Support iframe chatbot. (#3961)

### What problem does this PR solve?

#3909

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

api/db/services/canvas_service.py

@@ -14,6 +14,7 @@
 #  limitations under the License.
 #
 import json
+import traceback
 from uuid import uuid4
 from agent.canvas import Canvas
 from api.db.db_models import DB, CanvasTemplate, UserCanvas, API4Conversation
@@ -58,6 +59,8 @@ def completion(tenant_id, agent_id, question, session_id=None, stream=True, **kw
     if not isinstance(cvs.dsl, str):
         cvs.dsl = json.dumps(cvs.dsl, ensure_ascii=False)
     canvas = Canvas(cvs.dsl, tenant_id)
+    canvas.reset()
+    message_id = str(uuid4())
 
     if not session_id:
         session_id = get_uuid()
@@ -84,40 +87,24 @@ def completion(tenant_id, agent_id, question, session_id=None, stream=True, **kw
             return
         conv = API4Conversation(**conv)
     else:
-        session_id = session_id
         e, conv = API4ConversationService.get_by_id(session_id)
         assert e, "Session not found!"
         canvas = Canvas(json.dumps(conv.dsl), tenant_id)
-
-    if not conv.message:
-        conv.message = []
-    messages = conv.message
-    question = {
-        "role": "user",
-        "content": question,
-        "id": str(uuid4())
-    }
-    messages.append(question)
-    msg = []
-    for m in messages:
-        if m["role"] == "system":
-            continue
-        if m["role"] == "assistant" and not msg:
-            continue
-        msg.append(m)
-    if not msg[-1].get("id"):
-        msg[-1]["id"] = get_uuid()
-    message_id = msg[-1]["id"]
-
-    if not conv.reference:
-        conv.reference = []
-    conv.message.append({"role": "assistant", "content": "", "id": message_id})
-    conv.reference.append({"chunks": [], "doc_aggs": []})
+        canvas.messages.append({"role": "user", "content": question, "id": message_id})
+        canvas.add_user_input(question)
+        if not conv.message:
+            conv.message = []
+        conv.message.append({
+            "role": "user",
+            "content": question,
+            "id": message_id
+        })
+        if not conv.reference:
+            conv.reference = []
+        conv.reference.append({"chunks": [], "doc_aggs": []})
 
     final_ans = {"reference": [], "content": ""}
 
-    canvas.add_user_input(msg[-1]["content"])
-
     if stream:
         try:
             for ans in canvas.run(stream=stream):
@@ -141,6 +128,7 @@ def completion(tenant_id, agent_id, question, session_id=None, stream=True, **kw
             conv.dsl = json.loads(str(canvas))
             API4ConversationService.append_message(conv.id, conv.to_dict())
         except Exception as e:
+            traceback.print_exc()
             conv.dsl = json.loads(str(canvas))
             API4ConversationService.append_message(conv.id, conv.to_dict())
             yield "data:" + json.dumps({"code": 500, "message": str(e),