
#include "platform.h" #include <microhttpd.h> #include <sys/epoll.h> #include <sys/timerfd.h> #include <limits.h> #define TIMEOUT_INFINITE -1 struct Request { struct MHD_Connection *connection; int timerfd; }; int epfd; struct epoll_event evt; static int ahc_echo (void *cls, struct MHD_Connection *connection, const char *url, const char *method, const char *version, const char *upload_data, size_t *upload_data_size, void **ptr) { struct MHD_Response *response; int ret; struct Request* req; struct itimerspec ts; (void)url; /* Unused. Silent compiler warning. */ (void)version; /* Unused. Silent compiler warning. */ (void)upload_data; /* Unused. Silent compiler warning. */ (void)upload_data_size; /* Unused. Silent compiler warning. */ req = *ptr; if (!req) { req = malloc(sizeof(struct Request)); req->connection = connection; req->timerfd = 0; *ptr = req; return MHD_YES; } if (req->timerfd) { // send response (echo request url) response = MHD_create_response_from_buffer (strlen (url), (void *) url, MHD_RESPMEM_MUST_COPY); ret = MHD_queue_response (connection, MHD_HTTP_OK, response); MHD_destroy_response (response); return ret; } else { // create timer and suspend connection req->timerfd = timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK); if (-1 == req->timerfd) { printf("timerfd_create: %s", strerror(errno)); return MHD_NO; } evt.events = EPOLLIN; evt.data.ptr = req; if (-1 == epoll_ctl(epfd, EPOLL_CTL_ADD, req->timerfd, &evt)) { printf("epoll_ctl: %s", strerror(errno)); return MHD_NO; } ts.it_value.tv_sec = 1; ts.it_value.tv_nsec = 0; ts.it_interval.tv_sec = 0; ts.it_interval.tv_nsec = 0; if (-1 == timerfd_settime(req->timerfd, 0, &ts, NULL)) { printf("timerfd_settime: %s", strerror(errno)); return MHD_NO; } MHD_suspend_connection(connection); return MHD_YES; } } static int connection_done(struct MHD_Connection *connection, void **con_cls, enum MHD_RequestTerminationCode toe) { free(*con_cls); } int main (int argc, char *const *argv) { struct MHD_Daemon *d; const union MHD_DaemonInfo * info; int current_event_count; struct epoll_event events_list[1]; struct Request *req; uint64_t timer_expirations; if (argc != 2) { printf ("%s PORT\n", argv[0]); return 1; } d = MHD_start_daemon (MHD_USE_EPOLL | MHD_ALLOW_SUSPEND_RESUME, atoi (argv[1]), NULL, NULL, &ahc_echo, NULL, MHD_OPTION_NOTIFY_COMPLETED, &connection_done, NULL, MHD_OPTION_END); if (d == NULL) return 1; info = MHD_get_daemon_info(d, MHD_DAEMON_INFO_EPOLL_FD); if (info == NULL) return 1; epfd = epoll_create1(EPOLL_CLOEXEC); if (-1 == epfd) return 1; evt.events = EPOLLIN; evt.data.ptr = NULL; if (-1 == epoll_ctl(epfd, EPOLL_CTL_ADD, info->epoll_fd, &evt)) return 1; while (1) { int timeout; MHD_UNSIGNED_LONG_LONG to; if (MHD_YES != MHD_get_timeout (d, &to)) timeout = TIMEOUT_INFINITE; else timeout = (to < INT_MAX - 1) ? (int) to : (INT_MAX - 1); current_event_count = epoll_wait(epfd, events_list, 1, timeout); if (1 == current_event_count) { if (events_list[0].data.ptr) { // A timer has timed out req = events_list[0].data.ptr; // read from the fd so the system knows we heard the notice if (-1 == read(req->timerfd, &timer_expirations, sizeof(timer_expirations))) { return 1; } // Now resume the connection MHD_resume_connection(req->connection); } } else if (0 == current_event_count) { // no events: continue } else { // error return 1; } if (! MHD_run(d)) return 1; } return 0; }
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