7 files changed, 773 insertions, 0 deletions
diff --git a/activestorage/app/models/active_storage/attachment.rb b/activestorage/app/models/active_storage/attachment.rb
new file mode 100644
index 0000000000..9f61a5dbf3
--- /dev/null
+++ b/activestorage/app/models/active_storage/attachment.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require "active_support/core_ext/module/delegation"
+
+# Attachments associate records with blobs. Usually that's a one record-many blobs relationship,
+# but it is possible to associate many different records with the same blob. If you're doing that,
+# you'll want to declare with <tt>has_one/many_attached :thingy, dependent: false</tt>, so that destroying
+# any one record won't destroy the blob as well. (Then you'll need to do your own garbage collecting, though).
+class ActiveStorage::Attachment < ActiveRecord::Base
+  self.table_name = "active_storage_attachments"
+
+  belongs_to :record, polymorphic: true, touch: true
+  belongs_to :blob, class_name: "ActiveStorage::Blob"
+
+  delegate_missing_to :blob
+
+  after_create_commit :analyze_blob_later
+
+  # Synchronously purges the blob (deletes it from the configured service) and destroys the attachment.
+  def purge
+    blob.purge
+    destroy
+  end
+
+  # Destroys the attachment and asynchronously purges the blob (deletes it from the configured service).
+  def purge_later
+    blob.purge_later
+    destroy
+  end
+
+  private
+    def analyze_blob_later
+      blob.analyze_later unless blob.analyzed?
+    end
+end
diff --git a/activestorage/app/models/active_storage/blob.rb b/activestorage/app/models/active_storage/blob.rb
new file mode 100644
index 0000000000..3b48ee72af
--- /dev/null
+++ b/activestorage/app/models/active_storage/blob.rb
@@ -0,0 +1,328 @@
+# frozen_string_literal: true
+
+require "active_storage/analyzer/null_analyzer"
+
+# A blob is a record that contains the metadata about a file and a key for where that file resides on the service.
+# Blobs can be created in two ways:
+#
+# 1. Subsequent to the file being uploaded server-side to the service via <tt>create_after_upload!</tt>.
+# 2. Ahead of the file being directly uploaded client-side to the service via <tt>create_before_direct_upload!</tt>.
+#
+# The first option doesn't require any client-side JavaScript integration, and can be used by any other back-end
+# service that deals with files. The second option is faster, since you're not using your own server as a staging
+# point for uploads, and can work with deployments like Heroku that do not provide large amounts of disk space.
+#
+# Blobs are intended to be immutable in as-so-far as their reference to a specific file goes. You're allowed to
+# update a blob's metadata on a subsequent pass, but you should not update the key or change the uploaded file.
+# If you need to create a derivative or otherwise change the blob, simply create a new blob and purge the old one.
+class ActiveStorage::Blob < ActiveRecord::Base
+  class InvariableError < StandardError; end
+  class UnpreviewableError < StandardError; end
+  class UnrepresentableError < StandardError; end
+
+  self.table_name = "active_storage_blobs"
+
+  has_secure_token :key
+  store :metadata, accessors: [ :analyzed ], coder: JSON
+
+  class_attribute :service
+
+  has_many :attachments
+
+  has_one_attached :preview_image
+
+  class << self
+    # You can used the signed ID of a blob to refer to it on the client side without fear of tampering.
+    # This is particularly helpful for direct uploads where the client-side needs to refer to the blob
+    # that was created ahead of the upload itself on form submission.
+    #
+    # The signed ID is also used to create stable URLs for the blob through the BlobsController.
+    def find_signed(id)
+      find ActiveStorage.verifier.verify(id, purpose: :blob_id)
+    end
+
+    # Returns a new, unsaved blob instance after the +io+ has been uploaded to the service.
+    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.upload io
+      end
+    end
+
+    # Returns a saved blob instance after the +io+ has been uploaded to the service. Note, the blob is first built,
+    # then the +io+ is uploaded, then the blob is saved. This is done this way to avoid uploading (which may take
+    # time), while having an open database transaction.
+    def create_after_upload!(io:, filename:, content_type: nil, metadata: nil)
+      build_after_upload(io: io, filename: filename, content_type: content_type, metadata: metadata).tap(&:save!)
+    end
+
+    # Returns a saved blob _without_ uploading a file to the service. This blob will point to a key where there is
+    # no file yet. It's intended to be used together with a client-side upload, which will first create the blob
+    # in order to produce the signed URL for uploading. This signed URL points to the key generated by the blob.
+    # Once the form using the direct upload is submitted, the blob can be associated with the right record using
+    # the signed ID.
+    def create_before_direct_upload!(filename:, byte_size:, checksum:, content_type: nil, metadata: nil)
+      create! filename: filename, byte_size: byte_size, checksum: checksum, content_type: content_type, metadata: metadata
+    end
+  end
+
+
+  # Returns a signed ID for this blob that's suitable for reference on the client-side without fear of tampering.
+  # It uses the framework-wide verifier on <tt>ActiveStorage.verifier</tt>, but with a dedicated purpose.
+  def signed_id
+    ActiveStorage.verifier.generate(id, purpose: :blob_id)
+  end
+
+  # Returns the key pointing to the file on the service that's associated with this blob. The key is in the
+  # standard secure-token format from Rails. So it'll look like: XTAPjJCJiuDrLk3TmwyJGpUo. This key is not intended
+  # to be revealed directly to the user. Always refer to blobs using the signed_id or a verified form of the key.
+  def key
+    # We can't wait until the record is first saved to have a key for it
+    self[:key] ||= self.class.generate_unique_secure_token
+  end
+
+  # Returns an ActiveStorage::Filename instance of the filename that can be
+  # queried for basename, extension, and a sanitized version of the filename
+  # that's safe to use in URLs.
+  def filename
+    ActiveStorage::Filename.new(self[:filename])
+  end
+
+  # Returns true if the content_type of this blob is in the image range, like image/png.
+  def image?
+    content_type.start_with?("image")
+  end
+
+  # Returns true if the content_type of this blob is in the audio range, like audio/mpeg.
+  def audio?
+    content_type.start_with?("audio")
+  end
+
+  # Returns true if the content_type of this blob is in the video range, like video/mp4.
+  def video?
+    content_type.start_with?("video")
+  end
+
+  # Returns true if the content_type of this blob is in the text range, like text/plain.
+  def text?
+    content_type.start_with?("text")
+  end
+
+
+  # Returns an ActiveStorage::Variant instance with the set of +transformations+ provided. This is only relevant for image
+  # files, and it allows any image to be transformed for size, colors, and the like. Example:
+  #
+  #   avatar.variant(resize: "100x100").processed.service_url
+  #
+  # This will create and process a variant of the avatar blob that's constrained to a height and width of 100px.
+  # Then it'll upload said variant to the service according to a derivative key of the blob and the transformations.
+  #
+  # Frequently, though, you don't actually want to transform the variant right away. But rather simply refer to a
+  # specific variant that can be created by a controller on-demand. Like so:
+  #
+  #   <%= image_tag Current.user.avatar.variant(resize: "100x100") %>
+  #
+  # This will create a URL for that specific blob with that specific variant, which the ActiveStorage::VariantsController
+  # can then produce on-demand.
+  #
+  # Raises ActiveStorage::Blob::InvariableError if ImageMagick cannot transform the blob. To determine whether a blob is
+  # variable, call ActiveStorage::Blob#previewable?.
+  def variant(transformations)
+    if variable?
+      ActiveStorage::Variant.new(self, ActiveStorage::Variation.wrap(transformations))
+    else
+      raise InvariableError
+    end
+  end
+
+  # Returns true if ImageMagick can transform the blob (its content type is in +ActiveStorage.variable_content_types+).
+  def variable?
+    ActiveStorage.variable_content_types.include?(content_type)
+  end
+
+
+  # Returns an ActiveStorage::Preview instance with the set of +transformations+ provided. A preview is an image generated
+  # from a non-image blob. Active Storage comes with built-in previewers for videos and PDF documents. The video previewer
+  # extracts the first frame from a video and the PDF previewer extracts the first page from a PDF document.
+  #
+  #   blob.preview(resize: "100x100").processed.service_url
+  #
+  # Avoid processing previews synchronously in views. Instead, link to a controller action that processes them on demand.
+  # Active Storage provides one, but you may want to create your own (for example, if you need authentication). Here’s
+  # how to use the built-in version:
+  #
+  #   <%= image_tag video.preview(resize: "100x100") %>
+  #
+  # This method raises ActiveStorage::Blob::UnpreviewableError if no previewer accepts the receiving blob. To determine
+  # whether a blob is accepted by any previewer, call ActiveStorage::Blob#previewable?.
+  def preview(transformations)
+    if previewable?
+      ActiveStorage::Preview.new(self, ActiveStorage::Variation.wrap(transformations))
+    else
+      raise UnpreviewableError
+    end
+  end
+
+  # Returns true if any registered previewer accepts the blob. By default, this will return true for videos and PDF documents.
+  def previewable?
+    ActiveStorage.previewers.any? { |klass| klass.accept?(self) }
+  end
+
+
+  # Returns an ActiveStorage::Preview for a previewable blob or an ActiveStorage::Variant for a variable image blob.
+  #
+  #   blob.representation(resize: "100x100").processed.service_url
+  #
+  # Raises ActiveStorage::Blob::UnrepresentableError if the receiving blob is neither variable nor previewable. Call
+  # ActiveStorage::Blob#representable? to determine whether a blob is representable.
+  #
+  # See ActiveStorage::Blob#preview and ActiveStorage::Blob#variant for more information.
+  def representation(transformations)
+    case
+    when previewable?
+      preview transformations
+    when variable?
+      variant transformations
+    else
+      raise UnrepresentableError
+    end
+  end
+
+  # Returns true if the blob is variable or previewable.
+  def representable?
+    variable? || previewable?
+  end
+
+
+  # Returns the URL of the blob on the service. This URL is intended to be short-lived for security and not used directly
+  # with users. Instead, the +service_url+ should only be exposed as a redirect from a stable, possibly authenticated URL.
+  # Hiding the +service_url+ behind a redirect also gives you the power to change services without updating all URLs. And
+  # it allows permanent URLs that redirect to the +service_url+ to be cached in the view.
+  def service_url(expires_in: service.url_expires_in, disposition: "inline")
+    service.url key, expires_in: expires_in, disposition: disposition, filename: filename, content_type: content_type
+  end
+
+  # Returns a URL that can be used to directly upload a file for this blob on the service. This URL is intended to be
+  # short-lived for security and only generated on-demand by the client-side JavaScript responsible for doing the uploading.
+  def service_url_for_direct_upload(expires_in: service.url_expires_in)
+    service.url_for_direct_upload key, expires_in: expires_in, content_type: content_type, content_length: byte_size, checksum: checksum
+  end
+
+  # Returns a Hash of headers for +service_url_for_direct_upload+ requests.
+  def service_headers_for_direct_upload
+    service.headers_for_direct_upload key, filename: filename, content_type: content_type, content_length: byte_size, checksum: checksum
+  end
+
+  # Uploads the +io+ to the service on the +key+ for this blob. Blobs are intended to be immutable, so you shouldn't be
+  # using this method after a file has already been uploaded to fit with a blob. If you want to create a derivative blob,
+  # you should instead simply create a new blob based on the old one.
+  #
+  # Prior to uploading, we compute the checksum, which is sent to the service for transit integrity validation. If the
+  # checksum does not match what the service receives, an exception will be raised. We also measure the size of the +io+
+  # and store that in +byte_size+ on the blob record.
+  #
+  # Normally, you do not have to call this method directly at all. Use the factory class methods of +build_after_upload+
+  # and +create_after_upload!+.
+  def upload(io)
+    self.checksum  = compute_checksum_in_chunks(io)
+    self.byte_size = io.size
+
+    service.upload(key, io, checksum: checksum)
+  end
+
+  # Downloads the file associated with this blob. If no block is given, the entire file is read into memory and returned.
+  # That'll use a lot of RAM for very large files. If a block is given, then the download is streamed and yielded in chunks.
+  def download(&block)
+    service.download key, &block
+  end
+
+
+  # Extracts and stores metadata from the file associated with this blob using a relevant analyzer. Active Storage comes
+  # with built-in analyzers for images and videos. See ActiveStorage::Analyzer::ImageAnalyzer and
+  # ActiveStorage::Analyzer::VideoAnalyzer for information about the specific attributes they extract and the third-party
+  # libraries they require.
+  #
+  # To choose the analyzer for a blob, Active Storage calls +accept?+ on each registered analyzer in order. It uses the
+  # first analyzer for which +accept?+ returns true when given the blob. If no registered analyzer accepts the blob, no
+  # metadata is extracted from it.
+  #
+  # In a Rails application, add or remove analyzers by manipulating +Rails.application.config.active_storage.analyzers+
+  # in an initializer:
+  #
+  #   # Add a custom analyzer for Microsoft Office documents:
+  #   Rails.application.config.active_storage.analyzers.append DOCXAnalyzer
+  #
+  #   # Remove the built-in video analyzer:
+  #   Rails.application.config.active_storage.analyzers.delete ActiveStorage::Analyzer::VideoAnalyzer
+  #
+  # Outside of a Rails application, manipulate +ActiveStorage.analyzers+ instead.
+  #
+  # You won't ordinarily need to call this method from a Rails application. New blobs are automatically and asynchronously
+  # analyzed via #analyze_later when they're attached for the first time.
+  def analyze
+    update! metadata: metadata.merge(extract_metadata_via_analyzer)
+  end
+
+  # Enqueues an ActiveStorage::AnalyzeJob which calls #analyze.
+  #
+  # This method is automatically called for a blob when it's attached for the first time. You can call it to analyze a blob
+  # again (e.g. if you add a new analyzer or modify an existing one).
+  def analyze_later
+    ActiveStorage::AnalyzeJob.perform_later(self)
+  end
+
+  # Returns true if the blob has been analyzed.
+  def analyzed?
+    analyzed
+  end
+
+
+  # Deletes the file on the service that's associated with this blob. This should only be done if the blob is going to be
+  # deleted as well or you will essentially have a dead reference. It's recommended to use the +#purge+ and +#purge_later+
+  # methods in most circumstances.
+  def delete
+    service.delete(key)
+    service.delete_prefixed("variants/#{key}/") if image?
+  end
+
+  # Deletes the file on the service and then destroys the blob record. This is the recommended way to dispose of unwanted
+  # blobs. Note, though, that deleting the file off the service will initiate a HTTP connection to the service, which may
+  # be slow or prevented, so you should not use this method inside a transaction or in callbacks. Use +#purge_later+ instead.
+  def purge
+    delete
+    destroy
+  end
+
+  # Enqueues an ActiveStorage::PurgeJob job that'll call +purge+. This is the recommended way to purge blobs when the call
+  # needs to be made from a transaction, a callback, or any other real-time scenario.
+  def purge_later
+    ActiveStorage::PurgeJob.perform_later(self)
+  end
+
+  private
+    def compute_checksum_in_chunks(io)
+      Digest::MD5.new.tap do |checksum|
+        while chunk = io.read(5.megabytes)
+          checksum << chunk
+        end
+
+        io.rewind
+      end.base64digest
+    end
+
+
+    def extract_metadata_via_analyzer
+      analyzer.metadata.merge(analyzed: true)
+    end
+
+    def analyzer
+      analyzer_class.new(self)
+    end
+
+    def analyzer_class
+      ActiveStorage.analyzers.detect { |klass| klass.accept?(self) } || ActiveStorage::Analyzer::NullAnalyzer
+    end
+end
diff --git a/activestorage/app/models/active_storage/filename.rb b/activestorage/app/models/active_storage/filename.rb
new file mode 100644
index 0000000000..b9413dec95
--- /dev/null
+++ b/activestorage/app/models/active_storage/filename.rb
@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+
+# Encapsulates a string representing a filename to provide convenient access to parts of it and sanitization.
+# A Filename instance is returned by ActiveStorage::Blob#filename, and is comparable so it can be used for sorting.
+class ActiveStorage::Filename
+  include Comparable
+
+  def initialize(filename)
+    @filename = filename
+  end
+
+  # Returns the part of the filename preceding any extension.
+  #
+  #   ActiveStorage::Filename.new("racecar.jpg").base # => "racecar"
+  #   ActiveStorage::Filename.new("racecar").base     # => "racecar"
+  #   ActiveStorage::Filename.new(".gitignore").base  # => ".gitignore"
+  def base
+    File.basename @filename, extension_with_delimiter
+  end
+
+  # Returns the extension of the filename (i.e. the substring following the last dot, excluding a dot at the
+  # beginning) with the dot that precedes it. If the filename has no extension, an empty string is returned.
+  #
+  #   ActiveStorage::Filename.new("racecar.jpg").extension_with_delimiter # => ".jpg"
+  #   ActiveStorage::Filename.new("racecar").extension_with_delimiter     # => ""
+  #   ActiveStorage::Filename.new(".gitignore").extension_with_delimiter  # => ""
+  def extension_with_delimiter
+    File.extname @filename
+  end
+
+  # Returns the extension of the filename (i.e. the substring following the last dot, excluding a dot at
+  # the beginning). If the filename has no extension, an empty string is returned.
+  #
+  #   ActiveStorage::Filename.new("racecar.jpg").extension_without_delimiter # => "jpg"
+  #   ActiveStorage::Filename.new("racecar").extension_without_delimiter     # => ""
+  #   ActiveStorage::Filename.new(".gitignore").extension_without_delimiter  # => ""
+  def extension_without_delimiter
+    extension_with_delimiter.from(1).to_s
+  end
+
+  alias_method :extension, :extension_without_delimiter
+
+  # Returns the sanitized filename.
+  #
+  #   ActiveStorage::Filename.new("foo:bar.jpg").sanitized # => "foo-bar.jpg"
+  #   ActiveStorage::Filename.new("foo/bar.jpg").sanitized # => "foo-bar.jpg"
+  #
+  # Characters considered unsafe for storage (e.g. \, $, and the RTL override character) are replaced with a dash.
+  def sanitized
+    @filename.encode(Encoding::UTF_8, invalid: :replace, undef: :replace, replace: "�").strip.tr("\u{202E}%$|:;/\t\r\n\\", "-")
+  end
+
+  def parameters #:nodoc:
+    Parameters.new self
+  end
+
+  # Returns the sanitized version of the filename.
+  def to_s
+    sanitized.to_s
+  end
+
+  def as_json(*)
+    to_s
+  end
+
+  def to_json
+    to_s
+  end
+
+  def <=>(other)
+    to_s.downcase <=> other.to_s.downcase
+  end
+end
diff --git a/activestorage/app/models/active_storage/filename/parameters.rb b/activestorage/app/models/active_storage/filename/parameters.rb
new file mode 100644
index 0000000000..fb9ea10e49
--- /dev/null
+++ b/activestorage/app/models/active_storage/filename/parameters.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class ActiveStorage::Filename::Parameters #:nodoc:
+  attr_reader :filename
+
+  def initialize(filename)
+    @filename = filename
+  end
+
+  def combined
+    "#{ascii}; #{utf8}"
+  end
+
+  TRADITIONAL_ESCAPED_CHAR = /[^ A-Za-z0-9!#$+.^_`|~-]/
+
+  def ascii
+    'filename="' + percent_escape(I18n.transliterate(filename.sanitized), TRADITIONAL_ESCAPED_CHAR) + '"'
+  end
+
+  RFC_5987_ESCAPED_CHAR = /[^A-Za-z0-9!#$&+.^_`|~-]/
+
+  def utf8
+    "filename*=UTF-8''" + percent_escape(filename.sanitized, RFC_5987_ESCAPED_CHAR)
+  end
+
+  def to_s
+    combined
+  end
+
+  private
+    def percent_escape(string, pattern)
+      string.gsub(pattern) do |char|
+        char.bytes.map { |byte| "%%%02X" % byte }.join
+      end
+    end
+end
diff --git a/activestorage/app/models/active_storage/preview.rb b/activestorage/app/models/active_storage/preview.rb
new file mode 100644
index 0000000000..be5053edae
--- /dev/null
+++ b/activestorage/app/models/active_storage/preview.rb
@@ -0,0 +1,90 @@
+# frozen_string_literal: true
+
+# Some non-image blobs can be previewed: that is, they can be presented as images. A video blob can be previewed by
+# extracting its first frame, and a PDF blob can be previewed by extracting its first page.
+#
+# A previewer extracts a preview image from a blob. Active Storage provides previewers for videos and PDFs:
+# ActiveStorage::Previewer::VideoPreviewer and ActiveStorage::Previewer::PDFPreviewer. Build custom previewers by
+# subclassing ActiveStorage::Previewer and implementing the requisite methods. Consult the ActiveStorage::Previewer
+# documentation for more details on what's required of previewers.
+#
+# To choose the previewer for a blob, Active Storage calls +accept?+ on each registered previewer in order. It uses the
+# first previewer for which +accept?+ returns true when given the blob. In a Rails application, add or remove previewers
+# by manipulating +Rails.application.config.active_storage.previewers+ in an initializer:
+#
+#   Rails.application.config.active_storage.previewers
+#   # => [ ActiveStorage::Previewer::PDFPreviewer, ActiveStorage::Previewer::VideoPreviewer ]
+#
+#   # Add a custom previewer for Microsoft Office documents:
+#   Rails.application.config.active_storage.previewers << DOCXPreviewer
+#   # => [ ActiveStorage::Previewer::PDFPreviewer, ActiveStorage::Previewer::VideoPreviewer, DOCXPreviewer ]
+#
+# Outside of a Rails application, modify +ActiveStorage.previewers+ instead.
+#
+# The built-in previewers rely on third-party system libraries:
+#
+# * {ffmpeg}[https://www.ffmpeg.org]
+# * {mupdf}[https://mupdf.com]
+#
+# These libraries are not provided by Rails. You must install them yourself to use the built-in previewers. Before you
+# install and use third-party software, make sure you understand the licensing implications of doing so.
+class ActiveStorage::Preview
+  class UnprocessedError < StandardError; end
+
+  attr_reader :blob, :variation
+
+  def initialize(blob, variation_or_variation_key)
+    @blob, @variation = blob, ActiveStorage::Variation.wrap(variation_or_variation_key)
+  end
+
+  # Processes the preview if it has not been processed yet. Returns the receiving Preview instance for convenience:
+  #
+  #   blob.preview(resize: "100x100").processed.service_url
+  #
+  # Processing a preview generates an image from its blob and attaches the preview image to the blob. Because the preview
+  # image is stored with the blob, it is only generated once.
+  def processed
+    process unless processed?
+    self
+  end
+
+  # Returns the blob's attached preview image.
+  def image
+    blob.preview_image
+  end
+
+  # Returns the URL of the preview's variant on the service. Raises ActiveStorage::Preview::UnprocessedError if the
+  # preview has not been processed yet.
+  #
+  # This method synchronously processes a variant of the preview image, so do not call it in views. Instead, generate
+  # a stable URL that redirects to the short-lived URL returned by this method.
+  def service_url(**options)
+    if processed?
+      variant.service_url(options)
+    else
+      raise UnprocessedError
+    end
+  end
+
+  private
+    def processed?
+      image.attached?
+    end
+
+    def process
+      previewer.preview { |attachable| image.attach(attachable) }
+    end
+
+    def variant
+      ActiveStorage::Variant.new(image, variation).processed
+    end
+
+
+    def previewer
+      previewer_class.new(blob)
+    end
+
+    def previewer_class
+      ActiveStorage.previewers.detect { |klass| klass.accept?(blob) }
+    end
+end
diff --git a/activestorage/app/models/active_storage/variant.rb b/activestorage/app/models/active_storage/variant.rb
new file mode 100644
index 0000000000..e08a2271ec
--- /dev/null
+++ b/activestorage/app/models/active_storage/variant.rb
@@ -0,0 +1,132 @@
+# frozen_string_literal: true
+
+require "active_storage/downloading"
+
+# Image blobs can have variants that are the result of a set of transformations applied to the original.
+# These variants are used to create thumbnails, fixed-size avatars, or any other derivative image from the
+# original.
+#
+# Variants rely on {MiniMagick}[https://github.com/minimagick/minimagick] for the actual transformations
+# of the file, so you must add <tt>gem "mini_magick"</tt> to your Gemfile if you wish to use variants.
+#
+# Note that to create a variant it's necessary to download the entire blob file from the service and load it
+# into memory. The larger the image, the more memory is used. Because of this process, you also want to be
+# considerate about when the variant is actually processed. You shouldn't be processing variants inline in a
+# template, for example. Delay the processing to an on-demand controller, like the one provided in
+# ActiveStorage::VariantsController.
+#
+# To refer to such a delayed on-demand variant, simply link to the variant through the resolved route provided
+# by Active Storage like so:
+#
+#   <%= image_tag Current.user.avatar.variant(resize: "100x100") %>
+#
+# This will create a URL for that specific blob with that specific variant, which the ActiveStorage::VariantsController
+# can then produce on-demand.
+#
+# When you do want to actually produce the variant needed, call +processed+. This will check that the variant
+# has already been processed and uploaded to the service, and, if so, just return that. Otherwise it will perform
+# the transformations, upload the variant to the service, and return itself again. Example:
+#
+#   avatar.variant(resize: "100x100").processed.service_url
+#
+# This will create and process a variant of the avatar blob that's constrained to a height and width of 100.
+# Then it'll upload said variant to the service according to a derivative key of the blob and the transformations.
+#
+# A list of all possible transformations is available at https://www.imagemagick.org/script/mogrify.php. You can
+# combine as many as you like freely:
+#
+#   avatar.variant(resize: "100x100", monochrome: true, flip: "-90")
+class ActiveStorage::Variant
+  include ActiveStorage::Downloading
+
+  WEB_IMAGE_CONTENT_TYPES = %w( image/png image/jpeg image/jpg image/gif )
+
+  attr_reader :blob, :variation
+  delegate :service, to: :blob
+
+  def initialize(blob, variation_or_variation_key)
+    @blob, @variation = blob, ActiveStorage::Variation.wrap(variation_or_variation_key)
+  end
+
+  # Returns the variant instance itself after it's been processed or an existing processing has been found on the service.
+  def processed
+    process unless processed?
+    self
+  end
+
+  # Returns a combination key of the blob and the variation that together identifies a specific variant.
+  def key
+    "variants/#{blob.key}/#{Digest::SHA256.hexdigest(variation.key)}"
+  end
+
+  # Returns the URL of the variant on the service. This URL is intended to be short-lived for security and not used directly
+  # with users. Instead, the +service_url+ should only be exposed as a redirect from a stable, possibly authenticated URL.
+  # Hiding the +service_url+ behind a redirect also gives you the power to change services without updating all URLs. And
+  # it allows permanent URLs that redirect to the +service_url+ to be cached in the view.
+  #
+  # Use <tt>url_for(variant)</tt> (or the implied form, like +link_to variant+ or +redirect_to variant+) to get the stable URL
+  # for a variant that points to the ActiveStorage::VariantsController, which in turn will use this +service_call+ method
+  # for its redirection.
+  def service_url(expires_in: service.url_expires_in, disposition: :inline)
+    service.url key, expires_in: expires_in, disposition: disposition, filename: filename, content_type: content_type
+  end
+
+  # Returns the receiving variant. Allows ActiveStorage::Variant and ActiveStorage::Preview instances to be used interchangeably.
+  def image
+    self
+  end
+
+  private
+    def processed?
+      service.exist?(key)
+    end
+
+    def process
+      open_image do |image|
+        transform image
+        format image
+        upload image
+      end
+    end
+
+
+    def filename
+      if WEB_IMAGE_CONTENT_TYPES.include?(blob.content_type)
+        blob.filename
+      else
+        ActiveStorage::Filename.new("#{blob.filename.base}.png")
+      end
+    end
+
+    def content_type
+      blob.content_type.presence_in(WEB_IMAGE_CONTENT_TYPES) || "image/png"
+    end
+
+
+    def open_image(&block)
+      image = download_image
+
+      begin
+        yield image
+      ensure
+        image.destroy!
+      end
+    end
+
+    def download_image
+      require "mini_magick"
+      MiniMagick::Image.create { |file| download_blob_to(file) }
+    end
+
+    def transform(image)
+      variation.transform(image)
+    end
+
+    def format(image)
+      image.format("PNG") unless WEB_IMAGE_CONTENT_TYPES.include?(blob.content_type)
+    end
+
+    def upload(image)
+      File.open(image.path, "r") { |file| service.upload(key, file) }
+    end
+end
diff --git a/activestorage/app/models/active_storage/variation.rb b/activestorage/app/models/active_storage/variation.rb
new file mode 100644
index 0000000000..0046e6870b
--- /dev/null
+++ b/activestorage/app/models/active_storage/variation.rb
@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+# A set of transformations that can be applied to a blob to create a variant. This class is exposed via
+# the ActiveStorage::Blob#variant method and should rarely be used directly.
+#
+# In case you do need to use this directly, it's instantiated using a hash of transformations where
+# the key is the command and the value is the arguments. Example:
+#
+#   ActiveStorage::Variation.new(resize: "100x100", monochrome: true, trim: true, rotate: "-90")
+#
+# A list of all possible transformations is available at https://www.imagemagick.org/script/mogrify.php.
+class ActiveStorage::Variation
+  attr_reader :transformations
+
+  class << self
+    # Returns a Variation instance based on the given variator. If the variator is a Variation, it is
+    # returned unmodified. If it is a String, it is passed to ActiveStorage::Variation.decode. Otherwise,
+    # it is assumed to be a transformations Hash and is passed directly to the constructor.
+    def wrap(variator)
+      case variator
+      when self
+        variator
+      when String
+        decode variator
+      else
+        new variator
+      end
+    end
+
+    # Returns a Variation instance with the transformations that were encoded by +encode+.
+    def decode(key)
+      new ActiveStorage.verifier.verify(key, purpose: :variation)
+    end
+
+    # Returns a signed key for the +transformations+, which can be used to refer to a specific
+    # variation in a URL or combined key (like <tt>ActiveStorage::Variant#key</tt>).
+    def encode(transformations)
+      ActiveStorage.verifier.generate(transformations, purpose: :variation)
+    end
+  end
+
+  def initialize(transformations)
+    @transformations = transformations
+  end
+
+  # Accepts an open MiniMagick image instance, like what's returned by <tt>MiniMagick::Image.read(io)</tt>,
+  # and performs the +transformations+ against it. The transformed image instance is then returned.
+  def transform(image)
+    transformations.each do |name, argument_or_subtransformations|
+      image.mogrify do |command|
+        if name.to_s == "combine_options"
+          argument_or_subtransformations.each do |subtransformation_name, subtransformation_argument|
+            pass_transform_argument(command, subtransformation_name, subtransformation_argument)
+          end
+        else
+          pass_transform_argument(command, name, argument_or_subtransformations)
+        end
+      end
+    end
+  end
+
+  # Returns a signed key for all the +transformations+ that this variation was instantiated with.
+  def key
+    self.class.encode(transformations)
+  end
+
+  private
+    def pass_transform_argument(command, method, argument)
+      if eligible_argument?(argument)
+        command.public_send(method, argument)
+      else
+        command.public_send(method)
+      end
+    end
+
+    def eligible_argument?(argument)
+      argument.present? && argument != true
+    end
+end