feat!: removes the Tree method from the NMT wrapper and introduces …

…the `ProveRange` API (#1438)

## Overview
While reviewing the NMT wrapper implementation and attempting to write
its specs, came across a contradiction in the wrapper's design. On one
hand, the wrapper is intended to encapsulate an instance of NMT and
re-define/modify some of its APIs, specifically the
[`Push`](https://github.com/celestiaorg/celestia-app/blob/6d27b78aa64a749a808e84ea682352b8b551fbd7/pkg/wrapper/nmt_wrapper.go#L80)
method. On the other hand, the wrapper also exposes the
[`Tree()`](https://github.com/celestiaorg/celestia-app/blob/6d27b78aa64a749a808e84ea682352b8b551fbd7/pkg/wrapper/nmt_wrapper.go#L112)
method, which returns a pointer to the underlying NMT tree and thus
allows direct access to the tree and enables arbitrary manipulation of
the underlying tree outside of the intended control of the wrapper. For
instance, one can bypass the NMT wrapper `Push` method and directly call
the `Push` method of the underlying tree using `.Tree().Push()`. This
not only widens the error surface but also violates the initial
objective of the wrapper.

To address this issue, the `Tree()` API has been removed, and all
necessary methods of the underlying NMT are now exposed through the
wrapper directly.

## Checklist
- [x] New and updated code has appropriate documentation
- [x] New and updated code has new and/or updated testing
- [x] Required CI checks are passing
- [x] Visual proof for any user facing features like CLI or
documentation updates
- [x] Linked issues closed with keywords

pkg/proof/proof.go

@@ -162,7 +162,7 @@ func NewShareInclusionProof(
 		}
 
 		rawShares = append(rawShares, shares.ToBytes(row[startLeafPos:endLeafPos+1])...)
-		proof, err := tree.Tree().ProveRange(int(startLeafPos), int(endLeafPos+1))
+		proof, err := tree.ProveRange(int(startLeafPos), int(endLeafPos+1))
 		if err != nil {
 			return types.ShareProof{}, err
 		}

pkg/wrapper/nmt_wrapper.go

@@ -104,13 +104,14 @@ func (w *ErasuredNamespacedMerkleTree) Root() []byte {
 	return w.tree.Root()
 }
 
+// Prove returns a Merkle inclusion proof for the leaf at index `ind`.
 func (w *ErasuredNamespacedMerkleTree) Prove(ind int) (nmt.Proof, error) {
 	return w.tree.Prove(ind)
 }
 
-// Tree returns the underlying NamespacedMerkleTree
-func (w *ErasuredNamespacedMerkleTree) Tree() *nmt.NamespacedMerkleTree {
-	return w.tree
+// ProveRange returns a Merkle range proof for the leaf range [start, end] where `end` is non-inclusive.
+func (w *ErasuredNamespacedMerkleTree) ProveRange(start, end int) (nmt.Proof, error) {
+	return w.tree.ProveRange(start, end)
 }
 
 // incrementShareIndex increments the share index by one.

pkg/wrapper/nmt_wrapper_test.go

@@ -110,22 +110,6 @@ func TestComputeExtendedDataSquare(t *testing.T) {
 	assert.NoError(t, err)
 }
 
-// TestErasuredNamespacedMerkleTree verifies that Tree() returns the underlying
-// NMT tree.
-func TestErasuredNamespacedMerkleTree(t *testing.T) {
-	squareSize := 8
-	data := generateRandNamespacedRawData(squareSize, appconsts.NamespaceSize, appconsts.ShareSize-appconsts.NamespaceSize)
-	tree := NewErasuredNamespacedMerkleTree(uint64(squareSize), 0)
-
-	for _, d := range data {
-		tree.Push(d)
-	}
-
-	assert.Equal(t, tree.Tree(), tree.tree)
-	assert.Equal(t, tree.Tree().Root(), tree.tree.Root())
-	assert.Equal(t, appconsts.NamespaceSize, int(tree.Tree().NamespaceSize()))
-}
-
 // generateErasuredData generates random data and then erasure codes it. It
 // returns a slice that is twice as long as numLeaves because it returns the
 // original data + erasured data.