Make the library fork safe and drop the mutex

When forking, file descriptors are inherited and their state shared.

In the context of MiniMime this means that the offset of the file
opened by RandomAccessDb is shared across processes, so the
`seek + read` combo is subject to inter-process race conditions.

Of course that file is lazily opened, so assuming most applications
don't query MiniMime before fork, it's not a big problem.

However when reforking post boot (e.g. https://github.com/Shopify/pitchfork)
this becomes an issue.

Additionally, even if the file descriptor isn't shared across processes,
the file position is still process global requiring a Mutex.

By using `pread` instead of `seek + read` we can both make the library
fork safe and get rid of the need to synchronize accesses.

This also happens to fix an outstanding JRuby issue.

Fix: #37
Fix: #38

lib/mini_mime.rb

@@ -50,8 +50,6 @@ def binary?
   end
 
   class Db
-    LOCK = Mutex.new
-
     def self.lookup_by_filename(filename)
       extension = File.extname(filename)
       return if extension.empty?
@@ -60,18 +58,14 @@ def self.lookup_by_filename(filename)
     end
 
     def self.lookup_by_extension(extension)
-      LOCK.synchronize do
-        @db ||= new
-        @db.lookup_by_extension(extension) ||
-          @db.lookup_by_extension(extension.downcase)
-      end
+      @db ||= new
+      @db.lookup_by_extension(extension) ||
+        @db.lookup_by_extension(extension.downcase)
     end
 
     def self.lookup_by_content_type(content_type)
-      LOCK.synchronize do
-        @db ||= new
-        @db.lookup_by_content_type(content_type)
-      end
+      @db ||= new
+      @db.lookup_by_content_type(content_type)
     end
 
     class Cache
@@ -146,8 +140,7 @@ def lookup_uncached(val)
       end
 
       def resolve(row)
-        @file.seek(row * @row_length)
-        Info.new(@file.readline)
+        Info.new(@file.pread(@row_length, row * @row_length).force_encoding(Encoding::UTF_8))
       end
     end
 

test/fixtures/custom_content_type_mime.db

@@ -1,2 +1,2 @@
-liquid      application/x-liquid                                                      8bit
+liquid      application/x-liquid                                                      8bit            
 mp4         video/vnd.objectvideo                                                     quoted-printable

test/fixtures/custom_ext_mime.db

@@ -1,2 +1,2 @@
-lua         application/x-lua                                                         8bit
+lua         application/x-lua                                                         8bit            
 m4v         video/vnd.objectvideo                                                     quoted-printable