Background: Fuchsia and Android artifacts with content hashing require some extra work from scripts in the flutter repository. These do not have access to LUCI's "content_hash" variable. Fuchsia in particular handles the complete upload via build_fuchsia_artifacts.py

Problem: If two builds are in the merge queue with the same content hash; they can race to build and upload the fuchsia artifacts and tag them with the same content_hash:[a-f0-9]{40} tag. We tried to handle this here:

  already_exists = CheckCIPDPackageExists('flutter/fuchsia', content_tag)
  if already_exists:
    print('CIPD package flutter/fuchsia tag %s already exists!' % content_tag)
    # do not return; content hash can match multiple PRs (reverts, framework only)
  else:
    command.extend(['-tag', content_tag])

Instead we should upload the artifact and tag with the git_revision, and then check if the content_hash exists before tagging with a different call to RunCIPDCommandWithRetries