Provide an alternative PDF previewer based on Poppler

mutool is licensed under the Affero GPL, which has strict distribution requirements.

Poppler is licensed under the more liberal GPL, making it a good alternative for those who can't use mutool.

.travis.yml

@@ -24,6 +24,7 @@ addons:
       - ffmpeg
       - mupdf
       - mupdf-tools
+      - poppler-utils
 
 bundler_args: --without test --jobs 3 --retry 3
 before_install:

activestorage/app/models/active_storage/preview.rb

@@ -21,10 +21,9 @@
 #
 # 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] (version 1.8 or newer)
+# The built-in previewers rely on third-party system libraries. Specifically, the built-in video previewer requires
+# {ffmpeg}[https://www.ffmpeg.org]. Two PDF previewers are provided: one requires {Poppler}[https://poppler.freedesktop.org],
+# and the other requires {mupdf}[https://mupdf.com] (version 1.8 or newer). To preview PDFs, install either Poppler or mupdf.
 #
 # 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.

activestorage/lib/active_storage/engine.rb

@@ -3,7 +3,8 @@
 require "rails"
 require "active_storage"
 
-require "active_storage/previewer/pdf_previewer"
+require "active_storage/previewer/poppler_pdf_previewer"
+require "active_storage/previewer/mupdf_previewer"
 require "active_storage/previewer/video_previewer"
 
 require "active_storage/analyzer/image_analyzer"
@@ -14,7 +15,7 @@ module ActiveStorage
     isolate_namespace ActiveStorage
 
     config.active_storage = ActiveSupport::OrderedOptions.new
-    config.active_storage.previewers = [ ActiveStorage::Previewer::PDFPreviewer, ActiveStorage::Previewer::VideoPreviewer ]
+    config.active_storage.previewers = [ ActiveStorage::Previewer::PopplerPDFPreviewer, ActiveStorage::Previewer::MuPDFPreviewer, ActiveStorage::Previewer::VideoPreviewer ]
     config.active_storage.analyzers = [ ActiveStorage::Analyzer::ImageAnalyzer, ActiveStorage::Analyzer::VideoAnalyzer ]
     config.active_storage.paths = ActiveSupport::OrderedOptions.new
 

activestorage/lib/active_storage/previewer/mupdf_previewer.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module ActiveStorage
+  class Previewer::MuPDFPreviewer < Previewer
+    class << self
+      def accept?(blob)
+        blob.content_type == "application/pdf" && mutool_exists?
+      end
+
+      def mutool_path
+        ActiveStorage.paths[:mutool] || "mutool"
+      end
+
+      def mutool_exists?
+        return @mutool_exists unless @mutool_exists.nil?
+
+        system mutool_path, out: File::NULL, err: File::NULL
+
+        @mutool_exists = $?.exitstatus == 1
+      end
+    end
+
+    def preview
+      download_blob_to_tempfile do |input|
+        draw_first_page_from input do |output|
+          yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
+        end
+      end
+    end
+
+    private
+      def draw_first_page_from(file, &block)
+        draw self.class.mutool_path, "draw", "-F", "png", "-o", "-", file.path, "1", &block
+      end
+  end
+end

activestorage/lib/active_storage/previewer/pdf_previewer.rb

@@ -1,26 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveStorage
-  class Previewer::PDFPreviewer < Previewer
-    def self.accept?(blob)
-      blob.content_type == "application/pdf"
-    end
-
-    def preview
-      download_blob_to_tempfile do |input|
-        draw_first_page_from input do |output|
-          yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
-        end
-      end
-    end
-
-    private
-      def draw_first_page_from(file, &block)
-        draw mutool_path, "draw", "-F", "png", "-o", "-", file.path, "1", &block
-      end
-
-      def mutool_path
-        ActiveStorage.paths[:mutool] || "mutool"
-      end
-  end
-end

activestorage/lib/active_storage/previewer/poppler_pdf_previewer.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+module ActiveStorage
+  class Previewer::PopplerPDFPreviewer < Previewer
+    class << self
+      def accept?(blob)
+        blob.content_type == "application/pdf" && pdftoppm_exists?
+      end
+
+      def pdftoppm_path
+        ActiveStorage.paths[:pdftoppm] || "pdftoppm"
+      end
+
+      def pdftoppm_exists?
+        return @pdftoppm_exists unless @pdftoppm_exists.nil?
+
+        @pdftoppm_exists = system(pdftoppm_path, "-v", out: File::NULL, err: File::NULL)
+      end
+    end
+
+    def preview
+      download_blob_to_tempfile do |input|
+        draw_first_page_from input do |output|
+          yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
+        end
+      end
+    end
+
+    private
+      def draw_first_page_from(file, &block)
+        # use 72 dpi to match thumbnail  dimesions of the PDF
+        draw self.class.pdftoppm_path, "-singlefile", "-r", "72", "-png", file.path, &block
+      end
+  end
+end

activestorage/test/previewer/mupdf_previewer_test.rb

@@ -3,15 +3,15 @@
 require "test_helper"
 require "database/setup"
 
-require "active_storage/previewer/pdf_previewer"
+require "active_storage/previewer/mupdf_previewer"
 
-class ActiveStorage::Previewer::PDFPreviewerTest < ActiveSupport::TestCase
+class ActiveStorage::Previewer::MuPDFPreviewerTest < ActiveSupport::TestCase
   setup do
     @blob = create_file_blob(filename: "report.pdf", content_type: "application/pdf")
   end
 
   test "previewing a PDF document" do
-    ActiveStorage::Previewer::PDFPreviewer.new(@blob).preview do |attachable|
+    ActiveStorage::Previewer::MuPDFPreviewer.new(@blob).preview do |attachable|
       assert_equal "image/png", attachable[:content_type]
       assert_equal "report.png", attachable[:filename]
 

activestorage/test/previewer/poppler_pdf_previewer_test.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "database/setup"
+
+require "active_storage/previewer/poppler_pdf_previewer"
+
+class ActiveStorage::Previewer::PopplerPDFPreviewerTest < ActiveSupport::TestCase
+  setup do
+    @blob = create_file_blob(filename: "report.pdf", content_type: "application/pdf")
+  end
+
+  test "previewing a PDF document" do
+    ActiveStorage::Previewer::PopplerPDFPreviewer.new(@blob).preview do |attachable|
+      assert_equal "image/png", attachable[:content_type]
+      assert_equal "report.png", attachable[:filename]
+
+      image = MiniMagick::Image.read(attachable[:io])
+      assert_equal 612, image.width
+      assert_equal 792, image.height
+    end
+  end
+end