added core.update({ minLength: n }) and core.waitForLength(n)

index.js

@@ -626,11 +626,28 @@ module.exports = class Hypercore extends EventEmitter {
       if (!upgraded) upgraded = this.contiguousLength !== contig
     }
 
+    if (opts && typeof opts.minLength === 'number') {
+      await this.waitForLength(opts.minLength)
+    }
+
     if (!upgraded) return false
     if (this.snapshotted) return this._updateSnapshot()
     return true
   }
 
+  waitForLength (minLength = 0) {
+    return new Promise(resolve => {
+      if (this.length >= minLength) return resolve(this.length)
+      const onAppend = () => {
+        if (this.length >= minLength) {
+          this.removeListener('append', onAppend)
+          resolve(this.length)
+        }
+      }
+      this.on('append', onAppend)
+    })
+  }
+
   async seek (bytes, opts) {
     if (this.opened === false) await this.opening
 

test/replicate.js

@@ -840,3 +840,55 @@ test('sparse replication without gossiping', async function (t) {
     t.alike(await c.seek(4), [4, 0])
   })
 })
+
+test('sparse update with minLength', async function (t) {
+  const a = await create()
+  const b = await create(a.key)
+  replicate(a, b, t)
+
+  await a.append(['1', '2'])
+  await b.update()
+  t.is(b.length, 2)
+
+  const updateLength5 = t.test('updateLength5')
+  updateLength5.plan(1)
+
+  b.update({ minLength: 5 }).then(() => {
+    updateLength5.pass()
+  })
+
+  await a.append(['3'])
+  await eventFlush()
+  await a.append(['4'])
+  await eventFlush()
+  await a.append(['5'])
+
+  await updateLength5
+  t.is(b.length, 5)
+})
+
+test('non-sparse update with minLength', async function (t) {
+  const a = await create()
+  const b = await create(a.key, { sparse: false })
+  replicate(a, b, t)
+
+  await a.append(['1', '2'])
+  await b.update()
+  t.is(b.length, 2)
+
+  const updateLength5 = t.test('updateLength5')
+  updateLength5.plan(1)
+
+  b.update({ minLength: 5 }).then(() => {
+    updateLength5.pass()
+  })
+
+  await a.append(['3'])
+  await eventFlush()
+  await a.append(['4'])
+  await eventFlush()
+  await a.append(['5'])
+
+  await updateLength5
+  t.is(b.length, 5)
+})