Add preupgrade hook

index.js

@@ -78,6 +78,9 @@ module.exports = class Hypercore extends EventEmitter {
     this._preappend = preappend.bind(this)
     this._snapshot = null
     this._findingPeers = 0
+
+    this._preupgrade = opts.preupgrade || null
+    this._preupgrading = null
   }
 
   [inspect] (depth, opts) {
@@ -613,8 +616,39 @@ module.exports = class Hypercore extends EventEmitter {
     const activeRequests = (opts && opts.activeRequests) || this.activeRequests
     const req = this.replicator.addUpgrade(activeRequests)
 
+    if (this._preupgrade) this._updateSnapshot()
+
     let upgraded = await req.promise
 
+    if (this._preupgrade) {
+      if (this._preupgrading === null) {
+        const latest = this.session()
+
+        this._preupgrading = Promise.resolve(this._preupgrade(latest))
+        this._preupgrading
+          .catch(noop)
+          .then(() => latest.close())
+      }
+
+      const length = await this._preupgrading
+      const preupgradeLength = this._snapshot.length
+
+      this._preupgrading = null
+
+      if (typeof length === 'number' && length >= preupgradeLength && length < this.core.tree.length) {
+        this._snapshot = {
+          length,
+          byteLength: 0,
+          fork: this.core.tree.fork,
+          compatLength: length
+        }
+
+        return length !== preupgradeLength
+      }
+
+      return this._updateSnapshot()
+    }
+
     if (!this.sparse) {
       // Download all available blocks in non-sparse mode
       const start = this.length

test/preupgrade.js

@@ -0,0 +1,80 @@
+const test = require('brittle')
+const { create, replicate } = require('./helpers')
+
+test('upgrade to latest length', async function (t) {
+  const a = await create()
+  await a.append(['a', 'b', 'c'])
+
+  const b = await create(a.key, {
+    preupgrade (latest) {
+      t.is(b.length, 0)
+      t.is(latest.length, 3)
+
+      return latest.length
+    }
+  })
+
+  replicate(a, b, t)
+  t.is(b.length, 0)
+
+  t.ok(await b.update())
+  t.is(b.length, 3)
+})
+
+test('stay on previous length', async function (t) {
+  const a = await create()
+  await a.append(['a', 'b', 'c'])
+
+  const b = await create(a.key, {
+    preupgrade (latest) {
+      t.is(b.length, 0)
+      t.is(latest.length, 3)
+
+      return 0
+    }
+  })
+
+  replicate(a, b, t)
+  t.is(b.length, 0)
+
+  t.absent(await b.update())
+  t.is(b.length, 0)
+})
+
+test('return no length', async function (t) {
+  const a = await create()
+  await a.append(['a', 'b', 'c'])
+
+  const b = await create(a.key, {
+    preupgrade (latest) {
+      t.is(b.length, 0)
+      t.is(latest.length, 3)
+    }
+  })
+
+  replicate(a, b, t)
+  t.is(b.length, 0)
+
+  t.ok(await b.update())
+  t.is(b.length, 3)
+})
+
+test('return invalid length', async function (t) {
+  const a = await create()
+  await a.append(['a', 'b', 'c'])
+
+  const b = await create(a.key, {
+    preupgrade (latest) {
+      t.is(b.length, 0)
+      t.is(latest.length, 3)
+
+      return -1
+    }
+  })
+
+  replicate(a, b, t)
+  t.is(b.length, 0)
+
+  t.ok(await b.update())
+  t.is(b.length, 3)
+})