[lttng-dev] [PATCH lttng-tools] Better error reporting when a snapshot record size is too small

[Jérémie Galarneau]

Print an error message when a snapshot record is made with a
max size smaller than the subbuffers. This limitation is now
documented in the man page.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau at efficios.com>
---
 doc/man/lttng.1                   |  4 ++-
 src/bin/lttng/commands/snapshot.c | 59 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/doc/man/lttng.1 b/doc/man/lttng.1
index 972f71c..bf1b67e 100644
--- a/doc/man/lttng.1
+++ b/doc/man/lttng.1
@@ -787,7 +787,9 @@ List the output of a session. Attributes of the output are printed.
 Snapshot a session's buffer(s) for all domains. If an URL is specified, it is
 used instead of a previously added output. Specifying only a name or/and a max
 size will override the current output values. For instance, you can record a
-snapshot with a custom maximum size or with a different name.
+snapshot with a custom maximum size or with a different name. However, the
+max size must be high enough to contain a complete subbuffer. See the
+--subbuf-size switch for default subbuffer sizes.
 
 .nf
 $ lttng snapshot add-output -n mysnapshot file:///data/snapshot
diff --git a/src/bin/lttng/commands/snapshot.c b/src/bin/lttng/commands/snapshot.c
index c704eee..7fda949 100644
--- a/src/bin/lttng/commands/snapshot.c
+++ b/src/bin/lttng/commands/snapshot.c
@@ -125,6 +125,56 @@ static int count_arguments(const char **argv)
 	return i;
 }
 
+static uint64_t get_largest_subbuf()
+{
+	int domain_count;
+	int channel_count;
+	int domain_idx;
+	int channel_idx;
+	struct lttng_domain *domains;
+	uint64_t largest_subbuf = 0;
+
+	domain_count = lttng_list_domains(current_session_name, &domains);
+	if (domain_count < 0) {
+		ERR("Unable to list session %s's domains",
+			current_session_name);
+		goto end;
+	}
+
+	for (domain_idx = 0; domain_idx < domain_count; domain_idx++) {
+		struct lttng_channel *channels;
+		struct lttng_handle *handle = lttng_create_handle(
+			current_session_name, &domains[domain_idx]);
+
+		if (!handle) {
+			ERR("Unable to create handle for session %s",
+				current_session_name);
+			goto end;
+		}
+
+		channel_count = lttng_list_channels(handle, &channels);
+		if (channel_count < 0) {
+			ERR("Unable to list channels for session %s",
+				current_session_name);
+			goto end;
+		}
+
+		for (channel_idx = 0; channel_idx < channel_count;
+			channel_idx++) {
+			if (channels[channel_idx].attr.subbuf_size >
+				largest_subbuf) {
+				largest_subbuf =
+					channels[channel_idx].attr.subbuf_size;
+			}
+		}
+		free(channels);
+		lttng_destroy_handle(handle);
+	}
+end:
+	free(domains);
+	return largest_subbuf;
+}
+
 /*
  * Create a snapshot output object from arguments using the given URL.
  *
@@ -160,6 +210,15 @@ static struct lttng_snapshot_output *create_output_from_args(const char *url)
 	}
 
 	if (opt_max_size) {
+		/* Validate that the max size can contain one subbuffer. */
+		uint64_t largest_subbuf = get_largest_subbuf();
+		if (largest_subbuf == 0) {
+			goto error;
+		} else if (largest_subbuf > opt_max_size) {
+			ERR("Snapshot size must be greater or equal to the largest subbuffer's size (%zu).",
+				largest_subbuf);
+			goto error;
+		}
 		ret = lttng_snapshot_output_set_size(opt_max_size, output);
 		if (ret < 0) {
 			goto error;
-- 
1.9.1