util: add function spdk_fd_group_get_epoll_event

This patch introduces function spdk_fd_group_get_epoll_event, which
returns the epoll(7) event that caused the file descriptor group
callback function to execute. Rather than changing the signature of
spdk_fd_fn in order to pass the struct epoll_event, which would result
in a gigantic patch where there vast majority of users would simply have
to ignore the new argument, we introduce this new API that allows to
return the epoll_event only when really needed.

Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>
Suggested-by: John Levon <john.levon@nutanix.com>
Change-Id: I3debe1382d1c2bfec6ae4fea274ee38ed0b135fe
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/14935
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: John Levon <levon@movementarian.org>

CHANGELOG.md

@@ -55,6 +55,11 @@ Added spdk_rpc_set_allowlist to restrict allowed RPCs to the specified list.
 Promoted the application to example to match similar programs: fio_plugin and perf.
 It can now be found inside `examples/bdev/bdevperf`.
 
+### util
+
+New API `spdk_fd_group_get_epoll_event` that returns the epoll(7) event that
+caused a function callback in file descriptor group to execute.
+
 ## v22.09
 
 ### accel

include/spdk/fd_group.h

@@ -118,6 +118,22 @@ void spdk_fd_group_remove(struct spdk_fd_group *fgrp, int efd);
 int spdk_fd_group_event_modify(struct spdk_fd_group *fgrp,
 			       int efd, int event_types);
 
+/*
+ * Forward declaration of epoll_event to avoid having to conditionally compile
+ * spdk_fd_group_get_epoll_event on non-Linux systems.
+ */
+struct epoll_event;
+
+/**
+ * Copies the epoll(7) event that caused a callback function to execute.
+ * This function can only be called by the callback function, doing otherwise
+ * results in undefined behavior.
+ *
+ * \param event pointer to an epoll(7) event to copy to
+ * \return 0 on success, -errno on error
+ */
+int spdk_fd_group_get_epoll_event(struct epoll_event *event);
+
 #ifdef __cplusplus
 }
 #endif

lib/util/fd_group.c

@@ -60,6 +60,18 @@ spdk_fd_group_get_fd(struct spdk_fd_group *fgrp)
 
 #ifdef __linux__
 
+static __thread struct epoll_event *g_event = NULL;
+
+int
+spdk_fd_group_get_epoll_event(struct epoll_event *event)
+{
+	if (g_event == NULL) {
+		return -EINVAL;
+	}
+	*event = *g_event;
+	return 0;
+}
+
 int
 spdk_fd_group_add(struct spdk_fd_group *fgrp, int efd, spdk_fd_fn fn,
 		  void *arg, const char *name)
@@ -277,8 +289,10 @@ spdk_fd_group_wait(struct spdk_fd_group *fgrp, int timeout)
 		SPDK_DTRACE_PROBE4(interrupt_fd_process, ehdlr->name, ehdlr->fd,
 				   ehdlr->fn, ehdlr->fn_arg);
 
+		g_event = &events[n];
 		/* call the interrupt response function */
 		ehdlr->fn(ehdlr->fn_arg);
+		g_event = NULL;
 
 		/* It is possible that the ehdlr was removed
 		 * during this wait loop when it get executed.
@@ -295,6 +309,12 @@ spdk_fd_group_wait(struct spdk_fd_group *fgrp, int timeout)
 
 #else
 
+int
+spdk_fd_group_get_epoll_event(struct epoll_event *event)
+{
+	return -ENOTSUP;
+}
+
 int
 spdk_fd_group_add(struct spdk_fd_group *fgrp, int efd, spdk_fd_fn fn,
 		  void *arg, const char *name)

lib/util/spdk_util.map

@@ -150,6 +150,7 @@
 	# public functions in fd_group.h
 	spdk_fd_group_create;
 	spdk_fd_group_destroy;
+	spdk_fd_group_get_epoll_event;
 	spdk_fd_group_wait;
 	spdk_fd_group_add;
 	spdk_fd_group_remove;