From b68207610c57139d409ca7958f9c8386f9b8002e Mon Sep 17 00:00:00 2001
From: Braydon Fuller <courier@braydon.com>
Date: Wed, 1 May 2019 16:20:33 -0700
Subject: [PATCH] blockchain: do not accept forked chain older than last
 checkpoint

This has been patched in both bitcoind and btcd around February 20th, 2014. It was
shortly followed by a headers-first synchronization of blocks.

- https://github.com/bitcoin/bitcoin/commit/d8b4b49667f3eaf5ac16c218aaba2136ece907d8
- https://github.com/btcsuite/btcd/commit/50b6e10b5781772b0f6a506535f575d81a95dc7a
---
 lib/blockchain/chain.js | 10 ++++++++
 test/chain-test.js      | 53 +++++++++++++++++++++++++++++++++++------
 2 files changed, 56 insertions(+), 7 deletions(-)

diff --git a/lib/blockchain/chain.js b/lib/blockchain/chain.js
index 7e94c6a7..93271b0e 100644
--- a/lib/blockchain/chain.js
+++ b/lib/blockchain/chain.js
@@ -1104,6 +1104,16 @@ class Chain extends AsyncEmitter {
    */
 
   async saveAlternate(entry, block, prev, flags) {
+    // Do not accept forked chain older than the
+    // last checkpoint.
+    if (this.options.checkpoints) {
+      if (prev.height + 1 < this.network.lastCheckpoint)
+        throw new VerifyError(block,
+          'checkpoint',
+          'bad-fork-prior-to-checkpoint',
+          100);
+    }
+
     try {
       // Do as much verification
       // as we can before saving.
diff --git a/test/chain-test.js b/test/chain-test.js
index 9c915031..45cfbd10 100644
--- a/test/chain-test.js
+++ b/test/chain-test.js
@@ -121,17 +121,21 @@ chain.on('disconnect', (entry, block) => {
 describe('Chain', function() {
   this.timeout(process.browser ? 1200000 : 60000);
 
-  it('should open chain and miner', async () => {
+  before(async () => {
     await blocks.open();
     await chain.open();
     await miner.open();
-  });
 
-  it('should add addrs to miner', async () => {
     miner.addresses.length = 0;
     miner.addAddress(wallet.getReceive());
   });
 
+  after(async () => {
+    await miner.close();
+    await chain.close();
+    await blocks.close();
+  });
+
   it('should mine 200 blocks', async () => {
     for (let i = 0; i < 200; i++) {
       const block = await cpu.mineBlock();
@@ -900,9 +904,44 @@ describe('Chain', function() {
     assert(fmt.includes('chainwork'));
   });
 
-  it('should cleanup', async () => {
-    await miner.close();
-    await chain.close();
-    await blocks.close();
+  describe('Checkpoints', function() {
+    before(async () => {
+      const entry = await chain.getEntry(chain.tip.height - 5);
+      assert(Buffer.isBuffer(entry.hash));
+      assert(Number.isInteger(entry.height));
+
+      network.checkpointMap[entry.height] = entry.hash;
+      network.lastCheckpoint = entry.height;
+    });
+
+    after(async () => {
+      network.checkpointMap = {};
+      network.lastCheckpoint = 0;
+    });
+
+    it('will reject blocks before last checkpoint', async () => {
+      const entry = await chain.getEntry(chain.tip.height - 10);
+      const block = await cpu.mineBlock(entry);
+
+      let err = null;
+
+      try {
+        await chain.add(block);
+      } catch (e) {
+        err = e;
+      }
+
+      assert(err);
+      assert.equal(err.type, 'VerifyError');
+      assert.equal(err.reason, 'bad-fork-prior-to-checkpoint');
+      assert.equal(err.score, 100);
+    });
+
+    it('will accept blocks after last checkpoint', async () => {
+      const entry = await chain.getEntry(chain.tip.height - 4);
+      const block = await cpu.mineBlock(entry);
+
+      assert(await chain.add(block));
+    });
   });
 });