unixfs: integrate pb.Data into FSNode

To avoid duplicating fields and making the code easier to follow.

Remove all of `FSNode` previous fields in favor on a single `pb.Data` structure
that is not exported. Accessor methods are added only for the necessary internal
fields. This takes up more memory, `pb.Data` is always created inside `FSNode`
and it stays there instead of just being created and destroyed during the
(un)marshal operations.

The removed fields `Data`, `blocksizes` and `Type` had a direct counterpart in
the embedded `pb.Data` structure, in contrast (only) the `subtotal` field
doesn't have one, it was used as a temporary accumulator to track the
`Filesize`, which is now being kept updated on every modification (to ensure the
entire `FSNode` is always at a valid state), so `subtotal` could just be removed
without the addition of any other field (this temporary accumulator  was
obscuring how `Filesize` was computed).

To keep `Filesize` up to date a method was added (`UpdateFilesize()`) to adjust
its value in the two places where the file size could be modified, when changing
its data (in `SetData()`, accessor method added) and when adding or removing
child nodes (in `AddBlockSize()` and `RemoveBlockSize()`).

A constructor method was added (`NewFSNode()`) to initialize the required
fields, like `Type` which is explicitly set, this deprecates the previous
methodology of just calling `new(FSNode)` and relying in the default value of
`pb.Data_DataType` (`Data_Raw`) to avoid an explicit assignment. Also,
`Filesize` is initialized to avoid being left with a `nil` value before
marshaling empty nodes, which would result in a different hash from previous
versions, to be backwards compatible. Previous versions of `GetBytes()` always
set the `Filesize` value, even though it is reflected as an `optional` field in
the `.proto` file (this may be an inaccurate field rule).

Without the duplicated fields the functions `GetBytes()` and `FSNodeFromBytes()`
are now reduced to simple `Marshal()` and `Unmarshal()` operations respectively.

License: MIT
Signed-off-by: Lucas Molas <schomatis@gmail.com>

unixfs/unixfs_test.go

@@ -10,14 +10,13 @@ import (
 )
 
 func TestFSNode(t *testing.T) {
-	fsn := new(FSNode)
-	fsn.Type = TFile
+	fsn := NewFSNode(TFile)
 	for i := 0; i < 16; i++ {
 		fsn.AddBlockSize(100)
 	}
 	fsn.RemoveBlockSize(15)
 
-	fsn.Data = make([]byte, 128)
+	fsn.SetData(make([]byte, 128))
 
 	b, err := fsn.GetBytes()
 	if err != nil {