From: Christian Babeux <christian.babeux@efficios.com>
Date: Mon, 3 Dec 2012 01:41:43 +0000 (-0500)
Subject: Cygwin: Introduce new LTTNG_UST_STREAM_PIPE command to open wakeup pipe
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=840a73344734b584f7033a2b76628b28c2fa57bf;p=lttng-tools.git

Cygwin: Introduce new LTTNG_UST_STREAM_PIPE command to open wakeup pipe

Normally, the userspace tracer open the wakeup pipe, and the resulting
fd is passed to the consumer via the session daemon. Since we can't pass
fds via UNIX socket, the pipe need to be opened separately in the tracer
and in the consumer. The only way to open the write side of a named pipe
*before* the read side without blocking is to open it in read/write mode.
This is supported on Linux, but POSIX leave this behavior undefined [1].

The Cygwin named pipe implementation doesn't seem to allow multiple
readers/writers on a named pipe. Opening a pipe in RW mode in the tracer
and then opening the read side in the consumer won't work because the
consumer will get a "Device or ressource busy" error. Thus arise the need
to open the named pipe read side *before* the write side.

In order to accomplish this task, a new command must be introduced to signal
to the tracer that a specific named pipe should be opened in write mode.

Proper care must be taken to issue this command *after* the named pipe paths
have been sent to the consumer or else the sessiond or tracer will block
indefinitely.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
---

diff --git a/src/bin/lttng-sessiond/lttng-ust-abi.h b/src/bin/lttng-sessiond/lttng-ust-abi.h
index c54ef2fbf..2e08233e2 100644
--- a/src/bin/lttng-sessiond/lttng-ust-abi.h
+++ b/src/bin/lttng-sessiond/lttng-ust-abi.h
@@ -182,7 +182,7 @@ struct lttng_ust_calibrate {
 #define LTTNG_UST_STREAM			_UST_CMD(0x60)
 #define LTTNG_UST_EVENT			\
 	_UST_CMDW(0x61, struct lttng_ust_event)
-
+#define LTTNG_UST_STREAM_PIPE			_UST_CMD(0x62)
 /* Event and Channel FD commands */
 #define LTTNG_UST_CONTEXT			\
 	_UST_CMDW(0x70, struct lttng_ust_context)
diff --git a/src/bin/lttng-sessiond/lttng-ust-ctl.h b/src/bin/lttng-sessiond/lttng-ust-ctl.h
index a4f9c3fb4..43e3f8deb 100644
--- a/src/bin/lttng-sessiond/lttng-ust-ctl.h
+++ b/src/bin/lttng-sessiond/lttng-ust-ctl.h
@@ -26,6 +26,7 @@ int ustctl_create_session(int sock);
 int ustctl_open_metadata(int sock, int session_handle,
 		struct lttng_ust_channel_attr *chops,
 		struct lttng_ust_object_data **metadata_data);
+int ustctl_open_wait_pipe(int sock, struct lttng_ust_object *channel_data);
 int ustctl_create_channel(int sock, int session_handle,
 		struct lttng_ust_channel_attr *chops,
 		struct lttng_ust_object_data **channel_data);
diff --git a/src/bin/lttng-sessiond/ust-app.c b/src/bin/lttng-sessiond/ust-app.c
index 5380450f6..5c66776d4 100644
--- a/src/bin/lttng-sessiond/ust-app.c
+++ b/src/bin/lttng-sessiond/ust-app.c
@@ -2055,6 +2055,31 @@ int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
 		goto error_rcu_unlock;
 	}
 
+	/* Ask UST to open the write side of the wait pipe */
+	DBG("Asking UST to open metadata stream wait pipe path: %s\n",
+	    ua_sess->metadata->stream_obj->wait_pipe_path);
+
+	ret = ustctl_open_wait_pipe(app->sock, ua_sess->metadata->obj);
+
+	if (ret < 0) {
+		ERR("Asking UST to open wait_pipe failed");
+	}
+
+	/* each channel */
+	cds_lfht_for_each_entry(ua_sess->channels->ht, &iter.iter, ua_chan,
+				node.node) {
+
+		DBG("Asking UST to open channel wait pipe path: %s\n",
+		    ua_chan->obj->wait_pipe_path);
+
+		ret = ustctl_open_wait_pipe(app->sock, ua_chan->obj);
+
+		if (ret < 0) {
+			ERR("Asking UST to open wait_pipe failed for wait pipe: %s",
+			    ua_chan->obj->wait_pipe_path);
+		}
+	}
+
 skip_setup:
 	/* This start the UST tracing */
 	ret = ustctl_start_session(app->sock, ua_sess->handle);