[lttng-dev] [PATCH v2 lttng-tools] Filter: Notify the user when a filter is already enabled on event(s)

[Christian Babeux]

When using the enable-event command in conjunction with the --filter option,
a user can specify a filter expression to refine the trace output.

As stated in the lttng(1) man page, only the first activation of a filter on
an event will work. Subsequent activation of any filter expression on the
same event will fail.

e.g:

> lttng enable-event app:tp -s session -u --filter 'somefield > 42'

# Case: invalid filter expression
> lttng enable-event app:tp -s session -u --filter 'invalid expression'
  Error: Error setting filter
  Warning: Some command(s) went wrong

> ...

# Case: filter already enabled for event app:tp
> lttng enable-event app:tp -s session -u --filter 'someotherfield < 42'
  Error: Error setting filter
  Warning: Some command(s) went wrong

This commit differentiate the case where a filter was already set for the
specified event from the generic 'Error setting filter' error message.

e.g:

> lttng enable-event app:tp -s session -u --filter 'somefield > 42'

# Case: invalid filter expression
> lttng enable-event app:tp -s session -u --filter 'invalid expression'
  Error: Error setting filter
  Warning: Some command(s) went wrong

> ...

# Case: filter already enabled for event app:tp
> lttng enable-event app:tp -s session -u --filter 'someotherfield < 42'
  Error: Filter on event app:tp is already enabled (channel 0, session session)
  Warning: Some command(s) went wrong

Signed-off-by: Christian Babeux <christian.babeux at efficios.com>
---
 src/bin/lttng/commands/enable_events.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/src/bin/lttng/commands/enable_events.c b/src/bin/lttng/commands/enable_events.c
index a2c8a68..5a81fdb 100644
--- a/src/bin/lttng/commands/enable_events.c
+++ b/src/bin/lttng/commands/enable_events.c
@@ -384,7 +384,16 @@ static int enable_events(char *session_name)
 			ret = lttng_set_event_filter(handle, ev.name, channel_name,
 						opt_filter);
 			if (ret < 0) {
-				ERR("Error setting filter");
+				switch (-ret) {
+				case LTTCOMM_FILTER_EXIST:
+					ERR("Filter on events is already enabled (channel %s, session %s)",
+						channel_name, session_name);
+					break;
+				default:
+					ERR("Error setting filter");
+					break;
+				}
+
 				ret = -1;
 				goto error;
 			}
@@ -554,7 +563,16 @@ static int enable_events(char *session_name)
 			ret = lttng_set_event_filter(handle, ev.name,
 				channel_name, opt_filter);
 			if (ret < 0) {
-				ERR("Error setting filter");
+				switch (-ret) {
+				case LTTCOMM_FILTER_EXIST:
+					ERR("Filter on event %s is already enabled (channel %s, session %s)",
+						event_name, channel_name, session_name);
+					break;
+				default:
+					ERR("Error setting filter");
+					break;
+				}
+
 				ret = -1;
 				goto error;
 			}
-- 
1.7.11.4