Extract metadata from images and videos

13 files changed, 218 insertions, 30 deletions

diff --git a/activestorage/lib/active_storage.rb b/activestorage/lib/active_storage.rb
index 9ac1e7ab54..cfdb2a8acd 100644
--- a/activestorage/lib/active_storage.rb
+++ b/activestorage/lib/active_storage.rb
@@ -34,7 +34,10 @@ module ActiveStorage
   autoload :Attached
   autoload :Service
   autoload :Previewer
+  autoload :Analyzer
 
+  mattr_accessor :logger
   mattr_accessor :verifier
   mattr_accessor :previewers, default: []
+  mattr_accessor :analyzers, default: []
 end
diff --git a/activestorage/lib/active_storage/analyzer.rb b/activestorage/lib/active_storage/analyzer.rb
new file mode 100644
index 0000000000..837785a12b
--- /dev/null
+++ b/activestorage/lib/active_storage/analyzer.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require "active_storage/downloading"
+
+module ActiveStorage
+  # This is an abstract base class for analyzers, which extract metadata from blobs. See
+  # ActiveStorage::Analyzer::ImageAnalyzer for an example of a concrete subclass.
+  class Analyzer
+    include Downloading
+
+    attr_reader :blob
+
+    # Implement this method in a concrete subclass. Have it return true when given a blob from which
+    # the analyzer can extract metadata.
+    def self.accept?(blob)
+      false
+    end
+
+    def initialize(blob)
+      @blob = blob
+    end
+
+    # Override this method in a concrete subclass. Have it return a Hash of metadata.
+    def metadata
+      raise NotImplementedError
+    end
+
+    private
+      def logger
+        ActiveStorage.logger
+      end
+  end
+end
diff --git a/activestorage/lib/active_storage/analyzer/image_analyzer.rb b/activestorage/lib/active_storage/analyzer/image_analyzer.rb
new file mode 100644
index 0000000000..b25d2092b9
--- /dev/null
+++ b/activestorage/lib/active_storage/analyzer/image_analyzer.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module ActiveStorage
+  # Extracts width and height in pixels from an image blob.
+  #
+  # Example:
+  #
+  #   ActiveStorage::Analyzer::ImageAnalyzer.new(blob).metadata
+  #   # => { width: 4104, height: 2736 }
+  #
+  # This analyzer relies on the third-party [MiniMagick]{https://github.com/minimagick/minimagick} gem. MiniMagick requires
+  # the [ImageMagick]{http://www.imagemagick.org} system library. These libraries are not provided by Rails; you must
+  # install them yourself to use this analyzer.
+  class Analyzer::ImageAnalyzer < Analyzer
+    def self.accept?(blob)
+      blob.image?
+    end
+
+    def metadata
+      read_image do |image|
+        { width: image.width, height: image.height }
+      end
+    rescue LoadError
+      logger.info "Skipping image analysis because the mini_magick gem isn't installed"
+      {}
+    end
+
+    private
+      def read_image
+        download_blob_to_tempfile do |file|
+          require "mini_magick"
+          yield MiniMagick::Image.new(file.path)
+        end
+      end
+  end
+end
diff --git a/activestorage/lib/active_storage/analyzer/null_analyzer.rb b/activestorage/lib/active_storage/analyzer/null_analyzer.rb
new file mode 100644
index 0000000000..8ff7ce48e5
--- /dev/null
+++ b/activestorage/lib/active_storage/analyzer/null_analyzer.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module ActiveStorage
+  class Analyzer::NullAnalyzer < Analyzer # :nodoc:
+    def self.accept?(blob)
+      true
+    end
+
+    def metadata
+      {}
+    end
+  end
+end
diff --git a/activestorage/lib/active_storage/analyzer/video_analyzer.rb b/activestorage/lib/active_storage/analyzer/video_analyzer.rb
new file mode 100644
index 0000000000..408b5e58e9
--- /dev/null
+++ b/activestorage/lib/active_storage/analyzer/video_analyzer.rb
@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+require "active_support/core_ext/hash/compact"
+
+module ActiveStorage
+  # Extracts the following from a video blob:
+  #
+  # * Width (pixels)
+  # * Height (pixels)
+  # * Duration (seconds)
+  # * Angle (degrees)
+  # * Aspect ratio
+  #
+  # Example:
+  #
+  #   ActiveStorage::VideoAnalyzer.new(blob).metadata
+  #   # => { width: 640, height: 480, duration: 5.0, angle: 0, aspect_ratio: [4, 3] }
+  #
+  # This analyzer requires the {ffmpeg}[https://www.ffmpeg.org] system library, which is not provided by Rails. You must
+  # install ffmpeg yourself to use this analyzer.
+  class Analyzer::VideoAnalyzer < Analyzer
+    def self.accept?(blob)
+      blob.video?
+    end
+
+    def metadata
+      { width: width, height: height, duration: duration, angle: angle, aspect_ratio: aspect_ratio }.compact
+    end
+
+    private
+      def width
+        Integer(video_stream["width"]) if video_stream["width"]
+      end
+
+      def height
+        Integer(video_stream["height"]) if video_stream["height"]
+      end
+
+      def duration
+        Float(video_stream["duration"]) if video_stream["duration"]
+      end
+
+      def angle
+        Integer(tags["rotate"]) if tags["rotate"]
+      end
+
+      def aspect_ratio
+        if descriptor = video_stream["display_aspect_ratio"]
+          descriptor.split(":", 2).collect(&:to_i)
+        end
+      end
+
+
+      def tags
+        @tags ||= video_stream["tags"] || {}
+      end
+
+      def video_stream
+        @video_stream ||= streams.detect { |stream| stream["codec_type"] == "video" } || {}
+      end
+
+      def streams
+        probe["streams"] || []
+      end
+
+      def probe
+        download_blob_to_tempfile { |file| probe_from(file) }
+      end
+
+      def probe_from(file)
+        IO.popen([ "ffprobe", "-print_format", "json", "-show_streams", "-v", "error", file.path ]) do |output|
+          JSON.parse(output.read)
+        end
+      rescue Errno::ENOENT
+        logger.info "Skipping video analysis because ffmpeg isn't installed"
+        {}
+      end
+  end
+end
diff --git a/activestorage/lib/active_storage/attached/one.rb b/activestorage/lib/active_storage/attached/one.rb
index c66be08f58..dc19512484 100644
--- a/activestorage/lib/active_storage/attached/one.rb
+++ b/activestorage/lib/active_storage/attached/one.rb
@@ -59,12 +59,16 @@ module ActiveStorage
       def replace(attachable)
         blob.tap do
           transaction do
-            destroy
+            destroy_attachment
             write_attachment create_attachment_from(attachable)
           end
         end.purge_later
       end
 
+      def destroy_attachment
+        attachment.destroy
+      end
+
       def create_attachment_from(attachable)
         ActiveStorage::Attachment.create!(record: record, name: name, blob: create_blob_from(attachable))
       end
diff --git a/activestorage/lib/active_storage/downloading.rb b/activestorage/lib/active_storage/downloading.rb
new file mode 100644
index 0000000000..bcf42f610e
--- /dev/null
+++ b/activestorage/lib/active_storage/downloading.rb
@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module ActiveStorage
+  module Downloading
+    private
+      # Opens a new tempfile and copies blob data into it. Yields the tempfile.
+      def download_blob_to_tempfile # :doc:
+        Tempfile.open("ActiveStorage") do |file|
+          download_blob_to file
+          yield file
+        end
+      end
+
+      # Efficiently download blob data into the given file.
+      def download_blob_to(file) # :doc:
+        file.binmode
+        blob.download { |chunk| file.write(chunk) }
+        file.rewind
+      end
+  end
+end
diff --git a/activestorage/lib/active_storage/engine.rb b/activestorage/lib/active_storage/engine.rb
index 335eae8dd8..a01a14cd83 100644
--- a/activestorage/lib/active_storage/engine.rb
+++ b/activestorage/lib/active_storage/engine.rb
@@ -6,20 +6,22 @@ require "active_storage"
 require "active_storage/previewer/pdf_previewer"
 require "active_storage/previewer/video_previewer"
 
+require "active_storage/analyzer/image_analyzer"
+require "active_storage/analyzer/video_analyzer"
+
 module ActiveStorage
   class Engine < Rails::Engine # :nodoc:
     isolate_namespace ActiveStorage
 
     config.active_storage = ActiveSupport::OrderedOptions.new
     config.active_storage.previewers = [ ActiveStorage::Previewer::PDFPreviewer, ActiveStorage::Previewer::VideoPreviewer ]
+    config.active_storage.analyzers  = [ ActiveStorage::Analyzer::ImageAnalyzer, ActiveStorage::Analyzer::VideoAnalyzer ]
 
     config.eager_load_namespaces << ActiveStorage
 
     initializer "active_storage.logger" do
-      require "active_storage/service"
-
       config.after_initialize do |app|
-        ActiveStorage::Service.logger = app.config.active_storage.logger || Rails.logger
+        ActiveStorage.logger = app.config.active_storage.logger || Rails.logger
       end
     end
 
@@ -69,5 +71,11 @@ module ActiveStorage
         ActiveStorage.previewers = app.config.active_storage.previewers || []
       end
     end
+
+    initializer "active_storage.analyzers" do
+      config.after_initialize do |app|
+        ActiveStorage.analyzers = app.config.active_storage.analyzers || []
+      end
+    end
   end
 end
diff --git a/activestorage/lib/active_storage/log_subscriber.rb b/activestorage/lib/active_storage/log_subscriber.rb
index 0d00a75c0e..5cbf4bd1a5 100644
--- a/activestorage/lib/active_storage/log_subscriber.rb
+++ b/activestorage/lib/active_storage/log_subscriber.rb
@@ -27,7 +27,7 @@ module ActiveStorage
     end
 
     def logger
-      ActiveStorage::Service.logger
+      ActiveStorage.logger
     end
 
     private
diff --git a/activestorage/lib/active_storage/previewer.rb b/activestorage/lib/active_storage/previewer.rb
index 930b376067..460f6d5678 100644
--- a/activestorage/lib/active_storage/previewer.rb
+++ b/activestorage/lib/active_storage/previewer.rb
@@ -1,10 +1,14 @@
 # frozen_string_literal: true
 
+require "active_storage/downloading"
+
 module ActiveStorage
   # This is an abstract base class for previewers, which generate images from blobs. See
   # ActiveStorage::Previewer::PDFPreviewer and ActiveStorage::Previewer::VideoPreviewer for examples of
   # concrete subclasses.
   class Previewer
+    include Downloading
+
     attr_reader :blob
 
     # Implement this method in a concrete subclass. Have it return true when given a blob from which
@@ -24,37 +28,20 @@ module ActiveStorage
     end
 
     private
-      # Downloads the blob to a new tempfile. Yields the tempfile.
-      #
-      # Use this method to get a tempfile that you can provide to a drawing command.
-      def open # :doc:
-        Tempfile.open("input") do |file|
-          download_blob_to file
-          yield file
-        end
-      end
-
-      def download_blob_to(file)
-        file.binmode
-        blob.download { |chunk| file.write(chunk) }
-        file.rewind
-      end
-
-
       # Executes a system command, capturing its binary output in a tempfile. Yields the tempfile.
       #
-      # Use this method to shell out to system libraries (e.g. mupdf or ffmpeg) for preview image
+      # Use this method to shell out to a system library (e.g. mupdf or ffmpeg) for preview image
       # generation. The resulting tempfile can be used as the +:io+ value in an attachable Hash:
       #
       #   def preview
-      #     open do |input|
+      #     download_blob_to_tempfile do |input|
       #       draw "my-drawing-command", input.path, "--format", "png", "-" do |output|
       #         yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
       #       end
       #     end
       #   end
       def draw(*argv) # :doc:
-        Tempfile.open("output") do |file|
+        Tempfile.open("ActiveStorage") do |file|
           capture(*argv, to: file)
           yield file
         end
diff --git a/activestorage/lib/active_storage/previewer/pdf_previewer.rb b/activestorage/lib/active_storage/previewer/pdf_previewer.rb
index 31a2a8f120..a2f05c74a6 100644
--- a/activestorage/lib/active_storage/previewer/pdf_previewer.rb
+++ b/activestorage/lib/active_storage/previewer/pdf_previewer.rb
@@ -7,7 +7,7 @@ module ActiveStorage
     end
 
     def preview
-      open do |input|
+      download_blob_to_tempfile do |input|
         draw "mutool", "draw", "-F", "png", "-o", "-", input.path, "1" do |output|
           yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
         end
diff --git a/activestorage/lib/active_storage/previewer/video_previewer.rb b/activestorage/lib/active_storage/previewer/video_previewer.rb
index 840d87f100..49f128d142 100644
--- a/activestorage/lib/active_storage/previewer/video_previewer.rb
+++ b/activestorage/lib/active_storage/previewer/video_previewer.rb
@@ -7,11 +7,17 @@ module ActiveStorage
     end
 
     def preview
-      open do |input|
-        draw "ffmpeg", "-i", input.path, "-y", "-vcodec", "png", "-vf", "thumbnail", "-vframes", "1", "-f", "image2", "-" do |output|
+      download_blob_to_tempfile do |input|
+        draw_relevant_frame_from input do |output|
           yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
         end
       end
     end
+
+    private
+      def draw_relevant_frame_from(file, &block)
+        draw "ffmpeg", "-i", file.path, "-y", "-vcodec", "png",
+          "-vf", "thumbnail", "-vframes", "1", "-f", "image2", "-", &block
+      end
   end
 end
diff --git a/activestorage/lib/active_storage/service.rb b/activestorage/lib/active_storage/service.rb
index 1f012da1e7..aa150e4d8a 100644
--- a/activestorage/lib/active_storage/service.rb
+++ b/activestorage/lib/active_storage/service.rb
@@ -41,8 +41,6 @@ module ActiveStorage
     extend ActiveSupport::Autoload
     autoload :Configurator
 
-    class_attribute :logger
-
     class_attribute :url_expires_in, default: 5.minutes
 
     class << self