MediaFile: replace APNGInspector with ExifTool.

Replace our own handwritten APNG parser with ExifTool. This makes
ExifTool a hard requirement for handling APNGs.

app/logical/apng_inspector.rb

@@ -1,113 +0,0 @@
-# Parse an PNG and return whether or not it's animated.
-class APNGInspector
-  attr_reader :frames
-
-  PNG_MAGIC_NUMBER = ["89504E470D0A1A0A"].pack('H*')
-
-  def initialize(file_path)
-    @file_path = file_path
-    @corrupted = false
-    @animated = false
-  end
-
-  # PNG file consists of 8-byte magic number, followed by arbitrary number of chunks
-  # Each chunk has the following structure:
-  # 4-byte length (unsigned int, can be zero)
-  # 4-byte name (ASCII string consisting of letters A-z)
-  # (length)-byte data
-  # 4-byte CRC
-  #
-  # Any data after chunk named IEND is irrelevant
-  # APNG frame count is inside a chunk named acTL, in first 4 bytes of data.
-
-  # This function calls associated block for each PNG chunk
-  # parameters passed are |chunk_name, chunk_length, file_descriptor|
-  # returns true if file is read succesfully from start to IEND,
-  # or if 100 000 chunks are read; returns false otherwise.
-  def each_chunk
-    iend_reached = false
-    File.open(@file_path, 'rb') do |file|
-      # check if file is not PNG at all
-      return false if file.read(8) != PNG_MAGIC_NUMBER
-
-      chunks = 0
-
-      # We could be dealing with large number of chunks,
-      # so the code should be optimized to create as few objects as possible.
-      # All literal strings are frozen and read() function uses string buffer.
-      chunkheader = ''
-      while file.read(8, chunkheader)
-        # ensure that first 8 bytes from chunk were read properly
-        if chunkheader.nil? || chunkheader.length < 8
-          return false
-        end
-
-        current_pos = file.tell
-
-        chunk_len, chunk_name = chunkheader.unpack("Na4".freeze)
-        return false if chunk_name =~ /[^A-Za-z]/
-        yield chunk_name, chunk_len, file
-
-        # no need to read further if IEND is reached
-        if chunk_name == "IEND".freeze
-          iend_reached = true
-          break
-        end
-
-        # check if we processed too many chunks already
-        # if we did, file is probably maliciously formed
-        # fail gracefully without marking the file as corrupt
-        chunks += 1
-        if chunks > 100_000
-          iend_reached = true
-          break
-        end
-
-        # jump to the next chunk - go forward by chunk length + 4 bytes CRC
-        file.seek(current_pos + chunk_len + 4, IO::SEEK_SET)
-      end
-    end
-
-    iend_reached
-  end
-
-  def inspect!
-    actl_corrupted = false
-
-    read_success = each_chunk do |name, len, file|
-      if name == 'acTL'.freeze
-        framecount = parse_actl(len, file)
-        if framecount < 1
-          actl_corrupted = true
-        else
-          @animated = true
-          @frames = framecount
-        end
-      end
-    end
-
-    @corrupted = !read_success || actl_corrupted
-    self
-  end
-
-  def corrupted?
-    @corrupted
-  end
-
-  def animated?
-    !@corrupted && @animated
-  end
-
-  private
-
-  # return number of frames in acTL or -1 on failure
-  def parse_actl(len, file)
-    return -1 if len != 8
-    framedata = file.read(4)
-    if framedata.nil? || framedata.length != 4
-      return -1
-    end
-
-    framedata.unpack1("N".freeze)
-  end
-end

app/logical/media_file/image.rb

@@ -80,7 +80,7 @@ class MediaFile::Image < MediaFile
   end
 
   def is_animated_png?
-    file_ext == :png && APNGInspector.new(file.path).inspect!.animated?
+    file_ext == :png && metadata.fetch("PNG:AnimationFrames", 1) > 1
   end
 
   # @return [Vips::Image] the Vips image object for the file

config/initializers/zeitwerk.rb

@@ -1,6 +1,5 @@
 Rails.autoloaders.each do |autoloader|
   autoloader.inflector.inflect(
-    "apng_inspector" => "APNGInspector",
     "sftp" => "SFTP"
   )
 end

test/unit/apng_inspector_test.rb

@@ -1,63 +0,0 @@
-require "test_helper"
-
-class APNGInspectorTest < ActiveSupport::TestCase
-  def inspect(filename)
-    apng = APNGInspector.new("#{Rails.root}/test/files/apng/#{filename}")
-    apng.inspect!
-    apng
-  end
-  context "APNG inspector" do
-    should "correctly parse normal APNG file" do
-      apng = inspect('normal_apng.png')
-      assert_equal(3, apng.frames)
-      assert_equal(true, apng.animated?)
-      assert_equal(false, apng.corrupted?)
-    end
-
-    should "recognize 1-frame APNG as animated" do
-      apng = inspect('single_frame.png')
-      assert_equal(1, apng.frames)
-      assert_equal(true, apng.animated?)
-      assert_equal(false, apng.corrupted?)
-    end
-
-    should "correctly parse normal PNG file" do
-      apng = inspect('not_apng.png')
-      assert_equal(false, apng.animated?)
-      assert_equal(false, apng.corrupted?)
-    end
-
-    should "handle empty file" do
-      apng = inspect('empty.png')
-      assert_equal(false, apng.animated?)
-      assert_equal(true, apng.corrupted?)
-    end
-
-    should "handle corrupted files" do
-      apng = inspect('iend_missing.png')
-      assert_equal(false, apng.animated?)
-      assert_equal(true, apng.corrupted?)
-      apng = inspect('misaligned_chunks.png')
-      assert_equal(false, apng.animated?)
-      assert_equal(true, apng.corrupted?)
-      apng = inspect('broken.png')
-      assert_equal(false, apng.animated?)
-      assert_equal(true, apng.corrupted?)
-    end
-
-    should "handle incorrect acTL chunk" do
-      apng = inspect('actl_wronglen.png')
-      assert_equal(false, apng.animated?)
-      assert_equal(true, apng.corrupted?)
-      apng = inspect('actl_zero_frames.png')
-      assert_equal(false, apng.animated?)
-      assert_equal(true, apng.corrupted?)
-    end
-
-    should "handle non-png files" do
-      apng = inspect('jpg.png')
-      assert_equal(false, apng.animated?)
-      assert_equal(true, apng.corrupted?)
-    end
-  end
-end

test/unit/media_file_test.rb

@@ -190,6 +190,59 @@ class MediaFileTest < ActiveSupport::TestCase
     end
   end
 
+  context "a PNG file" do
+    context "that is not animated" do
+      should "not be detected as animated" do
+        file = MediaFile.open("test/files/apng/not_apng.png")
+
+        assert_equal(false, file.is_corrupt?)
+        assert_equal(false, file.is_animated?)
+      end
+    end
+
+    context "that is animated" do
+      should "be detected as animated" do
+        file = MediaFile.open("test/files/apng/normal_apng.png")
+
+        assert_equal(false, file.is_corrupt?)
+        assert_equal(true, file.is_animated?)
+      end
+    end
+
+    context "that is animated but with only one frame" do
+      should "not be detected as animated" do
+        file = MediaFile.open("test/files/apng/single_frame.png")
+
+        assert_equal(false, file.is_corrupt?)
+        assert_equal(false, file.is_animated?)
+      end
+    end
+
+    context "that is animated but malformed" do
+      should "be handled correctly" do
+        file = MediaFile.open("test/files/apng/iend_missing.png")
+        assert_equal(false, file.is_corrupt?)
+        assert_equal(true, file.is_animated?)
+
+        file = MediaFile.open("test/files/apng/misaligned_chunks.png")
+        assert_equal(true, file.is_corrupt?)
+        assert_equal(true, file.is_animated?)
+
+        file = MediaFile.open("test/files/apng/broken.png")
+        assert_equal(true, file.is_corrupt?)
+        assert_equal(true, file.is_animated?)
+
+        file = MediaFile.open("test/files/apng/actl_wronglen.png")
+        assert_equal(false, file.is_corrupt?)
+        assert_equal(true, file.is_animated?)
+
+        file = MediaFile.open("test/files/apng/actl_zero_frames.png")
+        assert_equal(false, file.is_corrupt?)
+        assert_equal(false, file.is_animated?)
+      end
+    end
+  end
+
   context "a corrupt GIF" do
     should "still read the metadata" do
       @file = MediaFile.open("test/files/test-corrupt.gif")