blockchain: HeightToHashRange function for pulling range of hashes.

This functionality is required to implement BIP 157.

blockchain/chain.go

@@ -1322,6 +1322,48 @@ func (b *BlockChain) HeightRange(startHeight, endHeight int32) ([]chainhash.Hash
 	return hashes, nil
 }
 
+// HeightToHashRange returns a range of block hashes for the given start height
+// and end hash, inclusive on both ends.  The hashes are for all blocks that are
+// ancestors of endHash with height greater than or equal to startHeight.  The
+// end hash must belong to a block that is known to be valid.
+//
+// This function is safe for concurrent access.
+func (b *BlockChain) HeightToHashRange(startHeight int32,
+	endHash *chainhash.Hash, maxResults int) ([]chainhash.Hash, error) {
+
+	endNode := b.index.LookupNode(endHash)
+	if endNode == nil {
+		return nil, fmt.Errorf("no known block header with hash %v", endHash)
+	}
+	if !b.index.NodeStatus(endNode).KnownValid() {
+		return nil, fmt.Errorf("block %v is not yet validated", endHash)
+	}
+	endHeight := endNode.height
+
+	if startHeight < 0 {
+		return nil, fmt.Errorf("start height (%d) is below 0", startHeight)
+	}
+	if startHeight > endHeight {
+		return nil, fmt.Errorf("start height (%d) is past end height (%d)",
+			startHeight, endHeight)
+	}
+
+	resultsLength := int(endHeight - startHeight + 1)
+	if resultsLength > maxResults {
+		return nil, fmt.Errorf("number of results (%d) would exceed max (%d)",
+			resultsLength, maxResults)
+	}
+
+	// Walk backwards from endHeight to startHeight, collecting block hashes.
+	node := endNode
+	hashes := make([]chainhash.Hash, resultsLength)
+	for i := resultsLength - 1; i >= 0; i-- {
+		hashes[i] = node.hash
+		node = node.parent
+	}
+	return hashes, nil
+}
+
 // locateInventory returns the node of the block after the first known block in
 // the locator along with the number of subsequent nodes needed to either reach
 // the provided stop hash or the provided max number of entries.

blockchain/chain_test.go

@@ -800,3 +800,95 @@ func TestLocateInventory(t *testing.T) {
 		}
 	}
 }
+
+// TestHeightToHashRange ensures that fetching a range of block hashes by start
+// height and end hash works as expected.
+func TestHeightToHashRange(t *testing.T) {
+	// Construct a synthetic block chain with a block index consisting of
+	// the following structure.
+	// 	genesis -> 1 -> 2 -> ... -> 15 -> 16  -> 17  -> 18
+	// 	                              \-> 16a -> 17a -> 18a (unvalidated)
+	tip := tstTip
+	chain := newFakeChain(&chaincfg.MainNetParams)
+	branch0Nodes := chainedNodes(chain.bestChain.Genesis(), 18)
+	branch1Nodes := chainedNodes(branch0Nodes[14], 3)
+	for _, node := range branch0Nodes {
+		chain.index.SetStatusFlags(node, statusValid)
+		chain.index.AddNode(node)
+	}
+	for _, node := range branch1Nodes {
+		if node.height < 18 {
+			chain.index.SetStatusFlags(node, statusValid)
+		}
+		chain.index.AddNode(node)
+	}
+	chain.bestChain.SetTip(tip(branch0Nodes))
+
+	tests := []struct {
+		name        string
+		startHeight int32            // locator for requested inventory
+		endHash     chainhash.Hash   // stop hash for locator
+		maxResults  int              // max to locate, 0 = wire const
+		hashes      []chainhash.Hash // expected located hashes
+		expectError bool
+	}{
+		{
+			name:        "blocks below tip",
+			startHeight: 11,
+			endHash:     branch0Nodes[14].hash,
+			maxResults:  10,
+			hashes:      nodeHashes(branch0Nodes, 10, 11, 12, 13, 14),
+		},
+		{
+			name:        "blocks on main chain",
+			startHeight: 15,
+			endHash:     branch0Nodes[17].hash,
+			maxResults:  10,
+			hashes:      nodeHashes(branch0Nodes, 14, 15, 16, 17),
+		},
+		{
+			name:        "blocks on stale chain",
+			startHeight: 15,
+			endHash:     branch1Nodes[1].hash,
+			maxResults:  10,
+			hashes: append(nodeHashes(branch0Nodes, 14),
+				nodeHashes(branch1Nodes, 0, 1)...),
+		},
+		{
+			name:        "invalid start height",
+			startHeight: 19,
+			endHash:     branch0Nodes[17].hash,
+			maxResults:  10,
+			expectError: true,
+		},
+		{
+			name:        "too many results",
+			startHeight: 1,
+			endHash:     branch0Nodes[17].hash,
+			maxResults:  10,
+			expectError: true,
+		},
+		{
+			name:        "unvalidated block",
+			startHeight: 15,
+			endHash:     branch1Nodes[2].hash,
+			maxResults:  10,
+			expectError: true,
+		},
+	}
+	for _, test := range tests {
+		hashes, err := chain.HeightToHashRange(test.startHeight, &test.endHash,
+			test.maxResults)
+		if err != nil {
+			if !test.expectError {
+				t.Errorf("%s: unexpected error: %v", test.name, err)
+			}
+			continue
+		}
+
+		if !reflect.DeepEqual(hashes, test.hashes) {
+			t.Errorf("%s: unxpected hashes -- got %v, want %v",
+				test.name, hashes, test.hashes)
+		}
+	}
+}