Support IO-like objects efficiently

Previously if we passed in an IO object which doesn't respond to #seek,
MimeMagic would read the whole IO into memory. And this is exactly the
type of objects that the Shrine file upload library deals with.

However, we can also "seek" by blank-reading that many bytes, and always
rewinding to the beginning, which is what we've implemented in this
change.

This change also doesn't require IOs to respond to #binmode.

lib/mimemagic.rb

@@ -109,28 +109,27 @@ class MimeMagic
   end
   end
 
 
   def self.magic_match(io, method)
   def self.magic_match(io, method)
-    if io.respond_to?(:seek) && io.respond_to?(:read)
-      io.binmode
-      io.set_encoding(Encoding::BINARY) if io.respond_to?(:set_encoding)
-      buffer = "".force_encoding(Encoding::BINARY)
-      MAGIC.send(method) { |type, matches| magic_match_io(io, matches, buffer) }
-    else
-      str = io.respond_to?(:read) ? io.read : io.to_s
-      magic_match(StringIO.new(str), method)
-    end
+    return magic_match(StringIO.new(io.to_s), method) unless io.respond_to?(:read)
+
+    io.binmode if io.respond_to?(:binmode)
+    io.set_encoding(Encoding::BINARY) if io.respond_to?(:set_encoding)
+    buffer = "".force_encoding(Encoding::BINARY)
+
+    MAGIC.send(method) { |type, matches| magic_match_io(io, matches, buffer) }
   end
   end
 
 
   def self.magic_match_io(io, matches, buffer)
   def self.magic_match_io(io, matches, buffer)
     matches.any? do |offset, value, children|
     matches.any? do |offset, value, children|
       match =
       match =
         if Range === offset
         if Range === offset
-          io.seek(offset.begin)
+          io.read(offset.begin, buffer)
           x = io.read(offset.end - offset.begin + value.bytesize, buffer)
           x = io.read(offset.end - offset.begin + value.bytesize, buffer)
           x && x.include?(value)
           x && x.include?(value)
         else
         else
-          io.seek(offset)
+          io.read(offset, buffer)
           io.read(value.bytesize, buffer) == value
           io.read(value.bytesize, buffer) == value
         end
         end
+      io.rewind
       match && (!children || magic_match_io(io, children, buffer))
       match && (!children || magic_match_io(io, children, buffer))
     end
     end
   end
   end

test/mimemagic_test.rb

 require 'bacon'
 require 'bacon'
 require 'mimemagic'
 require 'mimemagic'
 require 'stringio'
 require 'stringio'
+require 'forwardable'
 
 
 describe 'MimeMagic' do
 describe 'MimeMagic' do
   it 'should have type, mediatype and subtype' do
   it 'should have type, mediatype and subtype' do
@@ -116,12 +117,15 @@ describe 'MimeMagic' do
 
 
   it 'should handle different file objects' do
   it 'should handle different file objects' do
     MimeMagic.add('application/mimemagic-test', magic: [[0, 'MAGICTEST']])
     MimeMagic.add('application/mimemagic-test', magic: [[0, 'MAGICTEST']])
-    class ReadableObj
-      def read
-        'MAGICTEST'
+    class IOObject
+      def initialize
+        @io = StringIO.new('MAGICTEST')
       end
       end
+
+      extend Forwardable
+      delegate [:read, :size, :rewind, :eof?, :close] => :@io
     end
     end
-    MimeMagic.by_magic(ReadableObj.new).should.equal 'application/mimemagic-test'
+    MimeMagic.by_magic(IOObject.new).should.equal 'application/mimemagic-test'
     class StringableObject
     class StringableObject
       def to_s
       def to_s
         'MAGICTEST'
         'MAGICTEST'