Support File#truncate and File#ftruncate correctly

Turns out that #truncate needed to be a class method, so I moved
it to a class method of FileDescriptor.

Also added support for creating a DirectoryFileDescriptor. It
merely inherits from FileDescriptor now. We will see if it needs
any new behavior or if default behavior needs to be curtailed.

kernel/common/file.rb

@@ -1035,7 +1035,7 @@ def self.truncate(path, length)
 
     length = Rubinius::Type.coerce_to length, Integer, :to_int
 
-    prim_truncate(path, length)
+    FileDescriptor.truncate(path, length)
   end
 
   ##
@@ -1270,7 +1270,7 @@ def truncate(length)
 
     flush
     reset_buffering
-    prim_ftruncate(length)
+    @fd.ftruncate(length)
   end
 
   def inspect

kernel/common/io.rb

@@ -63,7 +63,7 @@ def self.choose_type(fd)
       when "socket"
         raise "cannot make socket yet"
       when "directory"
-        raise "cannot make a directory"
+        DirectoryFileDescriptor.new(fd, stat)
       when "blockSpecial"
         raise "cannot make block special"
       when "link"
@@ -110,6 +110,14 @@ def self.pagesize
       @pagesize ||= FFI::Platform::POSIX.getpagesize
     end
 
+    def self.truncate(name, offset)
+      raise RangeError, "bignum too big to convert into `long'" if offset.kind_of?(Bignum)
+
+      status = FFI::Platform::POSIX.truncate(name, offset)
+      Errno.handle("truncate(2) failed") if FFI.call_failed?(status)
+      return status
+    end
+
     def self.update_max_fd(new_fd)
       @@max_descriptors.get_and_set(new_fd)
     end
@@ -272,6 +280,11 @@ def write(str)
         left -= bytes_written
         buffer += bytes_written
         @offset += bytes_written
+
+        if @offset > @total_size
+          @total_size = @offset
+          @eof = false # only a failed read can set EOF!
+        end
       end
 
       Errno.handle("write failed") if error
@@ -469,14 +482,6 @@ def ftruncate(offset)
       return status
     end
 
-    def truncate(name, offset)
-      # FIXME: fail if +offset+ is too large, see C++ code
-
-      status = FFI::Platform::POSIX.truncate(name, offset)
-      Errno.handle("truncate(2) failed") if FFI.call_failed?(status)
-      return status
-    end
-
     ##
     # Returns true if ios is associated with a terminal device (tty), false otherwise.
     #
@@ -685,6 +690,9 @@ def sysseek(offset, whence=SEEK_SET)
     end
   end # class FIFOFileDescriptor
 
+  class DirectoryFileDescriptor < FileDescriptor
+  end # class DirectoryFileDescriptor
+
   # Encapsulates all of the logic necessary for handling #select.
   class Select
     #eval(Rubinius::Config['rbx.platform.timeval.class'])