Bugfix: treeHash for not-locally-available block

index.js

@@ -1047,12 +1047,16 @@ module.exports = class Hypercore extends EventEmitter {
     return this.core.append(buffers, { keyPair, signature, preappend })
   }
 
-  async treeHash (length) {
+  async treeHash (length, opts = {}) {
+    if (!this.opened) await this.ready()
+
     if (length === undefined) {
-      await this.ready()
       length = this.core.tree.length
     }
 
+    // Ensure the block corresponding with the length is locally available
+    if (length > 0) await this.get(length - 1, opts)
+
     const roots = await this.core.tree.getRoots(length)
     return this.crypto.tree(roots)
   }

test/basic.js

@@ -183,6 +183,34 @@ test('treeHash with default length', async function (t) {
   t.unlike(await core.treeHash(), await core2.treeHash())
 })
 
+test('treeHash of not-locally-available length works', async function (t) {
+  const core = new Hypercore(RAM)
+  await core.append('entry')
+  const remoteCore = new Hypercore(RAM, core.key)
+
+  const s1 = core.replicate(true, { keepAlive: false })
+  const s2 = remoteCore.replicate(false, { keepAlive: false })
+  s1.pipe(s2).pipe(s1)
+
+  await remoteCore.ready()
+  t.is(remoteCore.length, 0, 'Sanity check: initially unsynced')
+
+  const treeHash = await remoteCore.treeHash(1)
+  t.pass('No error when getting treeHash from unsynced core')
+  t.alike(treeHash, await core.treeHash(), 'Sanity check: correct treeHash')
+})
+
+test('treeHash of not-locally-available length works at length 0', async function (t) {
+  const core = new Hypercore(RAM)
+  const remoteCore = new Hypercore(RAM, core.key)
+
+  t.alike(
+    await core.treeHash(0),
+    await remoteCore.treeHash(0),
+    'Can get treeHash(0) on remote core (even without replication)'
+  )
+})
+
 test('snapshot locks the state', async function (t) {
   const core = new Hypercore(RAM)
   await core.ready()