Introduce ustctl_write_one_packet_to_channel

This function allows the consumer to write at most one packet of data to
the channel, that way we can push data to the ring buffer without
risking to block on subbuffer switch.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/include/lttng/ust-ctl.h b/include/lttng/ust-ctl.h
index f9315a6144c672a06006b0c06bb849fa3bc53af9..3c81e509a4af551e54ab0db2e0e6da9caad14370 100644 (file)
--- a/include/lttng/ust-ctl.h
+++ b/include/lttng/ust-ctl.h
@@ -164,6 +164,10 @@ int ustctl_write_metadata_to_channel(
                struct ustctl_consumer_channel *channel,
                const char *metadata_str,       /* NOT null-terminated */
                size_t len);                    /* metadata length */
+ssize_t ustctl_write_one_packet_to_channel(
+               struct ustctl_consumer_channel *channel,
+               const char *metadata_str,       /* NOT null-terminated */
+               size_t len);                    /* metadata length */
 
 /*
  * Send a NULL stream to finish iteration over all streams of a given

diff --git a/liblttng-ust-ctl/ustctl.c b/liblttng-ust-ctl/ustctl.c
index 0361f184e342619031ec51d837c509743c7bab77..28dee5e9cdf80118113f53522dc296460f66b1af 100644 (file)
--- a/liblttng-ust-ctl/ustctl.c
+++ b/liblttng-ust-ctl/ustctl.c
@@ -1054,6 +1054,40 @@ end:
        return ret;
 }
 
+/*
+ * Write at most one packet in the channel.
+ * Returns the number of bytes written on success, < 0 on error.
+ */
+ssize_t ustctl_write_one_packet_to_channel(
+               struct ustctl_consumer_channel *channel,
+               const char *metadata_str,       /* NOT null-terminated */
+               size_t len)                     /* metadata length */
+{
+       struct lttng_ust_lib_ring_buffer_ctx ctx;
+       struct lttng_channel *chan = channel->chan;
+       const char *str = metadata_str;
+       ssize_t reserve_len;
+       int ret;
+
+       reserve_len = min_t(ssize_t,
+                       chan->ops->packet_avail_size(chan->chan, chan->handle),
+                       len);
+       lib_ring_buffer_ctx_init(&ctx, chan->chan, NULL, reserve_len,
+                       sizeof(char), -1, chan->handle);
+       ret = chan->ops->event_reserve(&ctx, 0);
+       if (ret != 0) {
+               DBG("LTTng: event reservation failed");
+               assert(ret < 0);
+               reserve_len = ret;
+               goto end;
+       }
+       chan->ops->event_write(&ctx, str, reserve_len);
+       chan->ops->event_commit(&ctx);
+
+end:
+       return reserve_len;
+}
+
 int ustctl_channel_close_wait_fd(struct ustctl_consumer_channel *consumer_chan)
 {
        struct channel *chan;