Acquire read lock when marshalling container/task.

agent/api/container/json.go

@@ -0,0 +1,17 @@
+package container
+
+import (
+	"encoding/json"
+)
+
+// In order to override the MarshalJSON method while still using Go's marshalling logic inside, an alias type
+// is needed to avoid infinite recursion.
+type jContainer Container
+
+// MarshalJSON wraps Go's marshalling logic with a necessary read lock.
+func (c *Container) MarshalJSON() ([]byte, error) {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
+
+	return json.Marshal((*jContainer)(c))
+}

agent/api/task/json.go

@@ -0,0 +1,17 @@
+package task
+
+import (
+	"encoding/json"
+)
+
+// In order to override the MarshalJSON method while still using Go's marshalling logic inside, an alias type
+// is needed to avoid infinite recursion.
+type jTask Task
+
+// MarshalJSON wraps Go's marshalling logic with a necessary read lock.
+func (t *Task) MarshalJSON() ([]byte, error) {
+	t.lock.RLock()
+	defer t.lock.RUnlock()
+
+	return json.Marshal((*jTask)(t))
+}