[4/7] libtracefs: Add API tracefs_synth_snapshot()

diff --git a/Documentation/libtracefs-synth2.txt b/Documentation/libtracefs-synth2.txt
index a74e88ec0eda..194a4e021f62 100644
--- a/Documentation/libtracefs-synth2.txt
+++ b/Documentation/libtracefs-synth2.txt
@@ -4,7 +4,7 @@  libtracefs(3)
 NAME
 ----
 tracefs_synth_create, tracefs_synth_destroy, tracefs_synth_show,tracefs_synth_complete,
-tracefs_synth_get_start_hist,tracefs_synth_trace - Creation of synthetic events
+tracefs_synth_get_start_hist,tracefs_synth_trace,tracefs_synth_snapshot - Creation of synthetic events
 
 SYNOPSIS
 --------
@@ -23,6 +23,8 @@  struct tracefs_hist pass:[*]tracefs_synth_get_start_hist(struct tracefs_synth pa
 
 int tracefs_synth_trace(struct tracefs_synth pass:[*]synth,
 			enum tracefs_synth_handler type, const char pass:[*]var);
+int tracefs_synth_snapshot(struct tracefs_synth pass:[*]synth,
+			   enum tracefs_synth_handler type, const char pass:[*]var);
 --
 
 DESCRIPTION
@@ -75,6 +77,13 @@  when the given variable _var_ hits a new max for the matching keys. Or
 *TRACEFS_SYNTH_HANDLE_CHANGE* for when the _var_ changes. _var_ must be one of
 the _name_ elements used in *tracefs_synth_add_end_field*(3).
 
+*tracefs_synth_snapshot*() When the given variable _var_ is either a new max if
+_handler_ is TRACEFS_SYNTH_HANDLE_MAX_ or simpy changed if TRACEFS_SYNTH_HANDLE_CHANGE_
+then take a "snapshot" of the buffer. The snapshot moves the normal "trace" buffer
+into a "snapshot" buffer, that can be accessed via the "snapshot" file in the
+top level tracefs directory, or one of the instances.  _var_ changes. _var_ must be one of
+the _name_ elements used in *tracefs_synth_add_end_field*(3).
+
 RETURN VALUE
 ------------
 Returns zero on success or -1 on error.
diff --git a/include/tracefs.h b/include/tracefs.h
index 2faa564a860f..45a5bb2df2de 100644
--- a/include/tracefs.h
+++ b/include/tracefs.h
@@ -478,6 +478,8 @@  int tracefs_synth_append_end_filter(struct tracefs_synth *synth,
 				    const char *val);
 int tracefs_synth_trace(struct tracefs_synth *synth,
 			enum tracefs_synth_handler type, const char *field);
+int tracefs_synth_snapshot(struct tracefs_synth *synth,
+			   enum tracefs_synth_handler type, const char *field);
 bool tracefs_synth_complete(struct tracefs_synth *synth);
 struct tracefs_hist *tracefs_synth_get_start_hist(struct tracefs_synth *synth);
 int tracefs_synth_create(struct tracefs_instance *instance,
diff --git a/src/tracefs-hist.c b/src/tracefs-hist.c
index 562ec65088a9..2c9c37b8ec23 100644
--- a/src/tracefs-hist.c
+++ b/src/tracefs-hist.c
@@ -524,6 +524,7 @@  int tracefs_hist_append_filter(struct tracefs_hist *hist,
 enum action_type {
 	ACTION_NONE,
 	ACTION_TRACE,
+	ACTION_SNAPSHOT,
 };
 
 struct action {
@@ -1414,6 +1415,36 @@  int tracefs_synth_trace(struct tracefs_synth *synth,
 	return 0;
 }
 
+/**
+ * tracefs_synth_snapshot - create a snapshot command to the histogram
+ * @synth: The tracefs_synth descriptor
+ * @type: The type of handler to attach the snapshot action with
+ * @field: The field for handlers onmax and onchange
+ *
+ * Add the action to do a snapshot for handlers onmax and onchange.
+ *
+ * Returns 0 on succes, -1 on error.
+ */
+int tracefs_synth_snapshot(struct tracefs_synth *synth,
+			   enum tracefs_synth_handler type, const char *field)
+{
+	struct action *action;
+
+	if (!synth || !field || (type == TRACEFS_SYNTH_HANDLE_MATCH)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	action = create_action(type, synth, field);
+	if (!action)
+		return -1;
+
+	action->type = ACTION_SNAPSHOT;
+	action->handler = type;
+	add_action(synth, action);
+	return 0;
+}
+
 static char *create_synthetic_event(struct tracefs_synth *synth)
 {
 	char *synthetic_event;
@@ -1562,6 +1593,9 @@  static char *create_actions(char *hist, struct tracefs_synth *synth)
 		case ACTION_TRACE:
 			hist = create_trace(hist, synth);
 			break;
+		case ACTION_SNAPSHOT:
+			hist = append_string(hist, NULL, ".snapshot()");
+			break;
 		default:
 			continue;
 		}