Preview PDFs and videos

10 files changed, 135 insertions, 7 deletions

diff --git a/activestorage/lib/active_storage.rb b/activestorage/lib/active_storage.rb
index ccc1d4a163..44d9c25504 100644
--- a/activestorage/lib/active_storage.rb
+++ b/activestorage/lib/active_storage.rb
@@ -33,6 +33,8 @@ module ActiveStorage
 
   autoload :Attached
   autoload :Service
+  autoload :Previewer
 
   mattr_accessor :verifier
+  mattr_accessor :previewers, default: []
 end
diff --git a/activestorage/lib/active_storage/engine.rb b/activestorage/lib/active_storage/engine.rb
index 590a36a30a..335eae8dd8 100644
--- a/activestorage/lib/active_storage/engine.rb
+++ b/activestorage/lib/active_storage/engine.rb
@@ -3,11 +3,15 @@
 require "rails"
 require "active_storage"
 
+require "active_storage/previewer/pdf_previewer"
+require "active_storage/previewer/video_previewer"
+
 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.eager_load_namespaces << ActiveStorage
 
@@ -59,5 +63,11 @@ module ActiveStorage
         end
       end
     end
+
+    initializer "active_storage.previewers" do
+      config.after_initialize do |app|
+        ActiveStorage.previewers = app.config.active_storage.previewers || []
+      end
+    end
   end
 end
diff --git a/activestorage/lib/active_storage/previewer.rb b/activestorage/lib/active_storage/previewer.rb
new file mode 100644
index 0000000000..c91f64ac65
--- /dev/null
+++ b/activestorage/lib/active_storage/previewer.rb
@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+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
+    attr_reader :blob
+
+    # Implement this method in a concrete subclass. Have it return true when given a blob from which
+    # the previewer can generate an image.
+    def self.accept?(blob)
+      false
+    end
+
+    def initialize(blob)
+      @blob = blob
+    end
+
+    # Override this method in a concrete subclass. Have it yield an attachable preview image (i.e.
+    # anything accepted by ActiveStorage::Attached::One#attach).
+    def preview
+      raise NotImplementedError
+    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
+      # generation. The resulting tempfile can be used as the +:io+ value in an attachable Hash:
+      #
+      #   def preview
+      #     open 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|
+          capture *argv, to: file
+          yield file
+        end
+      end
+
+      def capture(*argv, to:)
+        to.binmode
+        IO.popen(argv) { |out| IO.copy_stream(out, to) }
+        to.rewind
+      end
+  end
+end
diff --git a/activestorage/lib/active_storage/previewer/pdf_previewer.rb b/activestorage/lib/active_storage/previewer/pdf_previewer.rb
new file mode 100644
index 0000000000..31a2a8f120
--- /dev/null
+++ b/activestorage/lib/active_storage/previewer/pdf_previewer.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+module ActiveStorage
+  class Previewer::PDFPreviewer < Previewer
+    def self.accept?(blob)
+      blob.content_type == "application/pdf"
+    end
+
+    def preview
+      open 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
+      end
+    end
+  end
+end
diff --git a/activestorage/lib/active_storage/previewer/video_previewer.rb b/activestorage/lib/active_storage/previewer/video_previewer.rb
new file mode 100644
index 0000000000..840d87f100
--- /dev/null
+++ b/activestorage/lib/active_storage/previewer/video_previewer.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+module ActiveStorage
+  class Previewer::VideoPreviewer < Previewer
+    def self.accept?(blob)
+      blob.video?
+    end
+
+    def preview
+      open do |input|
+        draw "ffmpeg", "-i", input.path, "-y", "-vcodec", "png", "-vf", "thumbnail", "-vframes", "1", "-f", "image2", "-" do |output|
+          yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
+        end
+      end
+    end
+  end
+end
diff --git a/activestorage/lib/active_storage/service.rb b/activestorage/lib/active_storage/service.rb
index b80fdea1ab..1f012da1e7 100644
--- a/activestorage/lib/active_storage/service.rb
+++ b/activestorage/lib/active_storage/service.rb
@@ -4,6 +4,7 @@ require "active_storage/log_subscriber"
 
 module ActiveStorage
   class IntegrityError < StandardError; end
+
   # Abstract class serving as an interface for concrete services.
   #
   # The available services are:
@@ -42,6 +43,8 @@ module ActiveStorage
 
     class_attribute :logger
 
+    class_attribute :url_expires_in, default: 5.minutes
+
     class << self
       # Configure an Active Storage service by name from a set of configurations,
       # typically loaded from a YAML file. The Active Storage engine uses this
@@ -113,5 +116,9 @@ module ActiveStorage
         # ActiveStorage::Service::DiskService => Disk
         self.class.name.split("::").third.remove("Service")
       end
+
+      def content_disposition_with(type: "inline", filename:)
+        (type.to_s.presence_in(%w( attachment inline )) || "inline") + "; #{filename.parameters}"
+      end
   end
 end
diff --git a/activestorage/lib/active_storage/service/azure_storage_service.rb b/activestorage/lib/active_storage/service/azure_storage_service.rb
index 895cc9c2f1..27dd192ce6 100644
--- a/activestorage/lib/active_storage/service/azure_storage_service.rb
+++ b/activestorage/lib/active_storage/service/azure_storage_service.rb
@@ -66,7 +66,7 @@ module ActiveStorage
           URI(base_url), false,
           permissions: "r",
           expiry: format_expiry(expires_in),
-          content_disposition: disposition,
+          content_disposition: content_disposition_with(type: disposition, filename: filename),
           content_type: content_type
         ).to_s
 
diff --git a/activestorage/lib/active_storage/service/disk_service.rb b/activestorage/lib/active_storage/service/disk_service.rb
index f600753a08..52eaba4e7b 100644
--- a/activestorage/lib/active_storage/service/disk_service.rb
+++ b/activestorage/lib/active_storage/service/disk_service.rb
@@ -64,9 +64,10 @@ module ActiveStorage
           if defined?(Rails.application)
             Rails.application.routes.url_helpers.rails_disk_service_path \
               verified_key_with_expiration,
-              filename: filename, disposition: disposition, content_type: content_type
+              filename: filename, disposition: content_disposition_with(type: disposition, filename: filename), content_type: content_type
           else
-            "/rails/active_storage/disk/#{verified_key_with_expiration}/#{filename}?content_type=#{content_type}&disposition=#{disposition}"
+            "/rails/active_storage/disk/#{verified_key_with_expiration}/#{filename}?content_type=#{content_type}" \
+              "&disposition=#{content_disposition_with(type: disposition, filename: filename)}"
           end
 
         payload[:url] = generated_url
diff --git a/activestorage/lib/active_storage/service/gcs_service.rb b/activestorage/lib/active_storage/service/gcs_service.rb
index 685dd61a0a..b4ffeeeb8a 100644
--- a/activestorage/lib/active_storage/service/gcs_service.rb
+++ b/activestorage/lib/active_storage/service/gcs_service.rb
@@ -24,12 +24,17 @@ module ActiveStorage
       end
     end
 
-    # FIXME: Add streaming when given a block
+    # FIXME: Download in chunks when given a block.
     def download(key)
       instrument :download, key do
         io = file_for(key).download
         io.rewind
-        io.read
+
+        if block_given?
+          yield io.read
+        else
+          io.read
+        end
       end
     end
 
@@ -54,7 +59,7 @@ module ActiveStorage
     def url(key, expires_in:, filename:, content_type:, disposition:)
       instrument :url, key do |payload|
         generated_url = file_for(key).signed_url expires: expires_in, query: {
-          "response-content-disposition" => disposition,
+          "response-content-disposition" => content_disposition_with(type: disposition, filename: filename),
           "response-content-type" => content_type
         }
 
diff --git a/activestorage/lib/active_storage/service/s3_service.rb b/activestorage/lib/active_storage/service/s3_service.rb
index e074269353..3e93cdd072 100644
--- a/activestorage/lib/active_storage/service/s3_service.rb
+++ b/activestorage/lib/active_storage/service/s3_service.rb
@@ -55,7 +55,7 @@ module ActiveStorage
     def url(key, expires_in:, filename:, disposition:, content_type:)
       instrument :url, key do |payload|
         generated_url = object_for(key).presigned_url :get, expires_in: expires_in.to_i,
-          response_content_disposition: disposition,
+          response_content_disposition: content_disposition_with(type: disposition, filename: filename),
           response_content_type: content_type
 
         payload[:url] = generated_url