Make #resize able to increase size

spec/mfile_spec.cr

@@ -29,4 +29,26 @@ describe MFile do
       file.delete
     end
   end
+
+  describe "#resize" do
+    it "can increase size" do
+      file = File.tempfile "mfile_spec"
+      file.print "hello world"
+      file.flush
+      data = "foo"
+      initial_size = file.size
+      MFile.open(file.path, initial_size) do |mfile|
+        mfile.capacity.should eq initial_size
+        expect_raises(IO::EOFError) { mfile.write data.to_slice }
+        mfile.resize(mfile.size + data.bytesize)
+        mfile.write data.to_slice
+        mfile.capacity.should eq(initial_size + data.bytesize)
+      end
+      file.size.should eq(initial_size + data.bytesize)
+      data = File.read(file.path)
+      data.should eq "hello worldfoo"
+    ensure
+      file.try &.delete
+    end
+  end
 end

src/lavinmq/mfile.cr

@@ -271,9 +271,17 @@ class MFile < IO
   end
 
   def resize(new_size : Int) : Nil
-    raise ArgumentError.new("Can't expand file larger than capacity, use truncate") if new_size > @capacity
-    @size = new_size.to_i64
-    @pos = new_size.to_i64 if @pos > new_size
+    if new_size > @capacity
+      raise File::Error.new("Can't resize readonly file", file: @path) if @readonly
+      unsafe_unmap
+      @capacity = new_size.to_i64
+      code = LibC.ftruncate(@fd, @capacity)
+      raise File::Error.from_errno("Error truncating file", file: @path) if code < 0
+      @buffer = mmap
+    else
+      @size = new_size.to_i64
+      @pos = new_size.to_i64 if @pos > new_size
+    end
   end
 
   # Read from a specific position in the file