Duplicated input indices

[TilBlechschmidt]

I am by no means an expert and just started reading into BuildKit and the inner workings of LLB but this looks not quite right:

The input is listed twice with the same index. In this specific case (buildkit-llb scratch example from current master) it doesn't cause any issues due to the equal digest but in the case of the highly-parallel example two different digests are occupying the same input index. I noticed some funky business with the index matching going on earlier and this is likely the cause for that.

Might be worth a look :)

{
  "Op": {
    "inputs": [
      {
        "digest": "sha256:4db4c9e0ad6c39235aa82237c23fc85ce11c5e1069cd86442c69019b66906f3b",
        "index": 0
      },
      {
        "digest": "sha256:4db4c9e0ad6c39235aa82237c23fc85ce11c5e1069cd86442c69019b66906f3b",
        "index": 0
      }
    ],
    "Op": {
      "file": {
        "actions": [
          {
            "input": 1,
            "secondaryInput": 0,
            "output": 0,
            "Action": {
              "copy": {
                "src": "/file0",
                "dest": "/file1",
                "mode": -1,
                "timestamp": -1
              }
            }
          },
          {
            "input": 2,
            "secondaryInput": 2,
            "output": 1,
            "Action": {
              "copy": {
                "src": "/file0",
                "dest": "/file2",
                "mode": -1,
                "timestamp": -1
              }
            }
          }
        ]
      }
    }
  },
  "Digest": "sha256:2debf72ab9472756432c19c2dd418b46b394ee8e5877ab8feae84bdd4da3ff14",
  "OpMetadata": {
    "description": {
      "llb.customname": "do multiple file system manipulations"
    },
    "caps": {
      "file.base": true
    }
  }
}

Command that produced this output:

cargo run --example=scratch | buildctl debug dump-llb | jq . | less