Support length opt for update

README.md

@@ -182,6 +182,7 @@ console.log('core was updated?', updated, 'length is', core.length)
 ``` js
 {
   wait: false,
+  length: 0 // minimum length (will hang until core.length is greater than or equal)
   activeRequests: undefined, // Advanced option. Pass requests for replicating blocks
   force: false, // Force an update even if core is writable.
 }

index.js

@@ -9,6 +9,7 @@ const Protomux = require('protomux')
 const id = require('hypercore-id-encoding')
 const safetyCatch = require('safety-catch')
 const unslab = require('unslab')
+const rrp = require('resolve-reject-promise')
 
 const Core = require('./lib/core')
 const Info = require('./lib/info')
@@ -681,6 +682,7 @@ class Hypercore extends EventEmitter {
 
     if (this.writable && (!opts || opts.force !== true)) return false
 
+    const minLength = opts?.length || 0
     const remoteWait = this._shouldWait(opts, this.core.replicator.findingPeers > 0)
 
     let upgraded = false
@@ -701,6 +703,25 @@ class Hypercore extends EventEmitter {
       }
     }
 
+    if (this.length < minLength) {
+      const { promise, resolve, reject } = rrp()
+      const onclose = () => {
+        reject(SESSION_CLOSED('Hypercore closed while waiting for length update'))
+      }
+      const onappend = () => {
+        if (this.length >= minLength) resolve()
+      }
+      this.on('close', onclose)
+      this.on('append', onappend)
+      try {
+        await promise
+      } finally {
+        this.off('close', onclose)
+        this.off('append', onappend)
+      }
+      upgraded = true
+    }
+
     if (!upgraded) return false
     return true
   }

package.json

@@ -58,6 +58,7 @@
     "protomux": "^3.5.0",
     "quickbit-universal": "^2.2.0",
     "random-array-iterator": "^1.0.0",
+    "resolve-reject-promise": "^1.1.0",
     "safety-catch": "^1.0.1",
     "sodium-universal": "^5.0.1",
     "streamx": "^2.12.4",

test/replicate.js

@@ -525,6 +525,50 @@ test('update with zero length', async function (t) {
   t.is(b.length, 0)
 })
 
+test('update with min length', async function (t) {
+  t.plan(2)
+  const a = await create(t)
+  const b = await create(t, a.key)
+
+  replicate(a, b, t)
+
+  let updateResolved = false
+  b.update({ length: 3 })
+    .then(() => {
+      updateResolved = true
+      t.is(b.length, 3, 'update resolved at length 3')
+    })
+    .catch((e) => {
+      console.error(e)
+      t.fail('unexpected error')
+    })
+
+  await a.append('block0')
+  await a.append('block1')
+  t.is(updateResolved, false, 'sanity check')
+  await a.append('block2')
+})
+
+test('update with min length throws if the core closes before reaching the length', async function (t) {
+  t.plan(1)
+  const a = await create(t)
+  const b = await create(t, a.key)
+
+  replicate(a, b, t)
+
+  b.update({ length: 3 })
+    .then(() => {
+      t.fail('should not trigger')
+    })
+    .catch((e) => {
+      t.is(e.code, 'SESSION_CLOSED')
+    })
+
+  await a.append('block0')
+  await a.append('block1')
+  await b.close()
+})
+
 test('basic multiplexing', async function (t) {
   const a1 = await create(t)
   const a2 = await create(t)