Ask Bazel for the hash of a target artifact, but not the artifact itself?

[njlr]

For context, I want to determine which artifacts are "new" and need to be pushed up to artifact stores, docker registries, etc. Here "new" means that the artifact has a hash that has not been seen before.

I was wondering if there is a way to ask Bazel for the hash of a given target's output with a minimal amount of building / fetching?

Typically, the target artifact is in the remote cache, but since it can be very large, I would like to avoid downloading it.

[meisterT]

That info is already available in the BEP. Example:

$ bazel build --config=remote --remote_download_minimal --build_event_json_file=/tmp/bep.json //src:bazel
[...]
INFO: Found 1 target...
Target //src:bazel up-to-date:
  bazel-bin/src/bazel
INFO: Elapsed time: 167.350s, Critical Path: 157.45s
INFO: 3523 processes: 1580 remote cache hit, 32 internal, 2 linux-sandbox, 1 local, 1908 remote.
INFO: Build completed successfully, 3523 total actions
INFO: Build Event Protocol files produced successfully.
$ ls -al bazel-bin/src/bazel
ls: cannot access 'bazel-bin/src/bazel': No such file or directory
$ cat /tmp/bep.json | jq '.completed | select( . != null ) | .importantOutput'
[
  {
    "name": "src/bazel",
    "uri": "bytestream://remotebuildexecution.googleapis.com/projects/bazel-untrusted/instances/default_instance/blobs/7a817ce9b686a93878f15a4ea93c8d7e018e7d753745c66695e5f21c6d0754a9/77189946",
    "pathPrefix": [
      "bazel-out",
      "k8-fastbuild",
      "bin"
    ],
    "digest": "7a817ce9b686a93878f15a4ea93c8d7e018e7d753745c66695e5f21c6d0754a9",
    "length": "77189946"
  }
]

[njlr]

Are there any important settings in --config=remote ?

Is there an easy way to tie the name back to a target?

[meisterT]

The label is also in the BEP, see:

$ cat /tmp/bep.json | jq 'select(.id | contains({"targetCompleted": {}}) )'
{
  "id": {
    "targetCompleted": {
      "label": "//src:bazel",
      "configuration": {
        "id": "7616bb24256a7f1bcce75764003040f2718e80f7f62d87bca6d909590fb4383a"
      }
    }
  },
  "completed": {
    "success": true,
    "outputGroup": [
      {
        "name": "default",
        "fileSets": [
          {
            "id": "0"
          }
        ]
      }
    ],
    "importantOutput": [
      {
        "name": "src/bazel",
        "uri": "bytestream://remotebuildexecution.googleapis.com/projects/bazel-untrusted/instances/default_instance/blobs/7a817ce9b686a93878f15a4ea93c8d7e018e7d753745c66695e5f21c6d0754a9/77189946",
        "pathPrefix": [
          "bazel-out",
          "k8-fastbuild",
          "bin"
        ],
        "digest": "7a817ce9b686a93878f15a4ea93c8d7e018e7d753745c66695e5f21c6d0754a9",
        "length": "77189946"
      }
    ]
  }
}

The --config=remote is just configuring building remotely for us, see https://github.com/bazelbuild/bazel/blob/master/.bazelrc#L25, nothing special in it.

[njlr]

Thank you! This gives me enough to go on