add assertion that checkout does not break prologue (#672)

Author: chm-diederichs

index.js

@@ -370,6 +370,9 @@ class Hypercore extends EventEmitter {
     if (this.state && checkout !== -1) {
       if (!opts.name && !opts.atom) throw ASSERTION('Checkouts must be named or atomized')
       if (checkout > this.state.length) throw ASSERTION('Invalid checkout ' + checkout + ' for ' + opts.name + ', length is ' + this.state.length)
+      if (this.state.prologue && checkout < this.state.prologue.length) {
+        throw ASSERTION('Invalid checkout ' + checkout + ' for ' + opts.name + ', prologue length is ' + this.state.prologue.length)
+      }
       if (checkout < this.state.length) await this.state.truncate(checkout, this.fork)
     }
 

test/sessions.js

@@ -117,3 +117,44 @@ test('sessions - cannot set checkout if name not set', async function (t) {
 
   await core.close()
 })
+
+test('sessions - checkout breaks prologue', async function (t) {
+  const storage = await createStorage(t)
+  const storage2 = await createStorage(t)
+
+  const core = new Hypercore(storage)
+
+  for (let i = 0; i < 10; i++) await core.append(b4a.from([i]))
+
+  const prologued = new Hypercore(storage2, {
+    manifest: {
+      ...core.manifest,
+      prologue: {
+        hash: await core.treeHash(),
+        length: core.length
+      }
+    }
+  })
+
+  await prologued.ready()
+  await prologued.core.copyPrologue(core.state)
+
+  process.on('uncaughtException', noop)
+
+  let session
+  try {
+    session = prologued.session({ name: 'fail', checkout: 7 })
+    await session.ready()
+    t.fail()
+  } catch (err) {
+    t.pass()
+  }
+
+  await session.close()
+  await prologued.close()
+  await core.close()
+
+  process.off('uncaughtException', noop)
+})
+
+function noop () {}