[PATCH 02/24] libxl: support multiple libxl__ev_fds for the same fd

[Ian Jackson <ian.jackson@eu.citrix.com>]

We need a slightly more sophisticated data structure to allow this,
where we record the slot not just for each fd but also for each
(fd,eventbit) where eventbit is POLLIN, POLLPRI, POLLOUT.

Document the new relaxed restriction.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

Changes since v6:
 * Fix typo
---
 tools/libxl/libxl_event.c    |   62 +++++++++++++++++++++++------------------
 tools/libxl/libxl_internal.h |   10 +++++--
 2 files changed, 42 insertions(+), 30 deletions(-)

diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c
index 5e1a207..149a4eb 100644
--- a/tools/libxl/libxl_event.c
+++ b/tools/libxl/libxl_event.c
@@ -635,10 +635,11 @@ static int beforepoll_internal(libxl__gc *gc, libxl__poller *poller,
 
 
     /*
-     * In order to be able to efficiently find the libxl__ev_fd
-     * for a struct poll during _afterpoll, we maintain a shadow
-     * data structure in CTX->fd_beforepolled: each slot in
-     * the fds array corresponds to a slot in fd_beforepolled.
+     * In order to be able to efficiently find the libxl__ev_fd for a
+     * struct poll during _afterpoll, we maintain a shadow data
+     * structure in CTX->fd_rindices: each fd corresponds to a slot in
+     * fd_rindices, and each element in the rindices is three indices
+     * into the fd array (for POLLIN, POLLPRI and POLLOUT).
      */
 
     if (*nfds_io) {
@@ -659,14 +660,16 @@ static int beforepoll_internal(libxl__gc *gc, libxl__poller *poller,
         });
 
         /* make sure our array is as big as *nfds_io */
-        if (poller->fd_rindex_allocd < maxfd) {
-            assert(maxfd < INT_MAX / sizeof(int) / 2);
-            int *newarray = realloc(poller->fd_rindex, sizeof(int) * maxfd);
-            if (!newarray) { rc = ERROR_NOMEM; goto out; }
-            memset(newarray + poller->fd_rindex_allocd, 0,
-                   sizeof(int) * (maxfd - poller->fd_rindex_allocd));
-            poller->fd_rindex = newarray;
-            poller->fd_rindex_allocd = maxfd;
+        if (poller->fd_rindices_allocd < maxfd) {
+            assert(ARRAY_SIZE_OK(poller->fd_rindices, maxfd));
+            poller->fd_rindices =
+                libxl__realloc(0, poller->fd_rindices,
+                               maxfd * sizeof(*poller->fd_rindices));
+            memset(poller->fd_rindices + poller->fd_rindices_allocd,
+                   0,
+                   (maxfd - poller->fd_rindices_allocd)
+                     * sizeof(*poller->fd_rindices));
+            poller->fd_rindices_allocd = maxfd;
         }
     }
 
@@ -677,8 +680,10 @@ static int beforepoll_internal(libxl__gc *gc, libxl__poller *poller,
             fds[used].fd = req_fd;
             fds[used].events = req_events;
             fds[used].revents = 0;
-            assert(req_fd < poller->fd_rindex_allocd);
-            poller->fd_rindex[req_fd] = used;
+            assert(req_fd < poller->fd_rindices_allocd);
+            if (req_events & POLLIN)  poller->fd_rindices[req_fd][0] = used;
+            if (req_events & POLLPRI) poller->fd_rindices[req_fd][1] = used;
+            if (req_events & POLLOUT) poller->fd_rindices[req_fd][2] = used;
         }
         used++;
     });
@@ -706,7 +711,6 @@ static int beforepoll_internal(libxl__gc *gc, libxl__poller *poller,
             *timeout_upd = our_timeout;
     }
 
- out:
     return rc;
 }
 
@@ -728,24 +732,28 @@ static int afterpoll_check_fd(libxl__poller *poller,
                               int fd, int events)
     /* returns mask of events which were requested and occurred */
 {
-    if (fd >= poller->fd_rindex_allocd)
+    if (fd >= poller->fd_rindices_allocd)
         /* added after we went into poll, have to try again */
         return 0;
 
-    int slot = poller->fd_rindex[fd];
+    int i, revents = 0;
+    for (i=0; i<3; i++) {
+        int slot = poller->fd_rindices[fd][i];
 
-    if (slot >= nfds)
-        /* stale slot entry; again, added afterwards */
-        return 0;
+        if (slot >= nfds)
+            /* stale slot entry; again, added afterwards */
+            continue;
 
-    if (fds[slot].fd != fd)
-        /* again, stale slot entry */
-        return 0;
+        if (fds[slot].fd != fd)
+            /* again, stale slot entry */
+            continue;
 
-    assert(!(fds[slot].revents & POLLNVAL));
+        assert(!(fds[slot].revents & POLLNVAL));
+        revents |= fds[slot].revents;
+    }
 
-    int revents = fds[slot].revents & (events | POLLERR | POLLHUP);
     /* we mask in case requested events have changed */
+    revents &= (events | POLLERR | POLLHUP);
 
     return revents;
 }
@@ -1009,7 +1017,7 @@ int libxl__poller_init(libxl_ctx *ctx, libxl__poller *p)
 {
     int r, rc;
     p->fd_polls = 0;
-    p->fd_rindex = 0;
+    p->fd_rindices = 0;
 
     r = pipe(p->wakeup_pipe);
     if (r) {
@@ -1036,7 +1044,7 @@ void libxl__poller_dispose(libxl__poller *p)
     if (p->wakeup_pipe[1] > 0) close(p->wakeup_pipe[1]);
     if (p->wakeup_pipe[0] > 0) close(p->wakeup_pipe[0]);
     free(p->fd_polls);
-    free(p->fd_rindex);
+    free(p->fd_rindices);
 }
 
 libxl__poller *libxl__poller_get(libxl_ctx *ctx)
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index af631fb..2ce4bb5 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -131,7 +131,11 @@ typedef void libxl__ev_fd_callback(libxl__egc *egc, libxl__ev_fd *ev,
    * events; otherwise revents contains only bits in events.  Contrary
    * to the documentation for poll(2), POLLERR and POLLHUP can occur
    * even if only POLLIN was set in events.  (POLLNVAL is a fatal
-   * error and will cause libxl event machinery to fail an assertion.) */
+   * error and will cause libxl event machinery to fail an assertion.)
+   *
+   * It is not permitted to listen for the same or overlapping events
+   * on the same fd using multiple different libxl__ev_fd's.
+   */
 struct libxl__ev_fd {
     /* caller should include this in their own struct */
     /* read-only for caller, who may read only when registered: */
@@ -257,8 +261,8 @@ struct libxl__poller {
     struct pollfd *fd_polls;
     int fd_polls_allocd;
 
-    int fd_rindex_allocd;
-    int *fd_rindex; /* see libxl_osevent_beforepoll */
+    int fd_rindices_allocd;
+    int (*fd_rindices)[3]; /* see libxl_osevent_beforepoll */
 
     int wakeup_pipe[2]; /* 0 means no fd allocated */
 };
-- 
1.7.2.5