active_storage_key_patches.rb

# Fixes for two ActiveStorage bugs which, separately and especially
# in combination, can lead to data being overwritten
module BlobKeyFixes
  # How many times we should try to regenerate the key when a collision is encountered
  TRIES_ON_NON_UNIQUE_KEY = 5

  # Overridden to actually save the blob before attempting to upload to it,
  # which makes the uniqueness constraint fire earlier if it gets triggered.
  # Is a fix for https://github.com/rails/rails/issues/34805
  def build_after_upload(io:, filename:, content_type: nil, metadata: nil)
    new.tap do |blob|
      blob.filename     = filename
      blob.content_type = content_type
      blob.metadata     = metadata
      blob.byte_size    = 0 # Default value to get past constraint
      blob.checksum     = "pending" # Default value to get past constraint
      tries_remaining = TRIES_ON_NON_UNIQUE_KEY
      begin
        blob.save! # Will raise a uniqueness error if the constraint is hit
      rescue ActiveRecord::RecordNotUnique
        tries_remaining -= 1
        raise if tries_remaining.zero?
        blob.key = blob.class.generate_unique_secure_token
        retry
      end
      blob.upload(io)
    end
  end

  # Overridden to generate a base36 lowercase token instead of
  # a base58 case-sensitive one.
  # Is a fix for https://github.com/rails/rails/issues/34804
  def generate_unique_secure_token
    # With base58 used in Rails by default, we get
    # [6] pry(main)> (58**24)
    # => 210025412090735248552623050583059191142809
    # possible values. The closest power to this with base36
    # offering "no less than" possible values is 28:
    # [14] pry(main)> (36**28)
    # => 37711171281396032013366321198900157303750656
    SecureRandom.base58(28).downcase
  end
end

# In the initializer the ActiveStorage::Blob model is not yet available,
# so delay our monkeypatch until it becomes available
Rails.application.config.after_initialize do
  Rails.logger.info { "ActiveStorage blob key override installed" }
  # We need to prepend because `build_after_upload` is defined on
  # the Blob class itself
  class << ::ActiveStorage::Blob
    prepend BlobKeyFixes
  end
end