[lttng-dev] [PATCH lttng-tools 1/2] Fix: add-context cannot be performed after a session has been started

[Jonathan Rajotte]

The following scenario lead to a corrupted trace/metadata layout problem:
  - lttng create test
  - lttng enable-channel -u test
  - lttng enable-event -u -a -c test
  - lttng start
  - ./instrumented-application
  - lttng stop
  - lttng add-context -u -t procname -c test
  - lttng start
  - ./instrumented-application
  - lttng stop
  - lttng view

Babeltrace 1.5.x will fail with:

[error] Unexpected end of packet. Either the trace data stream is corrupted or metadata description does not match data layout.
[error] Reading event failed.
Error printing trace.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien at efficios.com>
---
 src/bin/lttng-sessiond/cmd.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/bin/lttng-sessiond/cmd.c b/src/bin/lttng-sessiond/cmd.c
index a295059..1a60ce3 100644
--- a/src/bin/lttng-sessiond/cmd.c
+++ b/src/bin/lttng-sessiond/cmd.c
@@ -1650,6 +1650,17 @@ int cmd_add_context(struct ltt_session *session, enum lttng_domain_type domain,
 	int ret, chan_kern_created = 0, chan_ust_created = 0;
 	char *app_ctx_provider_name = NULL, *app_ctx_name = NULL;
 
+	/*
+	 * Don't try to add a context if the session has been started at
+	 * some point in time before. The tracer does not allow it and would
+	 * result in a corrupted trace.
+	 */
+	if (session->has_been_started) {
+		ret = LTTNG_ERR_TRACE_ALREADY_STARTED;
+		goto end;
+	}
+
+
 	if (ctx->ctx == LTTNG_EVENT_CONTEXT_APP_CONTEXT) {
 		app_ctx_provider_name = ctx->u.app_ctx.provider_name;
 		app_ctx_name = ctx->u.app_ctx.ctx_name;
-- 
2.7.4