BUG: Merge Node "Wait for both Inputs to arrive" is not waiting and gives false output

[prononext]

For quite a long time I was trying to make a demo workflow to explain this strange behavior and finally I got it together.
The behavior seems simple but I find it quite hard to explain and to demonstrate. Sorry for the complication and I hope you understand it:

Inside a Batch I am using a IF Node to recognize a empty output (for instance on a HTTP Request node on which the requested dateframe has no output and then you want to get the next batch for the next dateframe.

But most times you use the Sort Node behind a HTTP Request node to split out fields or other Nodes where you cannot use the environments like {{ $json.column1 }} and just have to use the plain column name "column1".

To make the workflow continue on a positive output of the HTTP Request Node, needing to use a Sort node after it, you will get a error if you place the sort node behind the IF Node and if you place it before the IF node the workflow will not continue.

To solve this Problem I thought I can check with the IF Node for a empty item and if there is a empty item I continue to the next batch. If not I will route the If node with to a Merge Node with "Choose Branch / Wait for both Inputs to arrive" (Choosing Input 1) on Input 2 and Route the Node Output from before the IF Node to input 1 so the Batch will continue untill it reaches another point which will trigger the next batch.

But the Batch Node does not wait for both Inputs, it just fires a empty item, or the choosen output branch, regardless if both Inputs have arrived.

I noticed that behavior of the Merge node on several occations, not only in Split in Batches situations and it was running crazy 💯
I was using the Merge node on several similar occasions on very complex workflows and thought my data did was the point of error. I was thinking a long time how to explain it without having to make a too complex example, hopefully that is clear.

On this Screenshot it is clearly visible that the empty check IF Node has no output, but the Merge Node is behaving like it has a output.

**To Reproduce just use my Demo Workflow **

{
  "meta": {
    "instanceId": "12345"
  },
  "nodes": [
    {
      "parameters": {
        "values": {
          "string": [
            {
              "name": "column1"
            },
            {
              "name": "column2",
              "value": "not-empty"
            }
          ]
        },
        "options": {}
      },
      "id": "2b3f8757-0c8f-4160-89a4-d3ac2530caf5",
      "name": "Column 1/2 empty",
      "type": "n8n-nodes-base.set",
      "typeVersion": 2,
      "position": [
        680,
        180
      ]
    },
    {
      "parameters": {
        "values": {
          "string": [
            {
              "name": "column1",
              "value": "not-empty"
            },
            {
              "name": "column2"
            }
          ]
        },
        "options": {}
      },
      "id": "25c133df-b2c1-4097-8141-d0adf1485eca",
      "name": "Column 2/2 empty",
      "type": "n8n-nodes-base.set",
      "typeVersion": 2,
      "position": [
        680,
        320
      ]
    },
    {
      "parameters": {},
      "id": "4a7648f2-bdc4-4c3b-a1cc-eb6ebc9a968d",
      "name": "Append",
      "type": "n8n-nodes-base.merge",
      "typeVersion": 2.1,
      "position": [
        900,
        260
      ]
    },
    {
      "parameters": {
        "batchSize": 1,
        "options": {}
      },
      "id": "eadc9da8-689a-4aec-adc3-23f0f4adc5f3",
      "name": "Split In Batches of 1",
      "type": "n8n-nodes-base.splitInBatches",
      "typeVersion": 2,
      "position": [
        1060,
        260
      ]
    },
    {
      "parameters": {
        "conditions": {
          "string": [
            {
              "value1": "={{ $json.column1 }}",
              "operation": "isNotEmpty"
            }
          ]
        }
      },
      "id": "74626e43-298f-4bd2-812b-2ad33cd3e777",
      "name": "Empty Check",
      "type": "n8n-nodes-base.if",
      "typeVersion": 1,
      "position": [
        1560,
        260
      ]
    },
    {
      "parameters": {
        "values": {
          "string": [
            {
              "name": "column3",
              "value": "not-empty"
            },
            {
              "name": "column4",
              "value": "not-empty"
            }
          ]
        },
        "options": {}
      },
      "id": "b12c0f8c-d5b3-460a-98c4-6aee4a122aa6",
      "name": "Column 3+4 add",
      "type": "n8n-nodes-base.set",
      "typeVersion": 2,
      "position": [
        1240,
        260
      ]
    },
    {
      "parameters": {},
      "id": "3e00f80e-bb8d-409e-99db-0b2a2d47b8e9",
      "name": "START",
      "type": "n8n-nodes-base.noOp",
      "typeVersion": 1,
      "position": [
        460,
        240
      ]
    },
    {
      "parameters": {},
      "id": "9219a0f1-a686-4d08-981c-9573a5d5772a",
      "name": "STOP",
      "type": "n8n-nodes-base.noOp",
      "typeVersion": 1,
      "position": [
        1800,
        480
      ]
    },
    {
      "parameters": {
        "keepOnlySet": true,
        "options": {}
      },
      "id": "3461ec69-dd67-4945-9358-02e5fe84f2c1",
      "name": "Insert Empty Item",
      "type": "n8n-nodes-base.set",
      "typeVersion": 2,
      "position": [
        1400,
        260
      ]
    },
    {
      "parameters": {},
      "id": "b5b5d9e4-c07c-495d-bb9a-46feb4c90765",
      "name": "Click on Execute",
      "type": "n8n-nodes-base.manualTrigger",
      "typeVersion": 1,
      "position": [
        300,
        240
      ]
    },
    {
      "parameters": {
        "mode": "chooseBranch"
      },
      "id": "4329ce10-2428-48b2-a3d2-c457da7c058a",
      "name": "Faulty Merge",
      "type": "n8n-nodes-base.merge",
      "typeVersion": 2.1,
      "position": [
        1780,
        20
      ]
    }
  ],
  "connections": {
    "Column 1/2 empty": {
      "main": [
        [
          {
            "node": "Append",
            "type": "main",
            "index": 0
          }
        ]
      ]
    },
    "Column 2/2 empty": {
      "main": [
        [
          {
            "node": "Append",
            "type": "main",
            "index": 1
          }
        ]
      ]
    },
    "Append": {
      "main": [
        [
          {
            "node": "Split In Batches of 1",
            "type": "main",
            "index": 0
          }
        ]
      ]
    },
    "Split In Batches of 1": {
      "main": [
        [
          {
            "node": "Column 3+4 add",
            "type": "main",
            "index": 0
          }
        ],
        [
          {
            "node": "STOP",
            "type": "main",
            "index": 0
          }
        ]
      ]
    },
    "Empty Check": {
      "main": [
        [
          {
            "node": "Faulty Merge",
            "type": "main",
            "index": 1
          }
        ]
      ]
    },
    "Column 3+4 add": {
      "main": [
        [
          {
            "node": "Insert Empty Item",
            "type": "main",
            "index": 0
          },
          {
            "node": "Faulty Merge",
            "type": "main",
            "index": 0
          }
        ]
      ]
    },
    "START": {
      "main": [
        [
          {
            "node": "Column 1/2 empty",
            "type": "main",
            "index": 0
          },
          {
            "node": "Column 2/2 empty",
            "type": "main",
            "index": 0
          }
        ]
      ]
    },
    "Insert Empty Item": {
      "main": [
        [
          {
            "node": "Empty Check",
            "type": "main",
            "index": 0
          }
        ]
      ]
    },
    "Click on Execute": {
      "main": [
        [
          {
            "node": "START",
            "type": "main",
            "index": 0
          }
        ]
      ]
    },
    "Faulty Merge": {
      "main": [
        [
          {
            "node": "Split In Batches of 1",
            "type": "main",
            "index": 0
          }
        ]
      ]
    }
  }
}

Expected behavior
The Merge node should wait till BOTH Inputs have arrived. If not both inputs arrive it should do nothing.

Environment :

• OS: Ubuntu Linux 22.04
• n8n Version 0.233.1
• Node.js Version v16.20.0

[janober]

Thanks a lot for reporting this and especially for taking the time to create a workflow to quickly reproduce the problem. Is very appreciated, and it would be amazing if all issues that get opened in our repository would be that great prepared and written.

But I have good and bad news.
Let's start with the bad. It is a known issue and got reported already a lot in the past.
The good news is, however, that it will be fixed with the upcoming n8n v1, which we will probably release next week. The PRs which fix that problem are #4238 #6507.

I just tested your workflow in that branch. In "legacy" mode it behaved wrong as you reported above. But in the new one it behaved correctly. Here is a screenshot of the workflow after I did execute it:

[prononext]

Thanks @janober that was bugging me a long time.
Hope to get the fix soon 👍

[janober]

Yes, me also! There were a lot of cases where that made workflow building very complicated. I am also looking forward to having it released soon.

[Joffcom]

Ah Jan beat me to it :) this is also a duplicate of #3949

As this is fixed in an upcoming release I am going to get this marked as solved for now, Thanks again for reporting this @prononext 🙌🏻

[prononext]

@janober more reason to release it today 🥇

[janober]

@prononext you can already start to test our pre release v1. Is on Docker as n8nio/n8n:1.0.1

[janober]

Fix got released with n8n@1.0.5