Mailing List Archive

Event-driven programs want to wait until the xs_fileno triggers for
reading, and then repeatedly call xs_check_watch.

Also xs_read_watch exposes a useless "num" out parameter, which should
always (if things aren't going hideously wrong) be at least 2 and
which the caller shouldn't be interested in. So xs_check_watch
doesn't have one of those.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
tools/xenstore/xs.c | 89 ++++++++++++++++++++++++++++++++++++++++++++-------
tools/xenstore/xs.h | 16 +++++++++
2 files changed, 93 insertions(+), 12 deletions(-)

diff --git a/tools/xenstore/xs.c b/tools/xenstore/xs.c
index df270f7..8e54fe0 100644
--- a/tools/xenstore/xs.c
+++ b/tools/xenstore/xs.c
@@ -132,7 +132,23 @@ struct xs_handle {

#endif

-static int read_message(struct xs_handle *h);
+static int read_message(struct xs_handle *h, int nonblocking);
+
+static void setnonblock(int fd, int nonblock) {
+ int esave = errno;
+ int flags = fcntl(fd, F_GETFL);
+ if (flags == -1)
+ goto out;
+
+ if (nonblock)
+ flags |= O_NONBLOCK;
+ else
+ flags &= ~O_NONBLOCK;
+
+ fcntl(fd, F_SETFL, flags);
+out:
+ errno = esave;
+}

int xs_fileno(struct xs_handle *h)
{
@@ -325,8 +341,16 @@ void xs_close(struct xs_handle* xsh)
xs_daemon_close(xsh);
}

-static bool read_all(int fd, void *data, unsigned int len)
+static bool read_all(int fd, void *data, unsigned int len, int nonblocking)
+ /* With nonblocking, either reads either everything requested,
+ * or nothing. */
{
+ if (!len)
+ return true;
+
+ if (nonblocking)
+ setnonblock(fd, 1);
+
while (len) {
int done;

@@ -334,18 +358,28 @@ static bool read_all(int fd, void *data, unsigned int len)
if (done < 0) {
if (errno == EINTR)
continue;
- return false;
+ goto out_false;
}
if (done == 0) {
/* It closed fd on us? EBADF is appropriate. */
errno = EBADF;
- return false;
+ goto out_false;
}
data += done;
len -= done;
+
+ if (nonblocking) {
+ setnonblock(fd, 0);
+ nonblocking = 0;
+ }
}

return true;
+
+out_false:
+ if (nonblocking)
+ setnonblock(fd, 0);
+ return false;
}

#ifdef XSTEST
@@ -374,7 +408,7 @@ static void *read_reply(
read_from_thread = read_thread_exists(h);

/* Read from comms channel ourselves if there is no reader thread. */
- if (!read_from_thread && (read_message(h) == -1))
+ if (!read_from_thread && (read_message(h, 0) == -1))
return NULL;

mutex_lock(&h->reply_mutex);
@@ -693,7 +727,8 @@ bool xs_watch(struct xs_handle *h, const char *path, const char *token)
* Returns array of two pointers: path and token, or NULL.
* Call free() after use.
*/
-char **xs_read_watch(struct xs_handle *h, unsigned int *num)
+static char **read_watch_internal(struct xs_handle *h, unsigned int *num,
+ int nonblocking)
{
struct xs_stored_msg *msg;
char **ret, *strings, c = 0;
@@ -707,14 +742,20 @@ char **xs_read_watch(struct xs_handle *h, unsigned int *num)
* we haven't called xs_watch. Presumably the application
* will do so later; in the meantime we just block.
*/
- while (list_empty(&h->watch_list) && h->fd != -1)
+ while (list_empty(&h->watch_list) && h->fd != -1) {
+ if (nonblocking) {
+ mutex_unlock(&h->watch_mutex);
+ errno = EAGAIN;
+ return 0;
+ }
condvar_wait(&h->watch_condvar, &h->watch_mutex);
+ }
#else /* !defined(USE_PTHREAD) */
/* Read from comms channel ourselves if there are no threads
* and therefore no reader thread. */

assert(!read_thread_exists(h)); /* not threadsafe but worth a check */
- if ((read_message(h) == -1))
+ if ((read_message(h, nonblocking) == -1))
return NULL;

#endif /* !defined(USE_PTHREAD) */
@@ -760,6 +801,24 @@ char **xs_read_watch(struct xs_handle *h, unsigned int *num)
return ret;
}

+char **xs_check_watch(struct xs_handle *h)
+{
+ unsigned int num;
+ char **ret;
+ ret = read_watch_internal(h, &num, 1);
+ if (ret) assert(num >= 2);
+ return ret;
+}
+
+/* Find out what node change was on (will block if nothing pending).
+ * Returns array of two pointers: path and token, or NULL.
+ * Call free() after use.
+ */
+char **xs_read_watch(struct xs_handle *h, unsigned int *num)
+{
+ return read_watch_internal(h, num, 0);
+}
+
/* Remove a watch on a node.
* Returns false on failure (no watch on that node).
*/
@@ -940,11 +999,17 @@ char *xs_debug_command(struct xs_handle *h, const char *cmd,
ARRAY_SIZE(iov), NULL);
}

-static int read_message(struct xs_handle *h)
+static int read_message(struct xs_handle *h, int nonblocking)
{
/* IMPORTANT: It is forbidden to call this function without
* acquiring the request lock and checking that h->read_thr_exists
* is false. See "Lock discipline" in struct xs_handle, above. */
+
+ /* If nonblocking==1, this function will always read either
+ * nothing, returning -1 and setting errno==EAGAIN, or we read
+ * whole amount requested. Ie as soon as we have the start of
+ * the message we block until we get all of it.
+ */

struct xs_stored_msg *msg = NULL;
char *body = NULL;
@@ -956,7 +1021,7 @@ static int read_message(struct xs_handle *h)
if (msg == NULL)
goto error;
cleanup_push(free, msg);
- if (!read_all(h->fd, &msg->hdr, sizeof(msg->hdr))) { /* Cancellation point */
+ if (!read_all(h->fd, &msg->hdr, sizeof(msg->hdr), nonblocking)) { /* Cancellation point */
saved_errno = errno;
goto error_freemsg;
}
@@ -966,7 +1031,7 @@ static int read_message(struct xs_handle *h)
if (body == NULL)
goto error_freemsg;
cleanup_push(free, body);
- if (!read_all(h->fd, body, msg->hdr.len)) { /* Cancellation point */
+ if (!read_all(h->fd, body, msg->hdr.len, 0)) { /* Cancellation point */
saved_errno = errno;
goto error_freebody;
}
@@ -1021,7 +1086,7 @@ static void *read_thread(void *arg)
struct xs_handle *h = arg;
int fd;

- while (read_message(h) != -1)
+ while (read_message(h, 0) != -1)
continue;

/* An error return from read_message leaves the socket in an undefined
diff --git a/tools/xenstore/xs.h b/tools/xenstore/xs.h
index 1cbe255..63f535d 100644
--- a/tools/xenstore/xs.h
+++ b/tools/xenstore/xs.h
@@ -135,6 +135,22 @@ bool xs_watch(struct xs_handle *h, const char *path, const char *token);
/* Return the FD to poll on to see if a watch has fired. */
int xs_fileno(struct xs_handle *h);

+/* Check for node changes. On success, returns a non-NULL pointer ret
+ * such that ret[0] and ret[1] are valid C strings, namely the
+ * triggering path (see docs/misc/xenstore.txt) and the token (from
+ * xs_watch). On error return value is NULL setting errno.
+ *
+ * Callers should, after xs_fileno has become readable, repeatedly
+ * call xs_check_watch until it returns NULL and sets errno to EAGAIN.
+ * (If the fd became readable, xs_check_watch is allowed to make it no
+ * longer show up as readable even if future calls to xs_check_watch
+ * will return more watch events.)
+ *
+ * After the caller is finished with the returned information it
+ * should be freed all in one go with free(ret).
+ */
+char **xs_check_watch(struct xs_handle *h);
+
/* Find out what node change was on (will block if nothing pending).
* Returns array containing the path and token. Use XS_WATCH_* to access these
* elements. Call free() after use.
--
1.7.2.5


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